Pectoral Girdle

Pachycormid taxonomy has accepted the twin lineage pattern of toothed and toothless tribes, leading respectively to the Late Cretaceous Protosphyraena and Bonnerichthys, for over ten years. Recent work on pectoral fin morphotypes has revealed a diversity of forms, reflecting specialization in swimming styles. Examination of pectoral girdle components led to an unexpectedly similar set of characteristics between these two genera, supposedly separated by a gap of over 100 million years since their common ancestor. This raises the question of whether these two taxa are as distantly related as current pachycormid phylogenetic hypotheses suggest.

Abstract:  A new, third, specimen of Onychoselache traquairi from the Viséan (Holkerian) of Scotland allows a significant revision of the anatomy of this stem-group elasmobranch. This first report of material from the Mumbie Quarry exposure of the Glencartholm fish beds presents a new reconstruction of Onychoselache showing broad-based cephalic and nuchal spines, and exceptionally large pectoral fins. Details of the jaws, braincase and postcranial skeleton demonstrate that Onychoselache is a well-characterized member of the Hybodontiformes. Comparisons of the pectoral skeleton with other early chondrichthyan examples, including new material of Tristychius arcuatus and Plesioselachus macracanthus, highlight a range of early chondrichthyan conditions that are incorporated into a revised hybodontiform phylogeny. Close resemblance between Onychoselache and Mesozoic and late Palaeozoic hybodonts implies that these clades diverged within the Carboniferous and Permian. Major differences between Onychoselache and the coeval Tristychius (a modified reconstruction of which is included) indicate that the Neoselachii-Hybodontiformes split is probably Late Devonian, consistent with records of isolated teeth. The pectoral fins of Onychoselache, while unique among Palaeozoic forms, resemble those of Recent bamboo and epaulette sharks (Orectolobiformes). The functional corollary of this convergence is that Onychoselache represents an instance of a non-tetrapod early vertebrate with a near-walking gait.

Pachycormid taxonomy has accepted the twin lineage pattern of toothed and toothless tribes, leading respectively to the Late Cretaceous Protosphyraena and Bonnerichthys, for over ten years. Recent work on pectoral fin morphotypes has revealed a diversity of forms, reflecting specialization in swimming styles. Examination of pectoral girdle components led to an unexpectedly similar set of characteristics between these two genera, supposedly separated by a gap of over 100 million years since their common ancestor. This raises the question of whether these two taxa are as distantly related as current pachycormid phylogenetic hypotheses suggest.

Among osteichthyans, basal actinopterygian fishes (e.g. paddlefish and bowfins) have paired fins with three endoskeletal components (pro-,meso- and metapterygia) articulating with polybasal shoulder girdles, while sarcopterygian fishes (lungfish, coelacanths and relatives) have paired fins with one endoskeletal component (metapterygium) articulating with monobasal shoulder girdles. In the fin–limb transition, the origin of the sarcopterygian paired fins triggered new possibilities of fin articulation and movement, and established the proximal segments (stylopod and zeugopod) of the presumptive tetrapod limb. Several authors have stated that the monobasal paired fins in sarcopterygians evolved from a primitive polybasal condition. However, the fossil record has been silent on whether and when the inferred transition took place. Here we describe threedimensionally preserved shoulder girdles of two stem sarcopterygians (Psarolepis and Achoania) from the Lower Devonian of Yunnan, which demonstrate that stem sarcopterygians have polybasal pectoral fin articulation as in basal actinopterygians. This finding provides a phylogenetic and temporal constraint for studying the origin of the stylopod, which must have originated within the stem sarcopterygian lineage through the loss of the propterygium and mesopterygium.

Pachycormid taxonomy has accepted the twin lineage pattern of toothed and toothless tribes, leading respectively to the Late Cretaceous Protosphyraena and Bonnerichthys, for over ten years. Recent work on pectoral fin morphotypes has revealed a diversity of forms, reflecting specialization in swimming styles. Examination of pectoral girdle components led to an unexpectedly similar set of characteristics between these two genera, supposedly separated by a gap of over 100 million years since their common ancestor. This raises the question of whether these two taxa are as distantly related as current pachycormid phylogenetic hypotheses suggest.

The Late Jurassic Slottsmøya Member Lagerstätte on Spitsbergen preserves a diverse array of marine reptiles, including four named taxa of ophthalmosaurid ichthyosaurs. One of these, , is based on the single holotype specimen (SVB 1451) with an incomplete skull. A newly discovered specimen (PMO 222.669) with a disarticulated but largely complete skull and anterior postcranium is described, which considerably expands our knowledge of this taxon. Two additional new ophthalmosaurid specimens with pectoral girdles from the same member are described. The taxonomic utility of the ophthalmosaurid pectoral girdle is contentious, and an assessment of seven pectoral girdles from the Slottsmøya Member provides a basis for addressing this question via a 2D landmark principal component analysis of baracromian coracoids. The analysis reveals a taxonomic signal in the coracoids but also highlights the degree of individual variation. Commonly used phylogenetic characters do not fully encapsulate the...