Platecarpus

Abstract: A new mosasaur, Russellosaurus coheni, is described from the Collignoniceras woollgari Zone (lower Middle Turonian) at Cedar Hill, Dallas County, Texas. At approximately 92 Ma, it is the oldest well-preserved mosasaur skull from North America. It possesses characters diagnostic of Plioplatecarpinae but retains numerous plesiomorphies as well. Phylogenetic analysis indicates a close relationship with Yaguarasaurus columbianus, and these two, together with Tethysaurus nopcsai, form a clade that occupies a position basal to the divergence of the subfamilies Tylosaurinae and Plioplatecarpinae. Russellosaurus coheni is proposed as the nominal taxon of a new mosasaur clade, parafamily taxon novum Russellosaurina, which includes Plioplatecarpinae, Tylosaurinae, their common ancestor and all descendants. Tethysaurus retains a plesiopedal limb and girdle morphology, and along with Russellosaurus and Yaguarasaurus, cranial plesiomorphies. Dallasaurus turneri, a temporally and geographically sympatric plesiopedal mosasaur, occupies a basal position within Mosasaurinae. This phyletic arrangement confirms that marine adaptations, such as development of paddle-like limbs, occurred independently in at least two lineages of mosasaurs, once within Mosasaurinae and once within Russellosaurina.

Abstract: Mosasaurs (family Mosasauridae) are a diverse group of secondarily aquatic lizards that radiated into marine environments during the Late Cretaceous (98–65 million years ago). For the most part, they have been considered to be simple anguilliform swimmers – i.e., their propulsive force was generated by means of lateral undulations incorporating the greater part of the body – with unremarkable, dorsoventrally narrow tails and long, lizard-like bodies. Convergence with the specialized fusiform body shape and inferred carangiform locomotory style (in which only a portion of the posterior body participates in the thrust-producing flexure) of ichthyosaurs and metriorhynchid crocodyliform reptiles, along with cetaceans, has so far only been recognized in Plotosaurus, the most highly derived member of the Mosasauridae. Here we report on an exceptionally complete specimen (LACM 128319) of the moderately derived genus Platecarpus that preserves soft tissues and anatomical details (e.g., large portions of integument, a partial body outline, putative skin color markings, a downturned tail, branching bronchial tubes, and probable visceral traces) to an extent that has never been seen previously in any mosasaur. Our study demonstrates that a streamlined body plan and crescent-shaped caudal fin were already well established in Platecarpus, a taxon that preceded Plotosaurus by 20 million years. These new data expand our understanding of convergent evolution among marine reptiles, and provide insights into their evolution's tempo and mode.

Abstract: A survey of mosasaur material from the Eutaw Formation (Santonian) and Selma Group (Late Santonian–Late Maastrichtian) of western and central Alabama has revealed significant stratigraphic segregation among taxa. Three distinct biozones are recognized based on this survey: the Tylosaurus Acme-zone (from the base of the Tombigbee Sand Member of the Eutaw Formation to approximately 12 m above the base of the Mooreville Chalk), the Clidastes Acme-zone (from approximately 12 m above the base of the Mooreville Chalk to the base of the Demopolis Chalk), and the Mosasaurus Acme-zone (from the base of the Demopolis Chalk to the K-T boundary at the top of the Prairie Bluff Chalk). In each case, the biozone is named for the genus comprising the majority of the specimens examined from that horizon. The previously-recognized Globidens alabamaensis Acme-zone, including the Arcola Limestone Member of the Mooreville Chalk, has been abandoned. The transition from the Tylosaurus biozone to the Clidastes biozone h...

Abstract: A new plioplatecarpine taxon, Latoplatecarpus, gen. nov., is proposed here to incorporate two species, L. willistoni, sp. nov., and L. nichollsae (comb. nov.). The new genus is currently restricted in distribution to the lower middle Campanian of North America. Phylogenetic analysis incorporating 97 morphological characters and 17 ingroup taxa indicates that (1) Ectenosaurus clidastoides is the basal-most member within the Plioplatecarpinae; (2) Platecarpus planifrons is generically distinct from P. tympaniticus; (3) Latoplatecarpus willistoni, gen. et sp. nov., constitutes the basal-most member within the clade that is sister to Platecarpus tympaniticus; and (4) as many as 17 unequivocal character changes support the clade comprising the three post-middle Campanian Plioplatecarpus species. Plesioplatecarpus, gen. nov., is erected to establish Plesioplatecarpus planifrons (comb. nov.). Further, we propose that the North American specimens referred to Platecarpus somenensis pertain to Latoplatecar...