Cranial structure in †Chondrosteus acipenseroides re-examined: implications for upper jaw homologies in acipenseriform fishes

The Early Jurassic family †Chondrosteidae is considered the sister lineage to all remaining acipenseriforms, the actinopterygian group that includes sturgeons, paddlefishes, and their immediate extinct relatives. As the best-known and oldest †chondrosteid, the Hettangian– Sinemurian †Chondrosteus acipenseroides plays a central role in understanding plesiomorphic acipenseriform conditions. Here we present a micro-computed tomographic examination of the cranial anatomy of †Chondrosteus based on a laterally compressed skull. We corroborate many aspects of the most recent account of this genus but find additional upper jaw bones not previously reported. These newly recognized ossifications, some dermal and some endoskeletal in origin, bear on hypothesized homologies of upper jaw components in acipenseriforms. First, †Chondrosteus possesses a series of up to three toothed dermal bones associated with the ventral margin of the palate. These bones correspond to the dermopalatines of other actinopterygians. We therefore reject identification of the marginal, strap-like bone of the acipenseriform upper jaw as a dermopalatine and accept the historical interpretation of that ossification as a maxilla. Second, two endoskeletal bones are associated with the aboral surface of the palate. The largest of these is a thin sheet matching the bone conventionally interpreted as an autopalatine in other acipenseriforms. It is joined by a smaller, more robust bone that lies at the anterior margin of the jaw and resembles the autopalatine of more generalized early actinopterygians, leading us to suggest the acipenseriform “autopalatine” is in fact the metapterygoid. These new observations support recently revised interpretations of homology based on living species. We conclude by reviewing past candidates for deeper members of the acipenseriform stem than †chondrosteids, including †saurichthyiforms, †Birgeria, †Errolichthys, †coccolepidids, †Eochondrosteus, and a handful of Paleozoic taxa. At present, we find no compelling evidence for an affinity between any of these and acipenseriforms. Paleontological and molecular data indicate that the long, barren acipenseriform stem spans the late Paleozoic and early Mesozoic; identification of its members poses an outstanding challenge in actinopterygian paleontology