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The new specimen of Tyrannosaurus rex, ‘Peter’, was found in the Lance Formation of Wyoming approximately thirty miles southwest of the town of Newcastle in Niobrara County (Fig. 4, page 4 of the report). A partial ilium from an Edmontosaurus and portions of the frill from a Triceratops were also found in the same layer as this new T. rex. The fossils were found in mudstone which are flood plain sediments, such as an oxbow lake setting that formed along the bend of an abandoned river channel. Precise stratigraphic information can be found using pollen analysis of the mudstone. It is estimated that the specimen may have come from the middle to upper part of the Lance Formation..
The Lance Formation of Wyoming (USA) is a Late Cretaceous rock unit where the first Tyrannosaurus rex bones were found by Hatcher in 1890. Subsequently, the University of Kansas Field Expedition of 1895, with Barnum Brown as part of the field party (Brown et al. 2004), recovered a large T. rex phalanx (Sundell 2003) and a Triceratops skull both of which have been on exhibit at the University of Kansas since that time. A few years later, Brown returned to Wyoming, now working for the American Museum of Natural History, and found more T. rex bones. Osborn described this discovery first as Dynamosaurus until Brown later found more complete remains in 1902 in the laterally equivalent beds of the Hell Creek Formation in Montana (Osborn 1905, Breithaupt 1997, Nudds and Selden 2008). The Lance Formation was originally termed the “Ceratops Beds” based on the many specimens of Triceratops that were found in these rocks (March 1899, Knowlton 1909, 1911; Breithaupt 1997) by Hatcher (1896). The first dinosaur found in the Lance Formation was a Triceratops in 1872 and was described by Cope as Agathaumas sylvestris). Since that time, the formation has produced hundreds of skulls (Derstler 1994). This area is also famous for the mummified duck-billed dinosaurs (Edmontosaurus) specimens collected by the Sternberg family (Lull and Wright 1942). Although the Lance Formation in Wyoming is seemingly rich in dinosaur remains (Breithaupt 1997; Derstler and Myers 2008), Tyrannosaurus rex fossils are historically and statistically rare (Derstler 1994, Dalman 2013, Stein 2019).
The Lance Formation comprises sandstones, and mudstones from ancient rivers, peat bogs, and coastal floodplains that formed in the riverine environment of the Late Cretaceous (Maastrichtian) during the retreat of the inland seaway. It was formed during a warm and subtropical environment (Dorf 1942) and fluvial and overbank sediments were deposited onto a flatland topography. The area was bounded to the west by the emergent Rocky Mountains and to the east by the seaway. The northern extent of the formation is about 600-750 m thick while it thickens to the south to nearly 1,000 m (Clemens 1963, Connor 1992, Derstler 1994, Breithaupt 1997). The Lance Formation also preserves the K/ Pg boundary, and the age is estimated to be 66.8 ± 1.2 Ma (Trumbull 1913; Clemens 1960; Breithaupt 1997; Dalman 2013).