Calcidermoid Scales: Proposal for a New Primary Fish Scale Category, Division into Types, Their Origin, Relation to Elasmoid Scales, and Taxonomic Distribution

Type 1A single-plate upright-spined (Figs. 2, 3).—

These calci-spinoid scales have one or more individual spines that rise vertically from a circular or approximately circular scale base (Figs. 1C, D, 2, 3). The scales of the psychrolutid T. pingelii above the lateral line exemplify the multi-spined 1A type (Figs. 1D, 2A–E). Contiguous scale plates have a small number of spines. The scale plates are tightly bound along their margin with no spaces between them (Fig. 2A). They are diverse in morphology and size. The multi-spined type 1A scales of T. pingelii fall into two size groups. Figure 2C shows the variation in each with tiny scales in a single row above two rows of large scales. In the row of tiny scales, the fifth scale has its inner surface outmost, while the other scales are mounted with their outer surfaces outermost. The inner surface of this scale shows the complete mineralization of the scale. Most scales of this species are small, but a line of large scales runs parallel to the dorsal fin, as seen in Figure 1D near the right-hand cut edge of skin preparation. These are the scales shown in the bottom two rows of Figure 2C. On the small scales, the spines in T. pingelii are either evenly distributed over a wide area excluding the marginal zones of the scale or arranged as a horseshoe centrally (Fig. 2B, D). The spines in the horseshoe shape flared outward or recurved backward toward the posterior margin. The remains of anchoring fiber bundles appear as tiny white dots, and their former presence as holes or short, dark streaks on the outer surfaces of the scales (Fig. 2B, D, E). Spines on the small scales lost during preparation leave shallow ring-shaped craters showing their former positions on the scale plate (Fig. 2A). The spines are grouped centrally in a tight crescent shape and point toward the margin on the large scales. The scales of the psychrolutid Icelus bicornis are multi-spined type 1A (Fig. 2F). They are like some of the small scales of T. pingelii and have cracking of the scale surface.

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A cracked scale surface occurs in the scales of T. pingelii and many other species (Fig. 2A, D, E). This cracking is a dehydration artifact created during specimen preparation. Although a product of the preparation method, this artifact is extremely useful in identifying the presence of an outer hypermineralized layer. However, for identification, it does have the limitation of reaching a certain thickness before it cracks. Hyaloine is a name created by Sire (1993) for the outer hypermineralized layer found on the scales of the armored catfish. For this reason, and because it does not correspond to any other hypermineralized outer layer, the hypermineralized outer layer in calcidermoid scales is called hyaloine in the current study.

Scales in the synanceiids are single-spined type 1A scales (Figs. 1C, 2G–O). The scales have foramina in the neck of the spine and base, indicating an odontode origin. The scale plates of synanceiid scales are about the same size on each fish, but the spines are variable. The variants are hollowed-out spines on one side on the scale bases of Adventor elongatus (Fig. 2G, H), bifurcate spines on the scale bases of Paraploactis pulvinus (Fig. 1C, J, K), and compact, unbranched spines on the scale bases of Aploactisoma milesii (Fig. 2M, N). In the three species, the spine has a foramen in its neck near the scale plate, and the scale plates have a foramen on the inner side, except in some scales on the same specimen of A. milesii, where it is compact and occupied by fiber bundles. In the three species, the outer sides of the scale plates have an outer hyaloine layer that is thicker in the scale center than in the marginal zone. Thus, the marginal zones tend to be brittle and break away entirely or partially from the rigid centers of the scales during their preparation (Fig. 2G, O). In A. elongatus, the marginal zones curl inward (Fig. 2I). The scale’s center is covered with organic material, remaining after scale preparation, making it dark and emphasizing the convex nature of the scale plate. Elasmodine rings are visible in the marginal zones. The formation of the Mandl’s corpuscles, seen as small ovoid particles, results in the mineralization of elasmodine (Fig. 2L, O). The tiny holes in the thicker scale centers show the positions where anchoring fibers have previously attached the scales of synanceiid species to the compact dermis (Fig. 2G, K, I, M, N).

Single-spined type 1A scales are found on the antennariid Antennarius commerson (Fig. 3A–C). In this species, the scales have bifurcate spines that arise from the scale plate subcentrally. The scale plate margin is either smoothly curved or scalloped. There is hyaloine over the whole surface, thicker in the center than in the marginal area. The holes in the hyaloine layer indicate the position of former attachment fibers. The images of the upper side of the scale show subsurface elasmodine rings.

The agonid Hemitripterus americanus has single-spined type 1A scales, which differ significantly from the description of this scale type above, particularly for the larger scales (Fig. 3D–F). On these scales, a superficial layer of spongy bone covers the elasmodine layer, and the spine is a honeycomb-like structure. The spongy bone is thicker in the center of the scale around the spine, with extensive cross-linking of trabeculae. In the marginal zone, the trabeculae are present mainly as radial struts. The elasmodine rings are close together and clearly defined. The scale plates of H. americanus are round with a scalloped margin (Fig. 3D, F). The elasmodine is entirely mineralized. Scale plates of the more minor scales lack spongy bone and have a layer of hyaloine (Fig. 3E). The bases of the spines of these scales appear to have tunnels going into the base and spine, but the upper parts of the spines are compact. Minute type 1A scales occur on larger scales (Fig. 3D). The agonid Blepsias cirrhosus has round single-spined type 1A scales with long central spines (no image). A hyaloine layer covers the scale plate centrally, and elasmodine rings are present.

Another variation of the type 1A scale with a single spine occurs in the congiopodid Zanclorhynchus spinifer (Fig. 3G, H). The scales are round with a deeply scalloped margin. They have central spines, sometimes long and attenuated, surrounded by thick ridges radiating from the spine to the margin. The ridges may branch, but they remain unconnected to each other. There are holes along the side of the ridges, left after removing attachment fibers. Elasmodine rings are visible. A cracked hypermineralized outer layer covers the entire surface, indicating the presence of hyaloine on the outer side (Fig. 3G). There is a large site on the scale’s inner side for attaching the scales to the dermis, with holes indicating the previous sites of attachment fibers (Fig. 3I).

Type 1B single-plate horizontal-spined (Fig. 4).—

This calci-spinoid scale type consists of a spine lying horizontally to a highly modified scale plate (Figs. 1F, 4). The scale plate appears as the base of the spine. The morphology of the scale plates of type 1B scales is variable within and between species. In the cottid Cottus asper, the margin of the scale plate is slightly to significantly lobate, with its outer and inner sides flattened against each other (Fig. 4A–C). As their cracked surfaces indicate, a hyaloine layer covers the spine and scale plate. Areas showing the former presence of attachment collagen fiber bundles are seen on the underside of the scale plate and inner side and lower part of the spine (Fig. 4B, C). In Cottus gobio, the margin of the scale plate is round and smooth (Fig. 4D). The scale plate is about half the size expected compared to a type 1A scale (Fig. 4E, F). The inner side of the scale plate is undivided or divided into compartments. Areas showing attachment sites for collagen fibers are on the outer and inner sides of the scale and the lower part of the spines. An outer hyaloine layer appears to be present, but there is only minimal cracking, perhaps indicating it is thinner than the severely cracked surface in Cottus asper. In the tetraodontid Torquigener pleurogramma, the spine is more prolonged and sharper than in the scales of the C. asper and C. gobio (Fig. 4G, I). The margin of the scale plate is either smooth or spined (Fig. 4G–I. The scale plate spines are short and sharp and form the extremity of the soft ridges on the outer scale plate surface (Fig. 4H). The outer and inner sides of the scale plate are flattened against each other (Fig. 4I).

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Type 1C single-plate blunt-spined.—

This calci-spinoid scale type is a plate with short, blunt spines. In the congiopodid Zanclorhynchus spinifer, they are represented by scales with a plate indistinguishable from the plate in the type 1A scales of this species but have stumps with flat surfaces.

Type 1D no-plate single-spined (literature).—

These calci-spinoid scales have a single spine with no scale plate or distinct base, and the scales are small, slender, elongate, needle-shaped, or lanceolate (Smith, 1970). Variants are found in the liparids Careproctus acanthodes, C. ostentum, C. rastrinus, and C. trachysoma, in which they occur in groups of 4–5 or 10 to 12 or more (Burke, 1930). Burke distinguished type 1D from type 1A scales, his thumbtack scales. He described the former as groups of spines arising close together in the dermis. Orr et al. (2015) reviewed the species of the Careproctus rastrinus complex, describing many species as having cactus-like prickles. Type 1D scales occur in the carangids Scomberoides lysan and Scomberoides tala (Smith, 1970) and in the macroramphosid Macroramphosus sp. (Burdak et al., 1986).

Type 1E micro-carapace (literature).—

Micro-carapace calci-spinoid scales have interdigitations along the scale edges, allowing them to articulate more tightly. Katayama and Matsuura (2016) used secondary SEM to describe the morphology of type 1E scales of the molids Masturus lanceolatus, Mola mola, and Ranzania laevis. In M. lanceolatus, the scales were elliptical or rounded and varied in size. The larger scales had central spines with ridges radiating from them, but not the smaller ones. In M. mola, the type 1E scales were round and in two sizes. Short upright spines in the center of the larger scale were surrounded by developed radial ridges that were absent around the poorly developed spines in the smaller scales. In Ranzania laevis, the scales are hexagonal and all about the same size. They have a row of 1–4 short and blunt spines in the central part of the scale.

Type 1F double-plate (Fig. 5).—

This scale type consists of two scale plates, one horizontal and a second plate projecting from the horizontal plate with robust spines growing out of its posterior margin or lateral surface. Type 1F scales are found ventral to the lateral line scales in the psychrolutid Triglops pingelii (Fig. 5A–C). The tiny scales are arranged linearly in oblique dermal folds (Fig. 1D). Only the spines on the posterior margins of the second plates in these scales are visible after removing the loose dermis. When isolated, the oblique plates arise from centers of horizontal plates over which they hang (Fig. 5A–C). The horizontal plate is convex. The scales of a 95 mm SL specimen of T. pingelii have three robust spines in the oblique, elevated second scale plate, so numerous scales are adjacent in each diagonal dermal fold (Figs. 1D, 5A, C).

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The scales of the agonid Hemilepidotus hemilepidotus include type 1F scales (Fig. 5D–F). They occur in scale bands with scaleless areas between the bands. Dorsal and ventral band scales fall into different calci-spinoid scale types. The difference between them lies in the curvature of the oblique second plate dorsally and ventrally, its spined margin, and the formation of a pedicel. Double-plate pedicel (type 1G) scales are in the anterior part of the dorsal band. Type 1F scales are in the central and posterior regions of the dorsal and ventral bands. In the type 1F scales of H. hemilepidotus, the oblique second plate’s posterior margin supports many robust spines growing laterally. The posterior margin is slightly curved and extends beyond the horizontal scale plate dorsally and ventrally. The oblique second plate arises from the center of the horizontal plate, and its posterior margin spines overhang the posterior margin of the convex horizontal plate.

Type 1F scales are present in scale bands in the skin of the psychrolutid Artedius notospilotus (Fig. 5G–I). They have an oblique second plate that arises near the anterior margin and supports a single row of many robust spines. In the center of the inner surface of the horizontal plate, there are two foramina. Around the foramen, there are many sites for the previous attachment of bundles of collagen fibers. Type 1F scales are found in the psychrolutids Icelinus borealis and I. quadriseriatus, where they occur in bands two scales wide (no images). They differ from the above type 1F scales in that the strongly spined margins of the oblique second scale plates face each other obliquely. The margin of the upper series is posterior and ventral, and the margin of the lower series is posterior and dorsal. If not for the swiveling of the oblique scale plate, they are like the least swiveled scale plates of A. notospilotus.

The zaniolepidid Zaniolepis latipinnis is wholly covered with type 1F scales of variable size, with tiny ones on the top of the lateral line scales (Figs. 1E, 5J–L). The second plate, which is oblique, supports a single row of many robust spines. There are curved edges anterior to the spines, nearly obliterated, indicating an earlier row of spines was present and resorbed. Unlike the above type 1F scales, the horizontal scale plate is hidden in the image of the outer surface of the scale. This observation indicates that the oblique second scale plate arises at the anterior margin of the horizontal scale plate. On the inner surface of scales of Z. latipinnis, there is a hyaloine layer covering the elasmodine rings and many attachment sites with the fibers mineralized at the level of the scale surface.

The trachichthyid Trachichthys australis is covered with type 1F scales (Fig. 5M–O). Superficially, it appears that type 3 elasmo-spinoid scales cover the fish because of the similarity of the arrangement of the exposed elevated posterior areas to the typical arrangement of upright spines in the exposed posterior fields of elasmoid scales and their superficial resemblance to an elasmoid scale arrangement on the fish. In Trachichthys australis, the elevated spined second plate rises sharply from the horizontal scale plate and curves over in the posterior direction. The spines covering the whole posterior area of the second plate are more delicate than those of other type 1F scales. The width of the posterior part of the horizontal plate is less than the anterior part of the plate (Fig. 5N). The inner sides of the spined second plate and horizontal plate of scales of Trachichthys australis are completely mineralized like the rest of the scale. Their surfaces appear smooth without fiber attachments or elasmodine bands (Fig. 5O).

Type 1G double-plate pedicel (Fig. 6).—

In this scale type, two horizontal plates are joined by a vertical pedestal. Two examples are found in the species examined, which are quite different from each other. In the agonid Hemilepidotus hemilepidotus, the type 1G scales occur in the anterior part of the dorsal band. The dorsal and ventral ends of the spined posterior margin of the scale are curved so that they almost meet or meet anteriorly at the midline (Fig. 6A–C). The upper scale plate’s posterior and anterior margins have robust spines. In side view, the pedestal between the two plates is a thick mass completely solid posteriorly or with a space indicating the junction where the edges of the strongly spined upper plate meet anteriorly (Fig. 6B, C). The pedestal exhibits two zones, a porous one nearer the top plate and one beneath it with a thick hyaloine outer layer that tends to peel away from the surface (Fig. 6B, C). A thinner hyaloine layer extends over the surface of the lower scale plate where there are elasmodine rings (Fig. 6A). The type 1G scales of the anoplogastrid Anoplogaster cornuta have an upper plate that matches the diameter of the lower plate over which it sits atop of a pedestal (Fig. 6D–F). Sharp ridges radiate from the center of the upper plate and its spine, ending in sharp points at the margin (Figs. 1G, 6D). Along the sharp radiating ridges, there are well-separated buttressed spines. A set of secondary elevations interconnects the spines along the radiating ridges. The lower plate has similar radiating and secondary interconnecting ridges but no spines (Fig. 6E). Outer and inner surfaces of both the upper and lower plates display elasmodine rings (Figs. 1G, 6D, F). The center of the inner layer of the scales shows the site where the scale was firmly attached to the dermis by attachment fibers (Fig. 6F).

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Type 1H single-plate marginal-spined (Fig. 7).—

This scale type is a single scale plate and a marginal row of spines or a single spine. Each species of psychrolutid with type 1H scales has one posterior marginal row of spines and at least one row of truncated spines resulting from spinal resorption of previous marginal rows. There are cavities anterior to the marginal spines whose nature is unclear in the psychrolutids Bero elegans, Alcichthys elongatus, and Myoxocephalus scorpius. The scales of B. elegans are round, with no anterior area ornamentation (Fig. 7A). The posterior region of the scales is slightly elevated. There is a row of spines on the posterior margin. Anterior to and in between the marginal spines are remnants of spines, of which the central one is in the process of resorption (Fig. 7B). Subsurface bands seen in the anterior area of the outer scale surface indicate the internal presence of elasmodine (Fig. 7A). Both outer and inner surfaces show numerous sites for fiber attachment (Fig. 7A, C). In the scales of A. elongatus, the posterior area is elevated over a slight extension of the anterior area under the scale (Fig. 7D). An inner surface view is unavailable to determine its exact nature. Some of the posterior margin spines in A. elongatus appear to have started their growth close to the large cavities across the central region of the scale (Fig. 7E, F). Truncated spines occur between the posterior margin spines. Numerous sites for attachment fibers are found all over the surface of the scales of A. elongatus and M. scorpius. Although the sex of specimens of M. scorpius was not determined, data indicate that Figure 7G represents scales from a male specimen and Figure 7H and I are single-plate marginal-spined scales from a female specimen of M. scorpius (Märss et al., 2010). Palmate-shaped scales in the male have a stout row of posterior margin spines (Fig. 7G), five in the scales from the specimen examined here, but 4–12 spines in one to two irregular marginal rows in scales studied by Märss et al. (2010). There does not appear to be a second row of spines in the scales considered here, but there is a row of cavities anterior to, and in line with, the bases of marginal spines marking their previous location. There is a second row of spines, truncated in scales from the female, which is also anterior to, and in line with, the posterior margin spines (Fig. 7I). An earlier generation of spines is also present in the scale of the assumed female M. scorpius (Fig. 7H, I). The growth of scales in females proceeds by adding new generations of spines as extensions from the posterolateral margin of the basal plate. Resorption of the older generation of spines occurred since there are osteoclastic resorption cavities on the surfaces of the spinal remnants (Fig. 7I).

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Another variant of type 1H scales is found in the synanceiid Synanceia verrucosa (Fig. 7J, K, L). The scales curve around dense, conical skin glands in an otherwise scaleless fish. They are thin, brittle, and easily damaged in attempts to flatten them during preparation. In one of the two scales examined, the spine had not risen at its origin but formed horizontally on top of the scale plate and extending past the posterior margin (Fig. 7J). On the other scale examined, a central bump, interpreted as a stunted spine, and a posteriorly directed spine lying horizontally on the scale plate had formed (Fig. 7K, L). The spines in both scales are not smooth. The scale plate displays concentric elasmodine rings. A hyaloine layer is present over the outer surface, particularly its central region.

Type 2A tubercloid (Fig. 7).—

The type 2A scale type is a small conical or pyramidal, well-mineralized structure, often with but not necessarily a sharp spine or spines at its apex and over its surface. They vary in size considerably on the body of the fish. The scales of the cyclopterid Cyclopterus lumpus represent one variant of the type 2A scale (Fig. 7M–O). The spines at the apex of the example shown have broken during preparation. They are sharp in the drawings of Ueno (1970), who also shows many facets and spines of type 2A scales from different species. Facets on the type 2A scales of C. lumpus are divided by faint ridges in the example shown here, whereas, in the type 2A scales of Eumicrotremus spinosus (no image) examined, they are well-defined ridges separating facets radiating from the apex (Fig. 7M, N). Type 2A scales in C. lumpus are conical structures composed of many type 1A scales. The suture junctions between the adjacent single-plate upright-spined scales are visible on the inner surface (Fig. 7O). The inner surface of the type 2A scale has cavities, each corresponding to the inner surface of a type 1A scale. There are many tiny holes in the mineralized inner surface of the type 1A scales, each representing the position of a former attachment collagen fiber. The bases of type 1A scales in C. lumpus do not overlap. Large type 1A scales are arranged in three lines with small type 2A scales in between them in C. lumpus.

Type 2B scutoid (Fig. 8).—

Type 2B scales are large, thick, single mineralized scale plates that are often roughly triangular but have irregular profiles and are juxtaposed with some overlap. Spines and pointed keels are either present or absent in type 2B variants (Fig. 8A–E). A pointed keel may be incorrectly interpreted as a spine when viewed directly from above in two dimensions. When viewed from the side, the nature of the keel can be better appreciated (Fig. 8D). The type 2B scale of the monocentrid Cleidopus gloriamaris consist of a superficial layer of thick vascularized and remodeled acellular bone that is ornamented anteriorly with spined radial ridges and posteriorly with fine, upright spines (Fig. 8A; Meunier and Saur, 2007). A series of overlapping posteriorly directed spines that appear as a single spine at low magnification bisects the scale plate except in its anterior third. The scale image in Figure 8A is from a 50 mm SL specimen. Scales from a 100 mm SL specimen show thicker anterior ridges that are more conspicuously spined and a posterior part in which the spines are enveloped by an outer hyaloine layer, partially obscuring them (no image). The type 2B scales of the triglid Peristedion liorhynchus have a featureless anterior part (Fig. 8B). Fine, upright spines occur in the posterior part of the scale on a layer of spongy bone. It has a median posteriorly directed keel, which rises sharply from the porous layer of the scale plate (Figs. 1H, 8B). The type 2B scales of the agonid Podothecus accipenserinus have a layer of dense mineralized tissue anteriorly with thick ridges under the point of the keel (Figs. 1I, 8C, D). The posterior part of type 2B scales of P. accipenserinus has a spongy bone layer with no fine spines but a prominent median, anteriorly directed keel (Fig. 8C, D). Concentric lines in the elasmodine layer are apparent in areas not covered by the spongy bone layer. The type 2B scales of the agonid Bathyagonus alascanus have a posteriorly directed pointed keel. The type 2B scales of the agonid Sarritor frenatus have a keel, but scale damage is too high to know anything else. The type 2B scales of the agonids Aspidophoroides monopterygius and Pallasina barbata have crests (Fig. 8E). A spongy bone layer was present in all agonid scales examined. Near the scale margins where spongy bone formation is incomplete, concentric elasmodine rings lie between trabeculae that are not cross-linked (Fig. 8D, F; Bhatthi, 1938; Sire, 1993; Sire and Huysseune, 1996).

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Type 2C macro-carapace (literature).—

Type 2C scales are a distinct armor consisting of tessellating, flat, mineralized plates that are polygonal (mostly hexagonal), strongly abutted, and forming interdigitated but disjoined sutures that lack connective tissues, giving a rigid tiling at the surface of the fish. In the ostraciid Lactoria cornuta, type 2C scales are predominantly hexagonal (Porter et al., 2017). Raised struts that radiate from the center toward the edges of each scale reinforce the scale (Yang et al., 2015). Below the mineralized plates is a dense, non-mineralized, collagenous matrix bounded by two mineralized plates, an upper and a lower plate, with vertical plates joining the two levels on their sides as found in three other ostraciids (Besseau and Bouligand, 1998).

Type 2D dermo-plateloid (literature).—

Type 2D scales are thick, mineralized scale plates dorsoventrally elongated to square or circular and overlap or interact with each other by articulating peg and socket joints. The type 2D scales in the callichthyid Corydoras arcuatus are dorsoventrally elongated and overlap anteroposteriorly within both rows and partially between rows (Sire, 1993). They are made of a layer of parallel-fibered lamellar bone covered by a layer of hyaloine. Type 2D scales are also found in the Scoloplacidae and Loricariidae (Nelson et al., 2016). Variants of type 2D scales in the gasterosteid Gasterosteus aculeatus are roundish in the small anteriormost scales that have no particular processes and dorsoventrally elongate in the larger lateral scales that have conspicuous processes extending from their otherwise smooth anterior edges and slight extensions of their otherwise serrated posterior edges (Lees et al., 2012). The surfaces of the lateral type 2D scales, except for their smooth anterior borders, are covered with nodular spongy bone. Lateral type 2D scales in the gasterosteid Pungitius pungitius are small, rhomboidal, or roundish (Lees et al., 2012). The edges of the scales are smooth. The lateral type 2D scales on the caudal peduncle are elongated to roundish and have a keel. Processes occur on the anterior and posterior margins of the scales. Trabeculae with nodules cover the surfaces of the lateral scales. Caudal peduncle scales have keels. Dorsal and ventral type 2D scales (median scales) elongate anteroposteriorly. Trabeculae decorate the scales penetrated by openings of various sizes. The lateral type 2D scales on the trunk of the gasterosteid Spinachia spinachia are oval or roundish (Lees et al., 2012). Processes that arise from a robust median crest extend over the anterior and posterior margins of the scales. The lateral scales are covered by trabeculae and penetrated by different-sized openings.

Type 2E subdermo-plateloid (literature).—

Type 2E scales are single scales that form ring-like segments around vertebrae and interconnect with each other by overlapping flat surfaces or are intercalating scales that cover spaces left between parts. Variants of these scales are present in the syngnathids, where the number of type 2E scales in the sections depends on segment location on the fish’s body. In the syngnathids Hippocampus reidi and H. kuda, there are seven type 2E scales in the torso, six in the torso-tail intersection, and four in the tail, with the scale size varying from larger to smaller (Porter et al., 2013, 2015, 2017; Neutens et al., 2014). Type 2E scales are attached to muscles around the vertebrae. The type 2E scales in Syngnathus typhle and Nerophis ophidion are large and elongated, or circular in the intercalating scales (Lees et al., 2012). A layer of trabecular bone with a small amount of spongy bone around a median keel occurs on the elongated scales of Syngnathus typhle. The elongated scales of Nerophis ophidion are embellished with a layer of spongy bone with trabecular bone at its periphery. The round intercalating scales in Syngnathus typhle have poorly developed trabecular bone, while the rounded scales of Nerophis ophidion have spongy bone centrally surrounded by trabeculae. In both species, the bone layers in the elongate and circular type 2E scales lie on a layer of elasmodine. The concentric lamellae rings are faintly visible in the secondary electron images (Lees et al., 2012). The centriscid Aeoliscus strigatus has thin type 2E scales, which appear as expansions of the vertebral column almost entirely covering the body Nelson et al. (2016).

Type 2F lacunoid (Fig. 8).—

Type 2F scales are thick, single mineralized scale plates divided posteriorly into upper and lower parts. The gap (lacuna) between the two parts accommodates the anterior part of an adjacent type 2F scale. Variants of the 2F scale type occur in the Dactylopteridae. The scales in the dactylopterid Dactyloptena macracantha are tightly interlocked with overlapping margins (Fig. 1J, K). The general shape of the scale is square (Fig. 8G–J). A posterior process, similar in form to the ganoid scale anterior process, is present (Fig. 8I, J). Although no lateral line scales occur on the specimen of Dactyloptena macracantha examined, there are two groups of scales, which in fishes with elasmoid scales would be consistent with scales dorsal and ventral to the lateral line scales. Scales tend to be larger and broader ventral to the lateral line than those dorsal (Fig. 8G, H). Strongly keeled scales are present on both groups of scales. They are featureless anteriorly. The keels extend to and are part of the posterior margin of the upper part. Dorsal scales have three prominent and several minor fold-like ridges posteriorly (Fig. 8H). Ventral scales have lines of very low, weak spines crossing the upper posterior part toward the posterior margin (Fig. 8G). Concentric elasmodine rings are visible in the upper posterior part of ventral scales. Views of the undersides of the scales are necessary to show the gap between the upper and lower parts (Fig. 8I, J). The posterior margin of the lower part is notched and bears the broadly-pointed posterior process. Sites for attachment fibers covering a large area are conspicuous on the inner surfaces of the scale plates (Fig. 8I, J). The generally oval-shaped scales of the dactylopterid D. papilio have a restricted keel in that it extends well short of the posterior margin and is flat over most of its length (Fig. 8K–M). The extent to which the keel does not extend to the anterior margin is different in the two scales imaged and presumably dependent on the morphology of the posterior margin, which would overlap the anterior area of the scale (Fig. 8K, L). The posterior margin of the upper part of some scales is a broad point, and a keel stops well short of the anterior margin (Fig. 8K). In other scales, a deeply notched posterior margin occurs, and the keel ends much closer to the anterior margin than those with broad-pointed posterior margins (Fig. 8L). The broadly-pointed posterior process is apparent in views of the underside of the scale (Fig. 8I, J). The gap between the upper and lower posterior parts in the scale of D. papilio is clear in lateral views (Fig. 8M). Elasmodine rings are visible both in the upper parts of the scale and scale plate. They are particularly strong in the upper parts (Fig. 8K, L). The underside of the scale plate has a large area for the attachment fibers.

Type 2G ganoine-lost (literature).—

Type 2G scales derived from ganoid scales by the loss of dentine and ganoine layers are ganoine-lost scales. Such scales are only formed of bone and are, therefore, calcidermoid scales. They are found in the stem teleosts, the pycnodonts †Eomesodon, †Gyrodus, †Gyronchus, †Macromesodon, †Mesturus, and †Proscines, and the aspidorhynchid †Aspidorhynchus (Schultze, 1996). The pachycormids †Asthenocormus, †Hypsocormus, †Orthocormus, and †Sauropsis have rounded scales with or without ganoine. †Orthocormus roeperi (Pachycormiformes) is covered by small, thin scales on the trunk and rays of the unpaired fins, and their scales lack ganoine (Arratia and Schultze, 2013). In the acipenserids Acipenser sturio and A. ruthenus, the caudal scales are roughly rhomboidal, fit closely together, and, at places, overlap (Smith, 1956). They are composed only of bone. Histological analysis found a cellular lamellar bone with concentric rings, indicating successive layers of bone over the whole scale (Smith, 1956).

Elasmoid scales with spines or spinal remnants in central foci (Figs. 9, 10).—

The focus is the first part of the scale to appear in the growth of the elasmoid scale. In original scales, as opposed to replacement scales, the focal area is very small. It is usually smooth and bound by the first bony ridge. However, the central focus is surrounded by an incomplete bony ridge on the scales of a 120 mm SL juvenile of the zeid Zeus faber and has an upright spine with a raised, well-mineralized area around it. If not for the central spine, the scales would be categorized only as an elasmoid cycloid scale because they have an external layer consisting of bony ridges, some incomplete, and the scales are thin with smooth margins (Fig. 9A). However, the central upright spine is crucial because it underlines the importance of their origin as type 1A scales. The scales of a 198 mm SL adult Zeus faber are categorized as cycloid scales (Fig. 9B). They reside in horizontal scale pockets that are tightly packed. However, a close look at the focus reveals a small spine lying horizontally across the inner bony ridges in some scales (Fig. 9C). In other scales, there is significant resorption of the central spines. However, they are still surrounded by the raised, highly mineralized areas, as seen in juvenile Zeus faber (Fig. 9D–H). The elasmodine of the inner scale layer is unmineralized. Distinct elasmodine rings surround the central area in which they are invisible or indistinct. This appearance is typical for growing elasmoid scales. In non-growing scales, the outermost elasmodine lamellae, which cover the whole inner side of the scale, become so numerous that they obscure the elasmodine rings on most or all of the scale.

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Larval scales of the anomalopid Kryptophanaron alfredi are type 1A scales (Baldwin and Johnson, 1995). The adult scales of this species are type 3 elasmo-spinoid since they have bony ridges and upright spines that cover the whole of the posterior field (Fig. 10A–C; Roberts, 1993). At the center of the scales, in the clearly defined focus, are the remains of an almost fully resorbed spine (Fig. 10A, B). The scales are thin and have a wholly mineralized elasmodine layer. Distinct elasmodine rings in which collagen lamellae alternate in orientation are visible on the inner surface, unlike the typical homogenous inner surface layer with uniformly oriented collagen fibrils typical of adult elasmoid scales.

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It is unknown whether type 1A scales occur in anomalopid Photoblepharon palpebratum larvae. However, focal spines in adult fish scales make it likely (Fig. 10D). These focal spines show no signs of resorption. Adult scales of P. palpebratum are elasmo-spinoid type 3 scales with upright spines covering the whole posterior field. The inner spines are much larger than those in the marginal zone. In these scales, the focus is not clearly defined. The elasmodine of the inner layer has lamellae rings. Mineralization occurs to the same extent as the scales of Kryptophanaron alfredi. In P. palpebratum, the most recent lamella is more mineralized (brighter in BSEMs) than the preceding lamellae (Fig. 10E). In surface views of the inner layer, distinct elasmodine rings have collagen fibrils oriented in different directions.

It is unknown if the anomalopid Anomalops katoptron has type 1A larval scales. Still, adult scales are type 3 elasmo-spinoid scales with spines in their focal areas; the focus is not clearly defined (Fig. 1B). Upright spines cover the whole posterior field. The elasmodine of the inner layer is mineralized except centrally, where dark ovoid areas in BSEMs indicate unmineralized lamellae (Fig. 10F). The mineralized zones show the different orientations of collagen fibrils, characterized by the cracking patterns that follow the direction of the fibrils. Several vascular canals penetrate the posterior fields of the scales.

The scales of the trachichthyid Gephyroberyx darwinii have spines in their focal area. It is not known whether larval scales are type 1A. Adult scales are categorized here as type 1–2 elasmo-spinoid because the spines form ridges crossing the posterior field (type 1), and there are submarginal spines that are slightly elevated but not upright (type 2; Fig. 10G, H). The spines in the outer posterior field line up with those in the inner posterior field and grow as ridges in which individual spines are recognizable. The elasmodine of the inner scale layer is largely unmineralized. Elasmodine rings are visible in the mineralized thick marginal rim around the scale underside (Fig. 10I).

The type 2 elasmo-spinoid scale of the trachichthyid Optivus agastos has its oldest and first spine in the focus, defined as the area inside the first bony ridge, and spines cover the whole posterior field (Fig. 10J, K). Type 1A scales are reported for Optivus sp. by Jordan and Bruce (1993). From the inner to the outer posterior field, the spines form lines in which they are distinct from each other. An outer layer that shows fine cracks (dehydration artifact) covers the area in and around the focus. The rest of the posterior field does not have this layer and also, in the absence of bony ridges, shows elasmodine rings that are much softer than ridges (Fig. 10K). The inner side of the scales of O. agastos does not show elasmodine rings, even in the sizeable mineralized area underneath the posterior field (Fig. 10L). The thin mineralized margin around the anterior part of the scale is typical for elasmoid scales. The anterior part of the inner layer is unmineralized. The mineralized part has vascular canals, some of which appear present in the unmineralized part, but the diameter of most is reduced here. There are slight indentations in the center of the dorsal and the ventral scale margins. As explained later, this indicates a connection with a type 1F scale.

The holocentrid Sargocentron diadema does not have spines in the focal area of its scales, with the focus likely to be at the apex of parallel bony ridges in the central part of the scale (Fig. 10M). After a period of growth, bony ridges that follow the scale’s outline surround these inner ridges. The scales of this species are type 1 elasmo-spinoid since they have a row of robust, acute spines confined to the posterior margin (Fig. 10M, N). Black and gray dots show the former position of fiber attachment on their undersides. The undersides of the marginal spines are brighter than the rest of the inner layer because of the lack of coverage by the elasmodine layer. The elasmodine layer of the inner side of the scales is entirely unmineralized and featureless.

Scales and spines from odontodes.—

The current study found the unexpected and surprising presence of foramina in the scale plate and neck of the spine in the type 1A scales of the synanceiids Adventor elongatus, Aploactisoma milesii, and Paraploactis pulvinus. These are odontode characteristics, and their presence in synanceiid scales has significant implications for the evolution of spines and scales. The subject of odontodes arises in connection with synanceiid scales and some features of spines and scales in other studies. Odontodes correspond closely to simple teeth (Francillon-Vieillot et al., 1990). They do not function like teeth and possibly have a protective function. They are hollow cones of dentine surrounding a pulp cavity. They are attached to the basal plate when embedded in the dermis, e.g., in the scyliorhinid Scyliorhinus canicular (Goodrich, 1909; Francillon-Vieillot et al., 1990). A round ligament connects them to an annular attachment bone, or pedicel, when attached to type 2D scales such as in the loricariids (Bhatti, 1938). They have one or two narrow openings or foramina in the base and the neck of the dentine cone through which pass blood vessels, nerves, and lymph channels (Goodrich, 1909; Francillon-Vieillot et al., 1990). In the synanceiids Adventor elongatus, Paraploactis pulvinus, and Aploactisoma milesii, the single-spined type 1A have foramina that recall the entrances to the pulp cavity in the basal plate and neck of odontodes. The upright spines of Adventor elongatus are hollowed out on one side and bifurcated in Paraploactis pulvinus and thus modified from the more conventional shape of a spine seen in Aploactisoma milesii. It is unknown if any of the synanceiids have odontode tooth-like histology in their spines. The foramina are present in some but not in all the scales of Aploactisoma milesii. In the odontodes of the callichthyid Corydoras, the pulp cavity does not contain blood vessels or nerves, and odontoblastic processes are not present in the dentine, interpreted as the result of a reduction process (Sire and Huysseune, 1996). Bridge (1904: 189) wrote that “on certain parts of the body of Centriscus sp., each scale consists of a rhombic basal plate, produced into a curved, backwardly inclined spine, the axis of which contains a pulp cavity opening on the inner surface of the basal plate.” He notes that some antennariid species have similar scales but with round basal plates and thick spines. In the psychrolutid Triglops pingelii described above, the loss of the spines on the minor multi-spined type 1A scales during sodium hypochlorite preparation left shallow ring-shaped craters, recalling the annular attachments of odontodes. Roberts (1993: fig. 16F) observed that on the elasmo-spinoid scales of the caproid Antigonia, the lateral spines are loosely attached to the central parts of the anterior and lateral fields. He noted that two lateral spines, dislodged by the cleaning action of sodium hypochlorite, left attachment scars. These findings suggest that they evolved from odontodes. The hypothesis that odontodes changed over time to transform into scales and spines ultimately can explain the above observations for calci- and elasmo-spinoid scales. Sire and Akimenko (2004: 234) state that in contrast to the chondrichthyans in which odontodes are conserved in a nearly unchanged form, “in the osteichthyan lineage, odontodes progressively modified into various types of scales.” These included ganoid scales, dermal bony scales, and elasmoid scales, the dermal bony scales being equivalent to the calcidermoid scales proposed in the current study.

Additional occurrences and variations of the eight types of calci-spinoid scales.—

The multi-spined and single-spined type 1A scales occur as isolated or joined scales. Spines can be numerous and cover almost the entire scale plate, like in the scales of the balistid Balistes (Bertin, 1958; Francillon-Vieillot et al., 1990). Multiple posteriorly directed spines arise in a semi-circle from the scale plate in the psychrolutid Astrocottus leprops. Many sharp spines in the liparid Careproctus seraphimae cover the whole scale plate (Burdak et al., 1986). Single and multi-spined type 1A scales occur in the xiphiid Xiphias gladius (Govoni et al., 2004). Scales of larvae, juveniles, and adults interdigitate, although some overlap along their margins. The stephanoberycid Acanthochaenus luetkenii has type 1A scales that are joined at their margins (pers. obs.). A thin bony connection on larger specimens joins multiple spines on the scale plate of the monacanthid Monacanthus where there are two to eight or more spines, none of them branched (Bertin, 1958; Berry and Voegle, 1961). Type 1A scales with a single spine occur in monacanthid species’ initial stage of scale development. Similarly, type 1A scales with a single spine appear initially in the monacanthid Stephanolepis and increase to three to eight closely joined spines with fish size (Berry and Voegle, 1961). The spines branch one to many times at their apices. The monacanthid Meuschenia scabra has multi-spined type 1A scales with ridges radiating from the spines (Roberts, 1993). The single spines in the type 1A scales are bifid in most antennariid genera, with some species having both bifid and trifid spines. There is typically full skin coverage with type 1A scales in antennariids (Paulin, 1978; Pietsch, 1984). The embedded horizontal plates are roughly oval or circular and enlarged in the psychrolutids placed in the Icelus spiniger group of Nelson (1984). Overlapping plate-like scales completely cover the dorsal body surface. These type 1A scales have a single robust spine on the scale plate. The type 1A scales in liparids resemble thumbtacks (Schmidt, 1904; Burke, 1930; Able and McAllister, 1980). Andriashev and Chernova (1997) shows densely packed type 1A scales in a skin sample of the liparid Careproctus aciculipunctatus. Fedrigo et al. (1996) reported that one of the types of scales on the centriscid Notopogon xenosoma was a type 1A scale. One variant had pointed thin spines and another wide spines with flattened cup-shaped tips. In the ogcocephalid genus Malthopsis, some species have type 1A scales in the interspaces between large and small type 2A scales on their trunk and ventral surfaces (Bradbury, 1988; Ho et al., 2013). The chiasmodontid Dysalotus alcocki is known to have the 1A scale type (Pietsch, 1989). In the acipenserid Acipenser ruthenus, between the five rows of well-separated large type 2B scales, numerous type 1A scales are arranged in oblique rows (Bridge, 1904: fig. 102B). Each scale consists of a basal plate embedded in the dermis and one or more projecting spines perforate the epidermis. In Acipenser sturio, the skin between the large type 2B scales is closely studded with isolated small plates (type 1A). These plates are entirely bony, rhomboidal, and often roughly diamond-shaped when seen from above, an elongated oval when seen from the side, with one or more spines on the outer surface (Kerr, 1952). The spines are vertical with epidermis rising up at their sides, so at most, only the tip is protruded. Smith (1958) reports that the skin of the synanceiids Ptarmus gallus and Cocotropus dermacanthus has minute fine prickles categorized as type 1A scales. The skin of the synanceiid Ablabys binotatus was reported as skin covered with minute spines (type 1A scales), each the external part of thin scattered scales below the skin. The skin of the scorpaenid Taenionotus triacanthus has spines which are mostly the external part of feeble, scattered scales below the skin. Type 1A larval scales are described as tack-like, with a single spine projecting outward from a circular plate. These scales occur in the larvae of many families of the Trachichthyiformes, in which adult fishes have scales in a different category (Johnson, 1984; Baldwin and Johnson, 1995). In the larvae of the monocentrid Monocentris, the single-spined type 1A scales have large round scale plates with pyramidal spines. In several trachichthyiform species, the scale number increased ontogenetically, but not in Monocentris. Single-spined type 1A scales occur in larval non-percoids such as the barbourisiid Barbourisia rufa and Xiphias in larvae and adults, but only in the larvae of Antigonia, chiasmodontids, acanthurids, some pleuronectiforms, tetraodontiforms, and scorpaeniforms (Johnson, 1984; Baldwin and Johnson, 1995). In larval percoids, multi-spined type 1A scales occur in the ephippid Chaetodipterus, the haemulid Conodon, malacanthids, pomacanthids, and scatophagids (Johnson, 1984).

Single-spined type 1A scales not only progress to multi-spined type 1A scales but also to many other types of calci-spinoid scales and other types of calcidermoid scales (see section on larval scales). Type 1B scales occur when there are significant modifications to the scale plate. As a result of these modifications, spines lie horizontally in the skin. Type 1B scales are found in the cottids Cottus asper and C. gobio and described and figured in the cottids C. gobio, C. sibiricus, C. spinulosus, and Paracottus kessleri by Koli (1969). As well occurring in the tetraodontid Torquigener pleurogramma, they embed in the skin of diodontids such as Diodon holocanthus, where they have long erectile spines with triradiate scale plates (Brainerd, 1994). The structure and development of type 1B scales in the tetraodontid Takifugu obscurus during larval growth and in adults are described by Byeon et al. (2011).

Koli (1969) reports some cottid species in which type 1C scales co-occur with single-spined type 1A and type 1B scales. In the current study, type 1C scales are present in the congiopodid Zanclorhynchus spinifer. They occur with type 1A scales in which there are spines of various lengths, including some significantly attenuated.

The type 1D scale that has a single spine but no scale plate or recognizable base is also found in the cottids described by Koli (1969). In C. poecilopus, there are type 1D scales near the lateral line next to type 1A scales with roundish plates and slender spines. Other variants of type 1D scales occur in the carangids (Smith, 1970), liparids (Burke, 1930), and centriscids (Burdak et al., 1986).

Type 1E scales are calci-spinoid scales with interdigitations along the scale edges, making them tightly articulate. The interlocking of the scales distinguishes them from other single-plate calci-spinoid scales. In the type 1E, two variants are identified. Ridges radiating from the central spines in the type 1A scales of molids Masturus lanceolatus and Mola mola end as rounded projections at the edge of the scale, and these interdigitate with one another (Katayama and Matsuura, 2016). The second variant is no radial ridges, such as in Ranzania laevis. The edges of the scales have a zigzag pattern with triangular projections. In the larval stages, long spikes cover the body of Mola (Gregory and Raven, 1934), but the spines in M. mola examined by Katayama and Matsuura (2016) are short and appear to be modified.

Type 1F scales consist of two scale plates, one horizontal and a second plate projecting from the horizontal plate. The second plate has robust spines growing out of its posterior margin or lateral surface. Fedrigo et al. (1996) identified, in the macroramphosid Notopogon xenosoma, four types of 1F scales according to morphology and the number of spines of the second plate. Type 1F scales had upright, straight, slightly curved, wide, or thin second plates with two to five spines. The horizontal base plate of the calci-spinoid scales differed according to the region of the fish, and there was a gradation between non-joined and partially nested bases. The upright nature of the second plates in the scales of Notopogon xenosoma is quite different from the oblique second plates of the type 1F scorpaeniform scales. The scales of the psychrolutids, previously the traditional marine Californian cottids, are described as having “flat, deeply embedded plates whose external surface bears a posteriorly inclined, strongly ctenoid, plate-like ridge” by Bolin (1947: 159). He noted that “they represent a type of scale which is widespread within the family and from which the more specialized scales are readily derived.” Variants of type 1F scales are described here for psychrolutids and illustrated by Bolin (1936) for Astrocottus leprops and Stlengis distoechus, and for Antipodocottus elegans by Nelson (1990). Ko and Park (2009) figure the type 1F scales of Icelus stenosomus. Nelson (1984) distinguished three patterns that separated Rastrinus scutiger from two groups of species of Icelus. In Rastrinus scutiger, there was complete body coverage with small scales with a second plate, referred to as an oblique ridge with small spinules. The horizontal base plates in all species of the genus Icelus are enlarged and plate-like and generally overlapping. The horizontal base plate is roughly circular in the I. bicornis group and I. gilberti, and the spined second plate traverses the lateral scale surface. In the I. euryops group and I. armatus, the horizontal base plates are expanded dorsoventrally so that they are elongated, and the oblique second plates with their marginal spines are displaced ventrally. Nelson (1984) was referenced by Roberts (1993: 86) for some cottids, now psychrolutids (Smith and Busby, 2014), as having spinoid type 5 scales that he defines in part as having “a bony pedicel elevated above the scale base supporting robust spines.” The oblique second plate in psychrolutid species in the genera Rastrinus and Icelus is not regarded here as a pedicel.

The trachichthyid Trachichthys australis has a distinctive variant of a type 1F scale. The second plate rises steeply from the horizontal base plate and curves over in the posterior direction. Shaw (1804) commented that the scales have processes that are so firmly and tightly inserted that it is impossible to use forceps to detach one from the rest without bringing away a small portion of the integument. The processes referred to by Shaw (1804) are the posterior parts of the horizontal plates. Stout spines cover the posterior area of the inclined second plate, and this coverage of short, robust spines is absent in any of the other type 1F variants examined. Notably, the spined area resembles the elasmoid scales of other trachichthyids. Trachichthys australis type 1F scales look like the type 1F mandibular scales of priacanthid Pristigenys alta (Starnes, 1988) even to the extent of a few spines projecting in straight line anteriorly from the anterior border of the spined posterior area. Larval acanthuroids have variants of type 1F scales (Johnson and Washington, 1987). Their scales are roughly ovoid (Luvaridae, Naso, and Zanclidae) or have elongated horizontal base plates (Acanthurus, Ctenochaetus, and Prionurus). They have vertically oriented fan-like (Luvaridae and Zanclidae) or triangular (Acanthurus, Ctenochaetus, and Prionurus) second plates that extend outward at ninety degrees from the horizontal plate. The second plates have one to several spines, usually three.

This study describes two variants of type 1G scales where a vertical pedestal joins two parallel plates, one in the agonid Hemilepidotus hemilepidotus and the other in the anoplogastrid Anoplogaster cornuta. They are very different from each other. Bertin (1958) illustrates the stages of growth of type 1G scales of the luvarid Luvarus sp. Roberts (1993) shows SEM images of the scales of the champsodontid Champsodon, which Pietsch (1989) places correctly as a type 1G scale. Their lower horizontal basal plates do not overlap. The upper second plate is offset anteriorly to the lower plate. The anterior margin of the upper plate is convex and smooth, whereas the posterior margin has robust spines. There is a high bump in the posterior region of the scale. The scales of centriscid Macroramphosus scolopax described by Fedrigo et al. (1996) are another example of a variant in type 1G scales. These scales’ lower horizontal base plates are star-shaped, enabling them to interlock with one another. The pedicel is short and sturdy, arising from the center of the lower horizontal base plate. The upper second plate has an anterior convex edge, and the posterior margin has spines that are continuations of parallel ridges crossing the posterior area of the scale, the central ridge extending the anterior border. The growth of the posterior region of the upper horizontal second plate is higher than that of the anterior area, such that it overhangs the anterior part of the adjacent scale.

Different variants of type 1H scales described here for the psychrolutids Alcichthys elongatus, Bero elegans, and Myoxocephalus scorpius, and the synanceiid Synanceia verrucose are each very distinctive scales with little resemblance to each other. Type 1H scales of the psychrolutids are also variants of a transforming spinoid scale type in which there is spinal resorption of the marginal spines and replacement by new spines (unpubl. data). There is evidence of spinal remnants and osteoclastic resorption cavities. There are two patterns of spine growth, alternating in A. elongatus and B. elegans and linear in M. scorpius. Both patterns are known for elasmo-spinoid scales (Roberts, 1993).

As well as the type 1A scales described above for Acipenser ruthenus, Sewertzoff (1926) describes the development of very minute scales in the trunk region of this species that match type 1H scales. Other researchers designated the scales as tiny grains of an irregular shape. Although they appear slightly varied and scattered irregularly, Sewertzoff found regularity in their shape and disposition (Sewertzoff, 1926: figs. 6–11). The longitudinal axis of the scales is transverse to the longitudinal axis of the fish’s body. In an early stage of development, on the posterior border of the scale, there is a low crest from which the free margin of the scale extends as a sharp spine. Similar scales with more developed single horizontal spines are numerous. There is a series of transitional shapes from a single spine to two and three spines on the scale’s posterior margin. Scales whose posterior border bears five spines occur with the three median spines more developed than those on either side. These type 1G scales from A. ruthenus have only slight anterior parts. Those of the psychrolutids A. elongatus, B. elegans, and M. scorpius and the synanceiid S. verrucose have well-developed anterior parts.

Like the psychrolutids, there is a well-developed anterior part in the type 1G scales of the polyodontids †Crossopholis, †Paleopsephurus, and †Protopsephurus (MacAlpin, 1947; Grande et al., 2002). The anterior parts of the scales of these three species have one to three anterolaterally projecting knobs on their outer surface. The number of spines on the posterior margins increases ontogenetically (Grande and Bemis, 1991). Overall, the scales are tiny, widely spaced, and non-overlapping.

Additional occurrences and variations of macro-calcidermoid scales.—

Type 2A scales are small, stiff, conical, or pyramidal, well-mineralized structures often with, but not necessarily, a sharp spine or spines at its apex and over its surface. The scales often lie side by side with slightly overlapping bases or not at all (Bradbury, 1967). There are different sizes of type 2A scales on individual fish and many variants (Bradbury, 1967; Ueno, 1970; Ho et al., 2013). Type 1A scales in the cyclopterid Cyclopterus are a developmental stage of type 2A scales (Clausen, 1959). Clusters of type 1A scales are recognizable in mature type 2A scales of Cyclopterus. Bradbury (1967: fig. 5) describes size-related changes in ogcocephalids. The ogcocephalid Malthopsis asperata has stout, pyramid-like, rough type 2A scales with prominent spines (Ho et al., 2013). In Malthopsis parva, type 2A scales are relatively blunt. Large, flat type 2A scales cover the ventral surface and caudal peduncle in Malthopsis mitrigera. The ogcocephalid Halieutaea stellata has strongly pointed tubercles on the dorsal surface and body margins. Enlarged type 2A scales on the edge of the body have 3–4 sharp spines. In the ogcocephalid Halieutopsis bathyoreos, type 2A scales on the body have 6–8 facets. Those at the edge of the body are large, with 2–3 spines. Few facets, generally 4–8, and few or no spines other than apical ones occur in the type 2A scales of the ogcocephalid genera Coelophrys, Dibranchus, and Halieutopsis. However, Dibranchus’ larger type 2A scales may have as many as 10–15 facets (Bradbury, 1967). Spines decorate the type 2A scale facets in Dibranchus atlanticus. Several facets occur in many species of the cyclopterid genus Eumicrotremus (Ueno, 1970). In the latter genus, type 2A scales have few to many spines. Type 2A scales in the scophthalmid Scophthalmus maximus start as small conical structures with flat, rounded apices and round bases (Märss et al., 2015). Small scales are shallowly concave, and the basal cavity is porous, with the central part having more prominent and more numerous pores. Zylberberg et al. (2003) and Märss et al. (2015) show that the type 2A scales of Scophthalmus maximus are quite different from variants in ogcocephalids and cyclopterids.

Type 2B scales are categorized as calcidermoid and, like all others in this category, have no scale pockets. They are large, thick, single mineralized structures, often roughly triangular, but may have irregular profiles and juxtaposed with some overlap, often with a keel. In the sturgeon Acipenser, the body is traversed by bony type 2B scales, as illustrated by Sewerztoff (1926) for A. ruthenus. Like the rhombic scales on the terminal part of the tail, they are furnished with ridges and projecting spines. The Notopteridae have ventral type 2B scales. Several lines of clupeomorphs, both fossil and extant, have members that are double-armored; that is, they have predorsal as well as ventral type 2B scales (Nelson et al., 2016). In some of these, there is only one or two predorsal type 2B scales (double-armored engraulids of the Indo-Pacific), while in others there is a series (†Clupanodon, †Diplomystus, †Ellimmichthys, Ethmidium, †Gosiutichthys, Hyperlophus, †Knightia, †Paraclupea, and Potamalosa). The double-armored feature has evolved or been lost independently several times, although most or all members of the superorder Clupeomorpha fossil-only order †Ellimmichthyiformes are double-armored, and some even have additional median type 2B scales behind the dorsal fin. The family †Paraclupeidae (= Ellimmichthyidae) has subrectangular dorsal type 2B scales and ventral type 2B scales. In the engraulid subfamily Coiliinae, type 2B scales are present in front of the pelvic fin and behind the pelvic fin, with pre-pelvic type 2B scales absent in some Coilia. In the subfamily Engraulinae, type 2B scales are present in front of the pelvic fin only in Encrasicholina and Stolephorus and absent behind the pelvic fin. In their research of small, armored acanthomorph teleosts, which have strong skin ossifications and are found in the Cretaceous of Mexico, González-Rodríguez et al. (2013) conducted a broad survey of calcidermoid scales and associated literature to classify their three fossil taxa. Noting that the terms scutes, plates, and shields are used interchangeably and have no clear definitions, they distinguished shields as overlapping structures and plates as structured bordering each other with straight or digitating sutures. Here, it is noted that their shields correspond to the type 2B scales of the Agonidae, Cleidopus, and Monocentris. It is also noted that they label spiny bony plates in photomicrographs of the scales of agonid Hypsagonus quadricornis when these structures do not conform to their definition but of necessity in the absence of a suitable term. The spiny bony plates correspond to type 1A scales. Researchers who have studied type 2B and type 2D scales suggest that they are secondary calcifications, not modified scales, and developed from a scaleless condition (Whitear and Mittal, 1986; Sire, 1993). However, Busby (1998) found that type 2B scales develop from type 1A scales. Lateral line scales in some species are called ‘scutes,’ but this practice should now change. These lateral line scales are now categorized as integrated 1 and 2 lateral line scales, which co-occur with ganoid or calcidermoid common scales, or as integrated 3 lateral line scales, which occur when scales are absent or elasmoid (Voronina and Hughes, 2018).

Type 2F scales have a gap or a lacuna between the posterior upper and lower parts of the thick, mineralized scale plates. The opening (lacuna) between the two parts accommodates the anterior part of an adjacent type 2F scale. The type 2F scale is found in dactylopterids Dactyloptena macracantha, D. papilio, and Dactylopterus volitans (Cockerell, 1912), but there is not enough detail in additional studies to determine if other dactylopterid species have this scale type (Eschmeyer, 1997; Sato et al., 2003).

The multiple origins of elasmoid scales from calcidermoid scales with examples.—

Type 2G scales are usually referred to as ganoid scales even though ganoine is absent, presumably because of the absence of any other scale category in which they may be placed. The current study solves this issue by creating a new primary fish scale category called calcidermoid and, within it, type 2G ganoid-lost. It is believed that elasmoid scales developed from ganoid scales after the complete disappearance (or after important reduction) of the superficial part of the latter (dentine and ganoine) as well as after modifications of its osseous basal plate (Schultze, 2015). It follows that elasmoid scales evolved from ganoid-scale derived calcidermoid scales, shown by Smith (1956) for acipenserids to be composed only of lamellar bone, a plywood-like tissue equivalent to elasmodine. Data from the current study show that elasmoid scales of the cycloid type can develop from type 1A scales, as do elasmoid scales of the spinoid type. The cycloid scale is created by the resorption of the upright spine and encasement in a horizontal scale pocket with the formation of a bony-ridge external layer. The elasmo-spinoid scale is created by the growth of the scale around central spines. Its creation from a type 1A calcidermoid scale explains the upright spines in the posterior field and complete mineralization of the elasmodine inner layer in the anomalopid Kryptophanaron alfredi. In other related species, there is reorientation of posterior field spines from upright to oblique to horizontal. There is eventual rearrangement and completion of the collagen layers in the elasmodine and reduction of elasmodine mineralization. It can also be deduced that elasmo-spinoid scales can develop from type 1F calci-spinoid scales. It has already been noted that the type 1F scales of the trachichthyid Trachichthys australis look the same as the type 1F mandibular scales of the priacanthid Pristigenys alta (Starnes, 1988), even to the extent of a few spines projecting in straight line anteriorly from the anterior border of the spined posterior area. In Trachichthys australis, the elevated and spined second plate rises sharply from the horizontal scale plate and curves in the posterior direction. The width of the posterior part of the horizontal plate is much less than the anterior part of the plate to the extent that Shaw (1804) commented that the scales have processes. Starnes (1988) described the body scales of the family Priacanthidae as more flattened than in the head, chin, and mandible, with a spine-bearing posterior (apical) field slightly flexed outward with no underlying structure to considerably elevated with an underlying prominence or flange. The elevated posterior fields of some priacanthid species correspond to the elevated second plates of type 1F scales. The anterior fields correspond to the anterior parts of the horizontal plate, and the prominence on their undersides corresponds to the posterior part of the horizontal plates. The scales of Trachichthys australis on the body of the fish appear to be organized in a similar pattern to elasmoid scales, and the elevated posterior field spines resemble those of type 3 elasmo-spinoid scales. In the outgroups of Hughes’s (1985) platycephalid study, elevated posterior fields occurred in the Berycidae, Cyttidae, Diretmidae, and Zeniontidae. Anterior scales dorsal to the lateral line are generally more elevated than scales ventral to it, which may not be elevated. The prominence on the scale’s underside is reduced to a pronounced mineralized ridge in the elasmo-spinoid scales of the triglid Lepidotrigla papilio, which has an elevated posterior field and large areas of mineralization on the undersides of the scales. The elevated posterior field of the elasmo-spinoid scales of the neosebastid Maxillicosta scabriceps, reported by Eschmeyer and Poss (1976), is elevated but there is no underside prominence. Still, their undersides are mineralized, as well as a large part of the undersides of the anterior fields. The undersides of the posterior fields of elasmo-spinoid scales are generally mineralized. The undersides of the transforming ctenoid scale posterior fields of species of Neosebastes are less elevated but also mineralized. The transforming ctenoid scales of the platycephalid Ratabulus tuberculatus also have elevated posterior fields and a broad band of mineralization extending dorsally and ventrally and in the anterior field on their undersides. Onigocia macrolepis has a wide band of mineralization on the undersides of its scales, but the posterior field is not elevated. A thin band of mineralization is present on the underside of the fields of Insidiator macracanthus, and the posterior field is flat like most of the other 53 flathead species examined by Hughes (1985) with no underside mineralization. Elevated posterior fields are associated with hexagonal scales with dorsal and ventral indentations, or as Bräger and Moritz (2016) term it, lateral fields are concave, such as those of the atherinid Atherina hepsetus. The extent of the indentations indicates the degree of elevation. So, it seems that underside mineralization supports the elevation of the posterior field due to a connection in the past with type 1F double-plate calci-spinoid scales. As mineralization decreases, the posterior field becomes flatter.

Histological structure of calcidermoid scales.—

Histological sections of the type 1A scales in the xiphiid Xiphias gladius show it to be composed of bone with growth rings (Govoni et al., 2004). In the acipenserid Acipenser sturio, between the larger type 2B scales, there are minute plates with vertical spines on the outer surface. In section, the bone in these type 1A scales is arranged as a series of concentric lamellae with corresponding flattened lacunae around a non-lamellar central region (Kerr, 1952). The type 1B scales of the diodontids Diodon holocanthus and D. hystrix are composed of radially aligned sheets of longitudinally aligned, mineralized collagen fibrils. In between these sheets are layers of radially aligned collagen fibrils. The spines also have concentric growth rings in the transverse cross section (Su et al., 2017). The type 1F scales of the centriscid Notopogon xenosoma are completely composed of avascular and acellular bone tissue. They have a parallel-fiber bone (or pseudo-lamellar), there is no evidence of the existence of a lamellar bone (Fedrigo et al., 1996). The type 1G scales of the centriscid Macroramphosus scolopax are also composed of avascular and acellular parallel-fiber bone with no existence of lamellar bone (Fedrigo et al., 1996).

Meunier et al. (1978) found that type 2B scales of the acipenserid Acipenser sturio have a homogeneous structure composed of cellular bone that is parallel-fibered. On their deep surface, large resorption cavities were sometimes observed. On the surface of these cavities, there was secondary bone with woven fibers. Whitear and Mittal (1986) found the type 2B scales of Agonus cataphractus are made of bone and have a girdered structure with radial and cross-bars inserted on the bone faces of a thin plate. The type 2B scales of the acipenserid Acipenser baerii consist of bone matrix that is entirely woven-fibered (fibrous; Leprevost et al., 2017). The type 2D scales of Corydoras arcuatus are composed of three layers: hyaloine overlying a layer of parallel-fiber bone on a layer of lamellar bone (Sire, 1993). The type 2D scales of the gasterosteid Gasterosteus aculeatus are composed of acellular lamellar bone (Sire et al., 2009). The ostraciid Lactoria cornuta has type 2C scales. The scales are natural composites, associating a fibrous network with a mineral deposit lying at two different levels of the scale, the ‘ceiling’ and the ‘floor,’ plus a set of similarly mineralized walls joining the two levels (Besseau and Bouligand, 1998). The scales are plates of bone, predominantly hexagonal in shape, that are reinforced with raised struts that extend from the center toward the edges of each scale (Yang et al., 2015). The type 2E scales of the syngnathid Hippocampus reidi are composed of acellular bone. External layers of harder mineralized tissue encase a softer interior that surrounds the hollow core (Porter et al., 2013). In summary, calcidermoid scales are composed of different types of bone and different types of mineralized collagen fiber structures.

Origin of calcidermoid and elasmoid scales from larval skin spines.—

Leis and Rennis (1983) describe the larval stages of Indo-Pacific coral reef fishes, eight of which have calcidermoid scales. In larval balistids, “by the start of inflection, small weak spines are present over the body, and these are the precursors of scales” (Leis and Rennis, 1983: 231). Balistid scales in adults are categorized here as type A1 scales. In monacanthid “larvae, small papillae form on the body, at about the time inflexion, and become tiny scales which together with existing head spinules, become tiny scales characteristic of monacanthids” (Leis and Rennis, 1983: 236). These tiny scales are categorized here in adult monacanthids as multi-spined type 1A scales. In tetraodontid larvae, spines in some species form at 2.9 mm, with most species retaining the spines as adults. In the tetraodontid Torquigener pleurogramma, the scales are categorized as type 1B. In diodontid larvae, body spines are present at the completion of inflexion. In the diodontid Diodon holocanthus, the spines are categorized here as type 1B. In the acanthurid Naso, larval “scales initially form at 5 mm as small body spines which eventually cover most of the body,” and “these turn into small triangular scales arrayed in vertical rows at 8 mm” (Leis and Rennis, 1983: 211). Adult scales of Naso are categorized here as type 1F scales. In ostraciid larvae, the body plates begin to form on the surface of the dermal sac before inflexion, the larvae becoming pelagic juveniles resembling miniature adults, the carapace spines in some species forming during the juvenile stage. In the adult ostraciid Lactoria cornuta, the scales are categorized here as type 2C. Leis and Rennis (1983: 50) found that the centriscid Aeoliscus strigatus develops a carapace that “forms as the bases of raised pyramidal spines on the trunk grow together.” The carapace of adults is categorized here as type 2E. The dactylopterid Dactyloptena at 6.5 mm develops weak spines over the trunk and tail, and by 10 mm, “these spines begin to form keeled scales” (Leis and Rennis, 1983: 66) categorized here in adults as type 2F. Govoni et al. (2004) found that spinoid scales appear early in the development of the xiphiid Xiphias gladius and are first discernable in the larval stage. Scales vary in form between two principal types: small single and multi-spined scales and large multi-spined scales. Unlike the typical teleostean condition, scales of Xiphias are attached along their base, not at their proximal end within scale pockets. The scales persisted in juveniles and adults. They conform best to the description of type 4 spinoid scales (Roberts, 1993; Govoni et al., 2004), that is, type 1A calci-spinoid scales. Byeon et al. (2011) studied the development of spines in the tetraodontid Takifugu obscurus. Spines first appear ten days after hatching and consist of a long tapering portion with two short processes at the tip until 9.6 mm TL. As development proceeds, the central long tapering portion fuses into one process, and the short supporting portion consists of three to six spines. The scales of the tetraodontid Takifugu obscurus are similar to the scales of another tetraodontid, Torquigener pleurogramma, which is categorized here as type 1B. Some cyclopterids develop dermal spines that become pronounced type 2A scales (Able et al., 1984). Busby (1998) found that the type 2B scales of agonid fishes are distinguishable in early (typically preflexion) larvae as small spines. He described how the small spines increased in length and the base broadened with bone covering the base in a radial pattern from the base of the spines as it increased in diameter. The type 2B scales grow until they overlap or fuse and eventually overlap with adjacent fused plates depending on which row of type 2B scales it lies. Type 1A larval scales are described as tack-like with a single spine projecting outward from a circular plate in the larvae of many families of Trachichthyiformes (Johnson, 1984; Baldwin and Johnson, 1995). In the larvae of the monocentrid Monocentris, the type 1A scales have large round scale plates with pyramidal spines, and they progress to type 2B scales. In the larvae of Optivus sp., small dermal spines appear on the body by 4.7 mm and develop in longitudinal rows over the entire body and dorsal- and anal-fin bases by 5.1 mm (Jordan and Bruce, 1993). By 8.0 mm, the base of every single dermal spine has transformed into a small elasmo-spinoid scale. A row of larger spines appears on the ventral surface between the anus and the pelvic fins by 7.2 mm and form the characteristic ventral type 2B scales of juveniles and adults (Jordan and Bruce, 1993). The process of formation of the initium of the type 2D scales of Corydoras arcuatus differs from that of the elasmoid scale and does not appear to involve skin spines (Sire, 1993).

In summary, the following eight calcidermoid scale types are recorded as having originated from skin spines: type 1A, type 1B, type 1F, type 2A, type 2B, type 2C, type 2E, and type 2F, as well as one elasmo-spinoid scale. Most of the skin spines would fit into the category of type 1A except perhaps the skin spines proceeding to type 2B.

Are calcidermoid scales true scales?—

Calcidermoid scales have often been thought of as “not true” scales because they don’t reside in scale pockets. However, calcidermoid scales are mineralized elements in the dermis, and therefore, they are part of the integumentary skeleton. Mineralized scale refers to a broad diversity of elements with different tissue composition and developmental origins (Sire et al., 2009). According to their origin, scales are named cosmoid, palaeoniscoid, polypteroid, and lepisosteoid, none of which reside in scale pockets, and elasmoid scales, which do reside in scale pockets, or the term “scale” is replaced altogether, e.g. dermal plate, scute, shield, etc., none of which reside in scale pockets. So calcidermoid scales, as part of the set of scales that do not reside in scale pockets, should be considered “true” scales, and the term “scale” should no longer be replaced by dermal plate, scute, shield, etc.

What is the origin of calcidermoid scales?—

Calcidermoid scales have always been thought of as originating from elasmoid scales, either cycloid or ctenoid. The assumption that calcidermoid scales are modified scales is well embedded in the ichthyological literature. In fact, some of these “modified scales” progress to elasmoid scales. It has been suggested in the current study that cycloid scales can originate from type 1A calcidermoid scales and that elasmo-spinoid scales can originate from type 1A and type 1F calcidermoid scales. Many descriptions above from larval fishes show that type 1A calcidermoid scales (skin spines) progress to the adult form of the relevant calcidermoid scale. There is no mention of elasmoid scales transforming into skin spines. So, many calcidermoid scales in adult fishes originate from larval calcidermoid scales. From where did type 1A calcidermoid scales originate? In the synanceiids Adventor elongatus, Aploactisoma milesii, and Paraploactis pulvinus, the type 1A scales are modified odontodes. They have foramina in the scale plate and neck of the spine. The foramen in the scale plate is the entrance to a pulp cavity. In these modified odontodes, the pulp cavity would be devoid of blood vessels and nerves, and odontoblastic processes would not be present in the dentine, interpreted as the result of a reduction process by Sire and Huysseune (1996). As noted in the section above on Scales and spines from odontodes, the type 1A scales on the body of Centriscus sp. also contain a pulp cavity that opens on the inner surface of the basal plate (Bridge, 1904). Bridge also noted that some antennariid species have similar scales. Whereas some of the type 1A scales in the synanceiid A. milesii have a pulp cavity, others do not. Those without a pulp cavity have progressed from modified odontodes to type 1A scales. In the osteichthyan lineage, odontodes have been progressively modified into various types of “scales,” including ganoid scales, dermal bony scales, and elasmoid scales, the dermal bony scales being equivalent to the calcidermoid scales proposed in the current study (Sire and Akimenko, 2004).

A strong argument against calcidermoid scales being modified elasmoid scales is that the ganoid scales of the chondrosteans are replaced by calcidermoid scales. Type 1A, 1H, type 2B, and type 2G scales are present on chondrosteans past and present. The Chondrostei predate the Holostei and the Teleostei in which elasmoid scales occur, presenting great difficulty in calling the calcidermoid scales of chondrosteans modified elasmoid scales. The caudal scales of the acipenserids Acipenser ruthenus and A. sturio are roughly rhomboidal, fit closely together, and, at places, overlap (Smith, 1956). In short, they resemble ganoid scales. However, they are composed only of cellular lamellar bone identified in the histological analysis of Smith (1956). The absence of ganoine in these scales that resemble outwardly ganoid scales makes them calcidermoid scales, categorized here as ganoine-lost (type 2G). Thus, another origin for calcidermoid scales, besides odontodes, is ganoid scales. Type 2G scales further progress to elasmoid cycloid scales.

Phylogenetic distribution of calcidermoid and elasmoid scales.—

Type 1A scales in adults are found in eight orders, and larval type 1A are known in 13 orders. In adults, type 2B scales are found in six orders, type 1G in five, type 1F and type 2G in four, type 1D, type 2A, and type 2D in three, type 1B, type 2D, and type 2H in two, and type 1C, type 1E, type 2C, type 2E, and type 2F in one. Overall, calcidermoid scales are distributed across 18 fish orders that span the entire phylogenetic classification of 67 orders (Nelson et al., 2016). Except for four orders (Scorpaeniformes, Acanthuriformes, Lophiiformes, and Tetraodontiformes) in the most phylogenetically advanced group of seven orders that have calcidermoid scales, the other 15 orders are well separated phylogenetically (Table 1). In the non-teleostean orders, there are four calcidermoid scale types, type A1, type 1H, type 2B, and type 2G. Each of these types can be found in the most advanced groups of teleosts, except for type 2G, which evolved into the elasmoid cycloid scales. The Clupeiformes, †Ellimmichthyiformes, and Osteoglossiformes do not have general coverage of calcidermoid scales, but dorsal, ventral, or abdominal type 2B scales (Table 1). †Ellimmichthyids are the only fishes that are double-armored with both dorsal and ventral type 2B scales. The formation of type 2B scales in many clupeiform families is greatly diminished, and cycloid scales are formed instead, showing the potential of the type 1A scales to progress down different development pathways. The next order with calcidermoid scales is the Siluriformes, in which three families have type 2D scales. Scales are absent in the numerous other families. Up until this point, cycloid scales dominate. More advanced are the next two orders with calcidermoid scales, the Trachichthyiformes with type 1F, type 1G, and type 2B scales, and the Beryciformes with type 1A scales. Elasmo-spinoid scales become common in the orders leading up to and within the Trachichthyiformes and the Beryciformes. The current study suggests that the elasmo-spinoid scale type originates from larval type 1A and from type 1F scales. The next more advanced order in which calcidermoid scales occur is the Istiophoriformes, closely followed by the Pleuronectiformes and the Syngnathiformes. Type 1A scales are present in adult xiphiid Xiphias in which they become hidden by the thickening of the dermis above the scale as larval and juvenile Xiphias grow (Govoni et al., 2004). The result is that only the tips of the scale spines protrude in adults. In one family of the Pleuronectiformes, the Scophthalmidae, type 2A scales occur, whereas the rest of the order has cycloid, elasmo-spinoid, and transforming ctenoid scales. The Syngnathiformes have five calcidermoid scale types, type 1A, type 1D, type 2B, type 2E, and type 2F. In the Trachiniformes, type 1G scales occur in the Champsodontidae.

Table 1.Distribution of calcidermoid scale types in fish orders. 1, calci-spinoid; 2, macro-calcidermoid. Type 1A, single-plate upright-spined; type 1B, single-plate horizontal-spined; type 1C, single-plate blunt-spined; type 1D, no-plate single-spined; type 1E, micro-carapace; type 1F, double-plate; type 1G, double-plate pedicel; type 1H, single-plate marginal-spined; type 2A, tubercloid; type 2B, scutoid; type 2C, macro-carapace; type 2D, dermo-plateloid; type 2E, subdermo-plateloid; type 2F, lacunoid; type 2G, ganoine-lost. Present in larvae = L, present in adults = +.

This table presents the distribution of the 15 calcidermoid scale types in larval and adult fishes in 22 orders. Columns represent scale types, rows show presence.

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The greatest diversity of calcidermoid scales is found in the order Scorpaeniformes sensu Smith et al. (2018) in the suborder Cottoidei plus the Gasterosteidae. There are six out of eight of the calci-spinoid types: type 1A, type 1B, type 1C, type 1F, type 1G, and type 1H; and three out of six macro-calcidermoid types: type 2A, type 2B, and type 2D. In the Scorpaenoidei, there are only type 1A, type 1H, and type 2B. In the Acanthuriformes, there are type 1F and type 1G. In the Lophiiformes, there are type 1A and type 2A scales. In the Tetraodontiformes, there are type 1A, type 2B, type 1E, and type 2C calcidermoid scales, the latter two types being only found in this order. There is no discernible trend in calcidermoid scale types looking overall at the complete phylogeny of fishes.

The classification used herein is based on Nelson et al. (2016) because of their more conservative approach to combining morphological data with molecular data. Another recent molecular phylogeny hypothesis is that of Betancur-R et al. (2017).

What might be the explanation for the fragmented distribution of calcidermoid scales? Although calcidermoid scales are very successful in some orders or suborders; overall, they occur in only about 27% (18/67) of orders. Compared to the distribution of cycloid transforming ctenoid scales that evolved from cycloid scales, they might be considered to be a less successful scale type. The current study suggests that type 1A scales transform into cycloid scales with central spines in juvenile Zeus faber and cycloid scales with resorbed spines in adult Zeus faber. In adult Zeus faber, the cycloid scales are minute and embedded. Nelson et al. (2016) noted for the scorpaenid Caracanthus that scales on the dorsal surface of the head are minute and bear a single spine. So it can be suggested that the occurrence of calcidermoid scales is reduced by their transformation into cycloid scales. Elasmo-spinoid scales, as shown in the current study, are derived from calcidermoid scales, either type 1A or type 1F, the double-plate scale. Derivation from the latter explains elevated posterior fields and underside mineralization found in some scales. They have uneven distribution with clustering around the lower eurypterygians. In summary, calcidermoid scales are not common because they have the propensity to transform into other scale types.

Conclusion.—

The name calcidermoid is proposed for a new scale category that is divided into 15 types assigned to two groups: 1. calci-spinoid with eight types and 2. macro-calcidermoid with seven types. Each type has a number of variants. The phylogenetic distribution of calcidermoid scales is fragmentary, and it is proposed that this is due to the propensity of calcidermoid scales to change into other scale types. Suggestions about how the scale types might have evolved from one type to others are made. 1. Calcidermoid scales are evolved from odontodes based on the similarity of type 1A scales to simple odontodes that are upright spines attached to a basal scale plate, the presence of foramina, and the pulp cavities in the type 1A scales of synanceiids and the pulp cavities reported in the type 1A scales of Centriscus and antennariid species; this conclusion is consistent with the statement by Sire and Akimenko (2004: 234) that “in the osteichthyan lineage, odontodes progressively modified into various types of scales” such as ganoid, calcidermoid, and elasmoid, and ganoid scales in which only bone remains after the loss of ganoine and dentine, leaving type 2G ganoid-lost scales. 2. Type 1A calcidermoid scales transform into many other types of calcidermoid scales, both calci-spinoid and macro-calcidermoid. This suggestion is based on several observations of type 1A scales (skin spines) in larval fishes transforming into various types of calcidermoid scales and the detailed description of type 2B scutoid scale development from small spines in larval agonid fishes. 3. Type 1A scales can transform into cycloid scales inferred from the finding of spines in the growth centers of a juvenile Zeus faber and resorbed spines in the centers of minute and embedded cycloid scales of adult Zeus faber, the observation that spines are the external part of feeble, scattered scales in the skin of three synanceiids and a scorpaenid, and that scales on the dorsal surface of the head are minute and bear a single spine in the scorpaenid Caracanthus. 4. Flat elasmo-spinoid scales are derived from type 1A scales inferred from the presence of type 1A scales in larval fishes of adults with spines in their growth centers and complete to incomplete elasmodine mineralization. 5. Elasmo-spinoid scales with elevated posterior fields, indented lateral fields, and underside mineralization are derived from or have a past connection with type 1F double-plate scales. This is based on the morphology of the type 1F scales in Trachichthys australis that explains these character states in the elasmo-spinoid scales of the priacanthids, the similar posterior field spines and organization of scales on the body to elasmoid scales, and the presence of indented lateral fields as seen in the image of the scale of Optivus in the current study.

Calcidermoid scales.—

Trachichthyiformes: Anoplogastridae: Anoplogaster cornuta, AMS I.20307-006; Monocentridae: Cleidopus gloriamaris, AMS I.16934-00, AMS IB.1150; Trachichthyidae: Trachichthys australis, AMS I.15912-007.

Elasmoid scales.—

Trachichthyiformes: Anomalopidae: Anomalops katoptron, AMS I.17293-001, AMS I.19291-001; Kryptophanaron alfredi, AMS I.21146-001; Trachichthyidae: Gephyroberyx darwinii, AMS I.20439-001; Optivus agastos, AMS 1.17178-046.