ANATOMY OF EARLY FISH: A VIRTUAL PERSPECTIVE

Fossils of early vertebrates were among the first subjects of tomographic studies in paleontology; as far back as the early 1900s, destructive techniques such as Sollas’ grinding method were employed to reconstruct the internal anatomy of primitive fishes. The advent of x-ray computed tomography (CT) allowed tomographic techniques to be applied more broadly and rapidly, with the non-destructive nature meaning that work is repeatable and specimens can be preserved. CT can be used to investigate the evolutionary history of entire systems, such as the vertebrate hard tissue complex, or specific morphological features of organisms. Here we employed lab-based CT and synchrotron radiation CT to investigate the structure of some of the earliest bony fishes, a clade which today includes nearly all living vertebrate diversity. Specifically, we targeted the Devonian ray-finned fish Cheirolepis, the sister-taxon to all other ray-fins and thus instrumental in our understanding of the primitive conditions in this clade. Despite its importance, the rarity of fossils preserving remains of the internal skeleton (endoskeleton) has marginalized this genus relative to other geologically younger ray-fins. Endoskeletal structure is particularly important in investigating the relationships of early jawed vertebrates, as hypotheses of homology can often be drawn more easily from endoskeletal structures (e.g., braincases) than the dermal skeleton, which varies considerably between primitive gnathostomes. Tomographic investigation of specimens of Cheirolepis from Old Red Sandstone localities of Scotland substantially alters previous accounts of endoskeletal structure in this genus, with important implications for understanding patterns of character evolution deep within the bony fish crown. In addition to elements of the hyoid arch and pectoral fin, a near-complete braincase was studied. Many aspects of the braincase are comparable to later ray-fins, such as a well-developed spiracular canal, but important exceptions likely represent plesiomorphic bony fish (and jawed vertebrate) features. Collectively, these new data help clarify primitive conditions within ray-finned fishes, which in turn have important implications for understanding features likely present in the last common ancestor of living bony fishes.