Systematic Implications of the Anterior Pectoral Basals in Squaliform Sharks (Chondrichthyes: Elasmobranchii)
This study investigates the skeletal architecture of the anteroproximal surface of the anterior pectoral basals in squaliform sharks, which presents distinctions at the insertion of one of the components of the depressor pectoralis muscle. Some squalomorph and galeomorph sharks studied have a different orientation of the propterygium or anteroproximal portion of the composite pectoral basal, as these project dorsally to form a distinct anterolateral plan for the insertion of the depressor pectoralis muscle. An exclusive condition of the anterior articular surface was observed in Centrophoridae, Dalatiidae, Oxynotidae, and Somniosidae, which have a hook-like process on the flattened and widened surface of the anteroproximal margin of the single or composite anterior pectoral basal. This distinct pectoral fin skeletal condition may represent evidence of a closer relationship among these taxa, whereas a dorsally projected anteriormost pectoral basal may represent the generalized condition among sharks.
Ventral (A, C, E), proximal (B), and medial-oblique (D, F) views of the pectoral fin skeleton (anterior to top in all except in B [anterior to right]) of representatives of the families Squalidae (A, B) and Etmopteridae (C–F), all presenting a flattened and widened surface on anteroproximal margin of propterygium or composite pectoral basal (indicated by white arrowhead), but lacking the hook-like process. (A, B) Squalus megalops (MZUSP 110973, female, 512 mm TL). Dorsal to top in B. (C, D) Centroscyllium fabricii (left figure, USNM 35579, female, 530 mm TL; right figure, UF 159844, female, 420 mm TL). (E, F) Etmopterus pusillus (USNM 221042, female, 530 mm TL). Scale bar = 10 mm.
Pectoral fin skeleton of Trigonognathus kabeyai (MZUSP uncat., male, 366 mm TL). (A) Ventral view. (B) Detail of anteroproximal portion of the composite pro+mesopterygium of Trigonognathus kabeyai (MZUSP uncat.) in medial-oblique view. Note that the anteroproximal region of the composite cartilage is not perpendicularly orientated nor does it present the flattened and widened surface for insertion of m. depressor pectoralis 1. White arrowhead indicates anteroproximal surface of composite pectoral basal; black arrows indicate pectoral basal articular surface with scapulocoracoid. Scale bar = 5 mm.
Ventral (A, C, E, G), proximal (B), and medial-oblique (D, F, H) views (anterior to top in all except in B [anterior to right]) of the pectoral fin skeleton of representatives of the families Lamnidae (A, B), Echinorhinidae (C, D), Scyliorhinidae (E, F), and Carcharhinidae (G, H), with a flattened and widened surface on anteroproximal portion of propterygium or composite pectoral basal. (A, B) Isurus oxyrinchus (USNM 201733, female, 745 mm TL). Note the dorsal position of the propterygium in relation to the other pectoral basals (dorsal to top in A, B). (C, D) Echinorhinus cookei (MZUSP uncat., female, approx. 1000 mm TL). (E, F) Scyliorhinus haeckelii (MZUSP 112024, male, 385 mm TL). (G, H) Rhizoprionodon lalandii (MZUSP uncat., female, 420 mm TL). White arrowheads indicate anteroproximal portion of single or composite pectoral basal dorsally oriented for the insertion of the depressor musculature. Black arrows indicate pectoral basal articular surface with scapulocoracoid. Scale bar = 20 mm.
Proximal surface of pectoral basals of representatives of the families Oxynotidae (A), Centrophoridae (B), and Somniosidae (C) depicting the concavity on anteroproximal portion of composite pectoral basal (indicated by an asterisk), forming a hook-like process (indicated by white arrowhead). (A) Pectoral basals of Oxynotus bruniensis (HUMZ 91383, male, 545 mm TL) in oblique proximal view depicting the anteroproximal flattened and widened surface of the composite pectoral basal associated with the hook-like process, and also the anteroproximal margin dorsally projected (indicated by a white arrow). (B) Deania calcea (MZUSP uncat., female, 1035 mm TL). (C) Centroscymnus owstonii (UERJ A.1460, female, 950 mm TL). Black arrow indicates pectoral basal articular surface with scapulocoracoid. Scale bar = 10 mm.
Ventral view (anterior to top) of pectoral fin skeleton of representatives of the families Centrophoridae (A) and Dalatiidae (B–E) depicting the flattened and widened surface on anteroproximal margin of propterygium (A) or composite (B–E) pectoral basal. White arrowheads indicate the presence of this pectoral character, which is also present in (C) but forming a faint knob in ventral view. (A) Centrophorus granulosus (USNM 220221, female, 395 mm TL). (B) Isistius brasiliensis (MNHN 1996-0465, male, 409 mm TL). (C) Dalatias licha (USNM 157844, female, 361 mm TL). (D) Euprotomicrus bispinatus (BPBM 40404, female, 255 mm TL). (E) Squaliolus laticaudus (UF 176375, female, 130 mm TL). (F) Detail of the anteroproximal margin of the composite pectoral basal in Isistius brasiliensis (MNHN 1996-0465, male, 409 mm TL), showing the flattened and widened surface for insertion of m. depressor pectoralis 1. White arrowhead indicates anteroproximal surface of composite pectoral basal; black arrows indicate pectoral basal articular surface with scapulocoracoid. Scale bar = 10 mm.
Ventral view (anterior to top) of pectoral fin skeleton of representatives of the family Somniosidae with the flattened and widened surface on anteroproximal margin of composite pectoral basal forming a faint hook-like process (indicated by white arrowheads). Condition visualized in (C) and (D) may represent an incipient state of the hook-like process observed in adult specimens of Centroscymnus. (A) Centroselachus crepidater (AMNH 58099, female, 299 mm TL). (B) Scymnodalatias albicauda (HUMZ 129360, male, 171 mm TL). (C) Centroscymnus owstoni (UF 27957, male, 311 mm TL). (D) Centroscymnus coelolepis (MZUSP uncat., female, 318 mm TL). White arrowhead indicates anteroproximal surface of composite pectoral basal; black arrows indicate pectoral basal articular surface with scapulocoracoid. Scale bar = 20 mm.
Ventral view (anterior to top) of pectoral fin skeleton of representatives of the family Centrophoridae presenting the flattened and widened surface on anteroproximal margin of composite pectoral basal (A), the marked concavity on anteroproximal portion of propterygium (B), or the composite (C) pectoral basal with the hook-like process indicated by white arrowheads. (A) Deania calcea (USNM 203495, female, 700 mm TL). (B) Deania calcea (MZUSP uncat., female, 1035 mm TL). (C) Deania quadrispinosum (SAIAB 188437, female, 540 mm TL). (D) Detail of the anteroproximal surface of propterygium with the hook-like process of Deania calcea (MZUSP uncat., female, 1035 mm TL), in medial-oblique view. White arrowhead indicates anteroproximal surface of composite pectoral basal; black arrows indicate pectoral basal articular surface with scapulocoracoid. Scale bar = 20 mm.
Ventral view (anterior to top) of pectoral fin skeleton of representatives of the families Oxynotidae (A, B) and Somniosidae (C–F) with marked concavity on anteroproximal portion of composite pectoral basal, forming a hook-like process (indicated by white arrowhead). (A) Oxynotus bruniensis (HUMZ 91383, male, 545 mm TL). (B) Detail of the composite pro+mesopterygium of Oxynotus bruniensis (HUMZ 91383) showing the marked concavity forming the hook-like process (hpr) and the insertion of the m. depressor pectoralis 1. (C) Somniosus rostratus (BMNH 1906.11.12.7, juvenile, unknown sex, pectoral skeleton only, approx. 500 mm TL). (D) Scymnodon ringens (BMNH 1991.7.9.686-690, female, 507 mm TL). (E) Centroscymnus owstonii (UERJ A 1460, female, 950 mm TL). (F) Zameus squamulosus (USNM 94520, female, 351 mm TL). White arrowhead indicates anteroproximal surface of composite pectoral basal; black arrows indicate pectoral basal articular surface with scapulocoracoid. Scale bar = 20 mm.
Hypotheses of phylogenetic relationships among extant elasmobranchs showing the distribution of the pectoral fin conditions. Red represents the anteroproximal surface of the first pectoral basal observed in surveyed taxa with a knob or hook-like process (hook-like process with dark contour). (A) Morphological phylogeny modified from Shirai (1992). (B) Morphological phylogeny modified from Carvalho (1996). (C) Morphological phylogeny modified from Adnet and Cappetta (2001). (D) Molecular phylogeny modified from Naylor et al. (2012).
Contributor Notes
Associate Editor: G. Arratia.