Variation in the Posttemporal-Supracleithrum Articulation in Euteleosts
A survey of the articulation between two of the dermal elements of the pectoral girdle, the posttemporal and supracleithrum, across 308 euteleost species in 40 orders and 181 families revealed considerable variation. This variation was categorized first by number of distinct contact points, which varied between zero and three. It was then subdivided further into a total of 17 morphotypes based on the observed patterns of variation in the form of these contact points, the plane in which they are oriented, and their relative positions on each bone. Given the distribution of these morphotypes across examined taxa, we hypothesize that a simple laminar morphotype, in which the posttemporal and supracleithrum are syndesmotically bound and no obvious contact points are discernable, represents the plesiomorphic condition. Establishing homology of contact points for many morphotypes remains challenging. However, some morphotypes appear to support previously recognized clades and thus represent putative synapomorphies; these include possession of two contact points (Acanthomorphata) and possession of a specialized three contact point morphotype (Acanthuridae + Zanclidae). The posttemporal-supracleithrum articulation is a variable character system that may be phylogenetically informative for future morphological investigations of acanthomorph clades.
Schematic diagrams of posttemporal-supracleithrum articulation morphotypes. The schematic diagrams utilize generalized outlines of the posttemporal and supracleithrum and are not meant to represent any specific taxon. A distinct contact point oriented in a sagittal plane is indicated with a dark gray circle; a distinct contact point oriented in a frontal plane is indicated by a pair of dark gray bars. The illustrated morphotypes are as follows: (A) simple laminar; (B) complete fusion; (C) one point, sagittal plane, restricted; (D) variant of one point, sagittal plane, restricted as it occurs in the aulopiform Parasudis; (E) one point, sagittal plane, elongate; (F) two points, both sagittal, without lateral line canal enclosure on supracleithrum; (G) two points, both sagittal, ventral contact point on anterodorsal portion of lateral line canal enclosure on supracleithrum; (H) two points, both sagittal, ventral contact point anteroventral to anterior opening of lateral line canal enclosure on supracleithrum; (I) two points, both frontal; (J) two points, dorsal sagittal, ventral frontal; (K) two points, dorsal frontal, ventral sagittal; (L) two points, both frontal, even with each other; (M) three points, all sagittal; (N) three points, dorsal frontal, middle and ventral sagittal; (O) three points, all frontal; and (P) three points, dorsal and middle frontal, ventral sagittal and on supracleithrum medial surface. Abbreviations: cp, contact point; llce, lateral line canal enclosure; pt, posttemporal; scl, supracleithrum.
Example of the morphotype “simple laminar.” The photograph shows the simple laminar morphotype as it occurs in the myctophid Diaphus dumerili (AMNH 27403). No distinct contact points or modified articular surfaces are present. Shown in lateral view with anterior facing left. Scale bar is 2 mm. Abbreviations: llce, lateral line canal enclosure; pt, posttemporal; scl, supracleithrum.
Example of the morphotype “complete fusion.” In the sternoptychid Argyropelecus affinis (FMNH 85165), a single element, presumed to represent a fusion of the posttemporal and supracleithrum (Baird, 1971), is present. Shown in lateral view with anterior facing left.
Examples of one-point morphotypes. (A) In the belonid Strongylura notate (FMNH 97508), a single distinct contact point with rugose surfaces is present. Shown in medial view with anterior facing right. (B) A single distinct contact point is also present in the morid gadiform Laemonema barbatulum (USNM 327734), but in this morphotype, the contact region is noticeably elongate relative to the “restricted” morphotype shown in (A). Shown in lateral view with anterior facing left. Scale bar is 2 mm. Abbreviations: cp, contact point; pt, posttemporal; scl, supracleithrum.
Examples of the morphotype “one point, condylar.” In (A) the balistoid Aluterus scriptus (KU 30391) and (B) the batrachoidiform Porichthys notatus (KU 18136), the concavity is on the scl, and the convexity is on the pt. In (C) the lampridiform Desmodema polystictum (SIO76-167) and (D) the beloniform Parexocoetus brachypterus (FMNH 64037), the concavity is on the pt, and the convexity is on the scl. All specimens are shown in lateral view with anterior facing left. Scale bar for A, B, and D is 2 mm; scale bar for C is 1 mm. Abbreviations: cp, contact point; pt, posttemporal; scl, supracleithrum.
Examples of two-point morphotypes. (A) The serranid scorpaeniform Diplectrum formosum (FMNH 64276) has two distinct contact points with rugose surfaces; both contact points are in a sagittal plane. The ventral contact point is on the anterodorsal portion of the supracleithrum lateral line canal enclosure. (B) The scombrolabracid scombriform Scombrolabrax heterolepis (UMML 7102) also has two distinct contact points in a sagittal plane, but in this morphotype, the ventral point is located anteroventral to the anterior opening of the supracleithrum lateral line canal enclosure. (C) The labrid Tautogolabrus adspersus (USNM 118350) has two distinct contact points, but in this taxon, they are situated in a frontal, rather than sagittal, plane. In (A) and (B), specimens are shown in lateral view with anterior facing left. In (C), the specimen is shown in medial view with anterior facing right. Scale bar is 2 mm. Abbreviations: cp, contact point; llce, lateral line canal enclosure; pt, posttemporal; scl, supracleithrum.
Examples of the morphotype “two points, both frontal, even with each other.” In (A) the tetraodontiform Hollardia hollardi (USNM 187811; epi-fluorescence, lateral view, in situ) and (B) the lophiiform Histrio histrio (FMNH 46140; posterolateral view), two contact points, indicated by brackets, are present. Both contact points are situated in a frontal plane (i.e., the posttemporal and supracleithrum articulate in an end-to-end fashion) with both points clearly visible in lateral view. (C) In the epi-fluorescence image of the lateral view of the zeiform Zenopsis conchifer (SIO uncataloged), only one contact point is visible, indicated by the arrow. The bones must be viewed in (D) medial view in order to see the second contact, indicated by the arrow. Scale bar is 2 mm. Abbreviations: cp, contact point; pt, posttemporal; scl, supracleithrum.
Examples of the morphotype “two points, anterior frontal, posterior condylar.” (A) In the caproiform Capros aper (AMNH 482), the concavity is on the scl, and the convexity is on the pt. (B) In the perciform Mene maculata (ANSP 158496), the concavity is on the pt, and the convexity is on the scl. Both specimens are shown in lateral view with anterior facing left. Scale bar is 2 mm. Abbreviations: cl, cleithrum; cp, contact point; pt, posttemporal; scl, supracleithrum.
Examples of three-point morphotypes. (A) The polynemid perciform Polydactylus octonemus (KU 7493) has three distinct rugose contact points, all of which are situated in a sagittal plane. The specimen is shown in lateral view with anterior facing left. Scale bar is 2 mm. The chaetodontid perciform Forcipiger flavissimus (USNM 340962), photographed via epi-fluorescence imaging in (B) lateral view and (C) medial view, has three distinct contact points, all of which are situated in a frontal plane. Abbreviations: cp, contact point; llce, lateral line canal enclosure; pt, posttemporal; scl, supracleithrum.
The unique three-contact point morphotype observed in Acanthuridae + Zanclidae. The condition is shown in Paracanthurus hepatus (AMNH 43116); in both (A) and (B), the left photograph is a lateral view with anterior to the right, and the right photograph is a medial view with anterior facing left. (A) The dorsal and middle contact points are in a frontal plane. The ventral point is in a sagittal plane but occurs on the medial surface of the supracleithrum rather than the lateral surface. This unusual arrangement is accommodated by a projection from the posteroventral edge of the posttemporal that reaches medial to the supracleithrum. (B) The black outline demarcates the outer border of the posttemporal. Scale bar is 2 mm. Abbreviations: cl, cleithrum; cp, contact point; pt, posttemporal; scl, supracleithrum.
Contributor Notes
Associate Editor: G. Arratia.