AN EXQUISITELY PRESERVED SKELETON REPRESENTING A PRIMITIVE STURGEON FROM THE UPPER CRETACEOUS JUDITH RIVER FORMATION OF MONTANA (ACIPENSERIFORMES: ACIPENSERIDAE: N. GEN. AND SP.)

Journal of Paleontology, Jul 2006 by Grande, Lance, Hilton, Eric J

ABSTRACT-

A new species of sturgeon (Acipenseridae). [dagger]Psammorhynchus longipinnis n. 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." [dagger]Psammorhynchus longipinnis is 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 [dagger]Protoscaphirhynchus squamosus Wilimovsky 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 ([dagger]Psammorhynchus longipinnis n. gen. and sp.; [dagger]Protoscaphirhynchus squamosus Wilimovsky; and a third, yet undescribed specimen) were apparently each independently discovered in the abdominal regions of hadrosaurian dinosaurs (as was the Cretaceous paddlefish [dagger]Paleopsephurus wilsoni MacAlpin). 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.

[dagger]Psammorhynchus longipinnis appears 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.

INTRODUCTION

"IN THE belly of the beast".-In 2003. J. Horner (Museum of the Rockies, Bozeman) sent the tirst author a roughly prepared fish that he and his field crew discovered in the abdominal cavity of a hadrosaur from the Upper Cretaceous Judith River Formation of Montana. It was preliminarily identified as a "gar" (Ancell et al.. 1998). After seeing a photograph of the specimen, we recognized it as a sturgeon, and asked permission to borrow the specimen for further preparation and description. Horner kindly sent us the specimen and allowed us to do so. This was serendipitous, as we had just initiated a project on the comparative osteology and systematics of sturgeons. After many months of additional line preparation of the specimen, it revealed ama/ingly detailed information on the skeletal anatomy of this Cretaceous species. The head region was only slightly dorsoventrally compressed due to the fossili/.ation process. Because of the relatively three-dimensional preservation of the skull and the fact that it was preserved in a soft, loosely consolidated sandstone, we were able to "dissect" its ventral surface once we had photographed and drawn the external bones in situ. Thus, even though there is only a single specimen of this species, we now know its osteology in relatively complete detail (at least for the head region). This is an extremely important specimen, in part because it is the first well-preserved, finely prepared articulated sturgeon fossil ever described. Also, it is one of the earliest known sturgeon fossils, giving us some insight into the early morphology, evolution, and biodiversity of the family Acipenseridae. The specimen shows a number of interesting morphological characters not known to occur in living sturgeons.

Today, the family Acipenseridae inhabits only temperate waters of the Northern Hemisphere. There are about 25 extant species in four genera, and they are either anadromous (marine forms that spawn in fresh water) or freshwater (see Bemis et al.. 1997 for a review of the family).

Fossil sturgeon species are also only known from the Northern Hemisphere (see Table 1). The fossil record of sturgeons (Acipenseridae) is notoriously poor. Although fossils are known as old as Upper Cretaceous, and are known from North America. Europe, and Asia, the fossils consist almost exclusively of isolated scutes or other isolated hones (e.g.. Table 1). Previously, the only described articulated skeleton of a fossil sturgeon was the Upper Cretaceous [dagger]Protoscaphirhvnchus squamosus Wilimovsky. 1956. which consists of a poorly preserved, badly prepared partial specimen showing very little skull anatomy (see Hilton and Grande. 2006). The specimen we describe here is the first articulated sturgeon fossil described with a clearly preserved, relatively complete skeleton. The scarcity of well-preserved articulated fossil sturgeon material is remarkable. Even the enormously productive and diverse lacustrine localities of the Eocene Green River Formation, which have produced well over one million complete articulated skeletons from a number of different freshwater paleoenvironments (Grande. 2001). have yet to produce a single sturgeon skeleton. The broad representation of sturgeon material in the fossil record (Cretaceous through Pleistocene: North America. Europe, Asia), combined with the consistent highly fragmentary mode of preservation, is probably a reflection of a long historical preference of higher energy environments for most species of the family (e.g., river or nearshore marine habitats). The uniquely well-preserved specimen described here represents an extremely rare window into the early history of Acipenseridae, thanks to a rare set of taphonomic circumstances.