Protopsephurus liui, a well-preserved primitive paddlefish (Acipenseriformes: Polyodontidae) from the Lower Cretaceous of China

Protopsephurus liui, from the Lower Cretaceous Yixian Formation of China, is both the oldest and the most primitive paddlefish (polyodontid) known. It was originally described based on a partial skeleton, a partial skull, and two partial trunk regions, leaving much of the species' osteology unknown. Here, based on the type specimens and many newly discovered well-preserved skeletons, we describe this species in detail. Protopsephurus is clearly a polyodontid based on several characters including: (1) the presence of stellate bones; (2) the presence of a series of long median dorsorostral and ventrorostral bones; (3) extreme elongation of the snout; (4) the presence of anterior, middle and posterior divisions of the fenestra longitudinalis; and (5) the presence of 'microctenoid' scales (defined here). Within Polyodontidae, Protopsephurus (which forms the monotypic series Protopsephuri) lacks several characters derived for all other Polyodontidae (=Polyodonti). In contrast to Protopsephuri, Polyodonti (including two fossil and two extant genera) have: (1) larger, more branched stellate bones; (2) a reduction of branchiostegals to a single element; (3) a sutural contact between posttemporal and dermosphenotic; (4) a longer anterior arm of the posttemporal; (5) a very large anterior division of the fenestra longitudinalis; (6) a loss of the pectoral fin spine; (7) the nasals reduced to little more than simple tubular ossifications; and (8) well-developed spine-like posterior projections of the subopercle. In addition, Protopsephurus and Paleopsephurus are apparently the only paddlefish species to have retained median elements at the rear of the skull roof (e.g., a median extrascapular bone). The presence of a sturgeon-like pectoral spine in Protopsephurus and some peipiaosteids suggest that paddlefishes primitively had this structure and that the mere presence of a pectoral fin spine is not synapomorphic for Acipenseridae (i.e., the pectoral fin spine could be a derived feature of Acipenseristomi sensu. Strong development of the pectoral fin spine can still be used as a derived character of Acipenseridae because it is unique to that family among Acipenseriformes. Our phylogenetic analysis here does not support the monophyly of Acipenseristomi(a group indicating Acipenseroidei to be more closely related to Peipiaosteidae than to Chondrosteidae).