Denversaurus

''Denversaurus'' (meaning "Denver lizard") is a genus of panoplosaurin nodosaurid dinosaur from the late Maastrichtian of Late Cretaceous Western North America. Although at one point treated as a junior synonym of '' Edmontonia'' by some taxonomists, current research indicates that it is its own distinct nodosaurid genus.

Discovery and naming

In 1986, the paleontologists Kenneth Carpenter and Brent Breithaupt described DMNH 468 which was a specimen of a late Maastrichtian nodosaurid, tentatively assigned to '' Edmontonia'' sp., discovered from the lower Hell Creek Formation of South Dakota.

In 1988, Robert Thomas Bakker decided to split the genus ''Edmontonia''. The species ''Edmontonia rugosidens'' was made into a separate genus named ''Chassternbergia'' and DMNH 468 was designated as a holotype of a new genus and species. The type species of this genus was ''Denversaurus schlessmani''. The generic name referred to the Denver Museum of Natural History at Denver, Colorado. The specific name honoured Lee E. Schlessman, a major benefactor of the museum and the founder of the Schlessman Family Foundation.Bakker, R.T. (1988). "Review of the Late Cretaceous nodosauroid Dinosauria: ''Denversaurus schlessmani'', a new armor-plated dinosaur from the Latest Cretaceous of South Dakota, the last survivor of the nodosaurians, with comments on Stegosaur-Nodosaur relationships". ''Hunteria'' 1(3): 1-23.(1988).

The holotype consists of a skull without the lower jaws and a number of osteoderms of the body armour. It is part of the collection of the Denver Museum of Nature and Science after which the genus was named. Bakker referred a second fossil to the species, specimen AMNH 3076, a skull found by Brown and American Museum of Natural History paleontologist Roland T. Bird at the Tornillo Creek in Brewster County, Texas, in a layer of the poorly dated Upper Cretaceous Aguja Formation, possibly from the Maastrichtian too. However, subsequent studies reassigned AMNH 3076 to '' Edmontonia'', probably ''E. longicpes''.

The validity of ''Denversaurus'' was disputed in a 1990 paper on ankylosaurian systematics by Kenneth Carpenter, who noted that Bakker's diagnosis of ''Denversaurus'' was based primarily on Bakker's artistic restoration of the holotype in an uncrushed state. Since DMNH 468 was found crushed, Carpenter assigned ''Denversaurus'' back to ''Edmontonia'' sp., even though he noted its similarity to ''Edmontonia rugosidens''.Carpenter, K. 1990. "Ankylosaur systematics: example using Panoplosaurus and Edmontonia (Ankylosauria: Nodosauridae)", In: Carpenter, K. & Currie, P.J. (eds) Dinosaur Systematics: Approaches and Perspectives, Cambridge University Press, Cambridge, pp. 281-298 A number of workers treated ''Denversaurus'' as synonymous with either ''E. rugosidens'' or ''E. longiceps'', or alternatively as a valid species of ''Edmontonia'': ''E. schlessmani''.Paul, G.S. (2010). The Princeton Field Guide to Dinosaurs, Princeton University Press.

In 1988, fossil hunters found a partial nodosaurid skeleton consisting of the fragmentary skull, parts of postcranial skeleton and seventy-five osteoderms from the Lance Formation of Niobrara County, Wyoming, nicknamed "Tank", which has been tentatively assigned to as ''Edmontonia'' sp. in 1994. It was part of the collection of the Rocky Mountain Dinosaur Resource Center under inventory number BHI 127327, referred to as ''Edmontonia schlessmani'' by Carpenter et al. (2013), and is now under the inventory number FPDM-V9673.

In the 2015 thesis, Michael Burns revisited the systematics of latest Cretaceous nodosaurids from the Western Interior. According to Burns, ''Denversaurus'' is likely a valid taxon based on its phylogenetic position. Recent phylogenetic analyses included ''Denversaurus'' as a valid genus closely related to ''Edmontonia''.

Description

In 2010, American paleontologist Gregory S. Paul estimated the length of ''Denversaurus'' at and its body mass at .

Robert T. Bakker considered ''Denversaurus'' distinct from ''Edmontonia'' and ''Chassternbergia'' in having a skull that was wide at the rear and a more rearward position of the eye sockets. The holotype skull has a length of 496 millimetres and a rear width of 346 millimetres. In the referred specimen AMNH 3076, these proportions are less extreme, measuring 395 millimetres long with a rear width of 220 millimetres. According to American paleontologist Kenneth Carpenter, the greater width of both the holotype and the referred specimen was due to crushing.

In 2015, vertebrate anatomist and paleontologist Michael Burns published an abstract concluding that ''Denversaurus'' was different from ''Edmontonia'', but similar to '' Panoplosaurus'' in having inflated, convex, cranial sculpturing with visible sulci, or troughs, between individual top skull armour elements, but is distinct from ''Panoplosaurus'' in having a relatively wider snout.

Classification

In 1988, Bakker placed ''Denversaurus'' within Edmontoniidae, the presumed sister group of Nodosauridae within Nodosauroidea that would not have been Ankylosauria, but the last surviving Stegosauria. However, these hypotheses have not been confirmed by modern cladistic analysis. Whether it presents a separate species or is identical to ''E. rugosidens'' or ''E. longiceps'', ''Denversaurus'' material is considered nodosaurid and ankylosaurian. Paul suggested that it was the direct descendant of ''E. longiceps''. Burns recovered ''Denversaurus'' as the sister species of ''Panoplosaurus''. ''Denversaurus'' is the latest known member of Thyreophora.

References

{{Taxonbar, from=Q21012043

Nodosauridae
Dinosaur genera
Maastrichtian dinosaurs
Hell Creek Formation
Lance Formation
Fossil taxa described in 1988
Taxa named by Robert T. Bakker
Dinosaurs of the United States