An Exquisitely Preserved Skeleton Representing a Primitive Sturgeon from the Upper Cretaceous Judith River Formation of Montana (Acipenseriformes: Acipenseridae: N. Gen. and Sp.)

Abstract

A new species of sturgeon (Acipenseridae), †Psammorhynchus longipinnisn. gen. and sp., is described based on a single well-preserved specimen from the Upper Cretaceous Judith River Formation of Montana. Although based on only a single individual, the osteological detail we are able to describe for this species is extensive because of the completeness of the specimen, the relatively three-dimensional preservation, and the soft, loosely consolidated sandstone matrix in which it was contained (allowing some dissection of the specimen to reveal the internal bones of the head region). We prepared the specimen completely out of the matrix, and described the head region in stages of “dissection.” †Psammorhynchus longipinnisis the only fossil sturgeon to date that provides a relatively complete, reliable look at its osteology. Although there have previously been numerous and widespread reports of fossil sturgeons, the material is notoriously fragmentary and poorly preserved, consisting mostly of isolated scutes or pectoral spines (summarized here in an appendix) and it is of very limited use to phylogenetic studies of living sturgeon taxa. The only other described articulated skeleton of a fossil sturgeon is †Protoscaphirhynchus squamosusWilimovsky from the Upper Cretaceous Hell Creek Formation of Montana. That specimen (the only one known for the species) is a badly preserved, very poorly prepared skeleton showing little detail of the head region. It is interesting to note that the only known articulated fossil sturgeon skeletons (†Psammorhynchus longipinnisn. gen. and sp.; †Protoscaphirhynchus squamosusWilimovsky; and a third, yet undescribed specimen) were apparently each independently discovered in the abdominal regions of hadrosaurian dinosaurs (as was the Cretaceous paddlefish †Paleopsephurus wilsoniMacAlpin). Perhaps hadrosaur carcasses provided the right combination of taphonomic factors to protect acipenseriform carcasses from damage in high energy aquatic environments predominantly preferred by sturgeons. In any case, based on known specimens, sediments within hadrosaurs appear to have been the main preservational medium for articulated fossil sturgeon skeletons in North America.†Psammorhynchus longipinnisappears to be the sister taxon to the rest of Acipenseridae based on our preliminary analysis here. We will reevaluate this relationship elsewhere in our (Hilton, Grande, and Bemis) ongoing study of fossil and living Acipenseridae.