The holotype pes of the Morrison dinosaur Camptonotus amplus Marsh, 1879 (Upper Jurassic, western USA) – is it Camptosaurus, Sauropoda or Allosaurus?
Galton, Peter M.; Carpenter, Kenneth; Dalman, Sebastian G.
published: Mar 1, 2015
The holotype of Camptonotus amplus Marsh, 1879, the right pes YPM VP.1879 from the Morrison Formation (Upper Jurassic) of Wyoming, USA, has long been referred to the Morrison ornithopod dinosaur Camptosaurus Marsh, 1885. However, the pointed proximal end of Mt I did not reach the tarsus, but was attached by ligaments to the mid-shaft of Mt II as in neotheropod dinosaurs, and a series of synapomorphic characters identify the pes to the Allosauroidea. The transversely compressed deep ungual phalanx, previously used to define the species amplus, is removed from YPM VP.1879 because it is the left pedal ungual of a juvenile neosauropod dinosaur. A partial skull with a few postcranial bones (YPM VP.1892) is probably part of the same individual and the postorbital and quadrate are very similar to those of Allosaurus. The articular condyle of Mt I of YPM VP.1879 is about twice the length of the rest of the non-tapering part, rather than being subequal as in the other allosaurians from the USA (Morrison allosaurids Allosaurus, Saurophaganax; Lower Cretaceous carcharodontosaurian Acrocanthosaurus). For distal Mt IV, the height is greater than the transverse width in Allosaurus, the plesiomorphic condition for Neotheropoda, whereas they are subequal in YPM VP.1879 and Saurophaganax. YPM VP.1879 is probably an allosaurid that is referred to as ?Allosaurus amplus (Marsh, 1879) pending further information on the range of variation in the pes of Allosaurus.