Pradel, A. | Search bioRxiv

bioRxiv2023

The pharynx of the iconic stem-group chondrichthyan Acanthodes (Agassiz, 1833) revisited with micro computed tomography.

Acanthodes has long been the primary source of information on the pharyngeal skeleton of acanthodians, a stem-group chondrichthyan grade. Because of this its anatomy has played an outsized role in attempts to understand the evolution of the jawed vertebrate pharynx and the clade as a whole. However, the anatomy of the pharynx of Acanthodes remains poorly understood and subject to several competing interpretations. We use computed tomography (CT) to image the articulated pharyngeal skeletons of three specimens of Acanthodes confusus from Lebach, Germany. Acanthodes had a melange of osteichthyan-like and chondrichthyan-like morphologies in its pharyngeal skeleton. Like other stem-chondrichthyans, Acanthodes had a basihyal with no hypohyals, and four pairs of posteriorly oriented pharyngobranchials. Like osteichthyans, Acanthodes possessed an interhyal, but lacked the separate infra- and supra-pharyngobranchial elements present in osteichthyans and the crown-chondrichthyan Ozarcus. Using this new data we build and animated a digital 3D model of the pharyngeal endoskeleton in Acanthodes, showing that the jaws would have swung outwards during the opening cycle, increasing the anteriorly facing area of the gape for suspension feeding. These new data provide a more definitive picture of the anatomy of a taxon that has long been of great significance in early vertebrate palaeontology.

paleontology↗

bioRxiv2020

The morphology and evolution of chondrichthyan cranial muscles: a digital dissection of the elephantfish Callorhinchus milii and the catshark Scyliorhinus canicula.

The anatomy of sharks, rays, and chimaeras (chondrichthyans) is crucial to understanding the evolution of the cranial system in vertebrates, due to their position as the sister group to bony fishes (osteichthyans). Strikingly different arrangements of the head in the two constituent chondrichthyan groups - holocephalans and elasmobranchs - have played a pivotal role in the formation of evolutionary hypotheses targeting major cranial structures such as the jaws and pharynx. However, despite the advent of digital dissections as a means of easily visualizing and sharing the results of anatomical studies in three dimensions, information on the musculoskeletal systems of the chondrichthyan head remains largely limited to traditional accounts, many of which are at least a century old. Here we use synchrotron tomography acquire 3D data which we used to carry out a digital dissection of a holocephalan and an elasmobranch widely used as model species: the elephantfish, Callorhinchus milii, and the small-spotted catshark, Scyliorhinus canicula. We describe and figure the skeletal anatomy of the head, labial, mandibular, hyoid, and branchial cartilages in both taxa as well as the muscles of the head and pharynx. We make new observations, particularly regarding the branchial musculature of Callorhinchus, revealing several previously unreported or previously ambiguous structures. Finally, we review what is known about the evolution of chondrichthyan cranial muscles from their fossil record and discuss the implications for muscle homology and evolution, broadly concluding that the holocephalan pharynx is likely derived from a more elasmobranch-like form. This dataset has great potential as a resource, particularly for researchers using these model species for zoological research, functional morphologists requiring models of musculature and skeletons, as well as for palaeontologists seeking comparative models for extinct taxa.

evolutionary biology↗