Tuesday 16 April 2019

Evidence of large Tyranosauroid Dinosaurs living on the East Coast of North America shortly before the End Cretaceous Extinction.

The End Cretaceous Extinction, 66 million years ago, saw the end of all non-Avian Dinosaurs, as well as a wide range of other animals, including the flying Pterosaurs and several Bird groups, a wide range of Marine Reptiles and the ubiquitous Mesozoic Ammonites. However it is unclear whether these extinctions all occurred simultaneously, due to the impact of a large extra-terrestrial body into the Yucatan Peninsula of Mexico, or whether the process was more gradual, with the Earth suffering a long period of declining biodiversity associated with declining environmental conditions associated with the emplacement of the Deccan Traps volcanic strata in India over several million years. In order to understand this, palaeontologists are very interested in the diversity of groups such as the Dinosaurs over the last few million years of the Cretaceous, however relatively few fossil locations preserve material from this time, with almost all End-Cretaceous Dinosaurs known coming from the Western Part of North America.

In a paper published in the journal The Mososaur (the journal of the Delaware Valley Paleontological Society) in December 2018, Chase Brownstein of the Department of Collections and Exhibitions at the Stamford Museum and Nature Center, described three bones from the foot of a large Tyranosauroid Dinosaur from the Latest Cretaceous Navesink Formation of Big Brook New Jersey, on the East Coast of North America.

The Navesink Formation comprises dark glauconitic sandstones (‘greensands’) with several distinct shell bed horizons. Close to the top of this formation is a fossil bed interpreted as having been deposited close to the End Cretaceous Extinction, which produces fossils from a range of taxa, with Shark teeth and invertebrates being the most common. The Navesink Formation overlies the slightly older Mount Laurel Formation unconformably, i.e. there is a gap between the two formations in which no deposition occurred and there may have been erosion, which has been interpreted as evidence of fluctuating sea levels close to the End of the Cretaceous, possibly caused by glaciation.

The three bones described are a pedal phalanxes II-1, II-2, and III-2, from a large Theropod Dinosaur. These may have come from the same individual; they are similar in colour and are appropriately sized, though only one bone (the large II-1 phalanx) was found in situ within the bed, the other two specimens having been recovered from a gravel bar at Ramanessin Brook.

Pedal phalanx II-1 in lateral (A), medial (B), dorsal (C), ventral (D), distal (E), and proximal (F) views. Scale bar is 50 mm. Brownstein (2018).

Based upon the morphology (shape) of the bones they are assessed to have come from a non-Tyranosaurid Tyranosauroid, rather than a Tyranosaurid Tyranosauroid such as the gigantic Tyrannosaurus rex of western North America, or Tarbosaurus bataar of Mongolia, both of which reached around nine metres in length, although the size of the bones suggests they may have come from an animal of similar size, possibly revealing the presence of a previously unknown lineage of large Tyranosauroids in eastern North America.

Comparison of Late Cretaceous Tyrannosauroid pedal anatomy. Reconstructed pes of Navesink Tyrannosaur in (A) lateral view. Reconstructed pes of generalized Tyrannosauroid in (B) lateral view. Brownstein (2018).

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