Paleobiota of the Hell Creek Formation

Restesia[28] R. americana[28] Middle Hell Creek Formation[10] 5 specimens previously assigned to Squatirhina[11] A wobbegong-like shark.

Here is the most recent estimate of the proportions of the eight most common dinosaurian families in the Hell Creek Formation, based on detailed field studies by White, Fastovsky and Sheehan.

The top and bottom sections were the focus of the PLoS One report, and within each portion many remains of Triceratops, Edmontosaurus, and Tyrannosaurus were found.

HC-530 and HC-312) in the Apex sandstone and Jen-rex sand where individual bones were counted and (2) in two thirds of the formation reflected in L3 and U3 records of dinosaur skeletons only.

[34] A trackway made by mid-sized theropod, possibly a small tyrannosaurid individual, was discovered in South Dakota in 1997, and in 2014 these footprints were named Wakinyantanka styxi.

[40] Pachycephalosaur remains have been unearthed in Montana as in the case of Platytholus and the now invalid genus Stenotholus kohleri, which is now a junior synonym of Pachycephalosaurus.

[11] In 2012, a new unidentified species of chasmosaur ceratopsian with noticeable differences from Triceratops was unearthed in South Dakota by a fossil hunter named John Carter.

Because the teeth of different ceratopsians are so similar to one another, its hard to differentiate between genera/species, but based on the abundance of identifiable bones belonging to Triceratops in Lancian-aged North American formations, especially the Hell Creek, isolated ceratopsid teeth from the lower and middle Hell Creek Formation have a high likelihood of originating from T. horridus.

[citation needed] Because the teeth of different ceratopsians are so similar to one another, its hard to differentiate between genera/species, but based on the abundance of identifiable bones belonging to Triceratops in Lancian-aged North American formations, especially the Hell Creek, isolated ceratopsid teeth from the upper 1/3 of the Hell Creek Formation have a high likelihood of originating from T. prorsus.

[39] A trackway from South Dakota, named Wakinyantanka, was made by a mid-sized theropod with three slender toes, possibly a small tyrannosaurid.

[35] A second footprint that may have been made by a specimen of Tyrannosaurus was first reported in 2007 by British paleontologist Phil Manning, from the Hell Creek Formation of Montana.

Oviraptorosaur fossils have been found at the Hell Creek Formation for many years, most notably from isolated elements until the discovery of Anzu.

Historically, numerous teeth have been attributed to various dromaeosaurid and troodontid taxa with known body fossils from only older formations, including Saurornithoides, Zapsalis, Dromaeosaurus, Saurornitholestes, and Troodon.

However, in a 2013 study, Evans et al. concluded that there is little evidence for more than a single dromaeosaurid taxon, Acheroraptor, in the Hell Creek-Lance assemblages, which would render these taxa invalid for this formation.

[87] Fossilized teeth of various troodontids and coelurosaurs are common throughout the Hell Creek Formation; the best known examples include Paronychodon, Pectinodon and Richardoestesia, respectively.

Teeth previously referred to various Campanian dromaeosaurids Saurornitholestes and Dromaeosaurus, frequently found throughout the formation, probably belong to this one species.

Quetzalcoatlus[103] Indeterminate Borealosuchus[104] Extinct genus of crocodylians that lived from the Late Cretaceous to the Eocene in North America.

Altacreodus A. magnus a possible creodont, formerly a species of Cimolestes[122] Alostera[116] A. saskatchewanensis[116] A eutherian of uncertain phylogenetic placement.

Scollardius S. propalaeoryctes A eutherian of uncertain classification, formally a species of Cimolestes The Hell Creek Formation was a low floodplain at the time before the sea retreated, and in the wet ground of the dense woodland, the diversity of angiosperms and conifers were present.

The evidence of the forested environment is overwhelmingly supported by petrified wood, rooted gley paleosols,[123] and ubiquitous tree leaves.

The Hell Creek Formation of this layer contains over 300 tablets, of which angiosperms are by far the most diverse and dominant flora of the entire population, about 90 percent, followed by about 5% of conifers, 4% of ferns, and others.

In sharp contrast to the Great Plains today, the presence of many thermophilous taxa such as palm trees and gingers meant the climate was warmer and wetter then.

The plants of the Hell Creek Formation generally represent angiosperm-dominated riparian forests of variable diversity, depending on stratigraphic position and sedimentary environment.

For this reason, Kirk Johnson and Leo Hickey divided it into five zones and described them as HCIa, HCIb, HCIIa, HCIIb, and HCIII as a reflection of floral change through time.

[124] For example, the HCIa zone is dominated by "Dryophyllum" subfalcatum, Leepierceia preartocarpoides, "Vitis" stantonii, and "Celastrus" taurenensis, and is located 55 to 105 meters below the K-Pg boundary layer.

Although the HCIb zone is a very thin layer, about 5 meters of rock, it bears unusually high diversity of herbaceous and shrubby plants, including Urticaceae, Ranunculaceae, Rosaceae, and Cannabaceae.

Among the localities, the Mud Buttes, located in Bowman County, North Dakota, is probably the richest megaflora assemblage known and the most diverse leaf quarry from the Hell Creek Formation.

[125] C. lobata C. puilasokensis Cobbania C. corrugata A prehistoric species of water lettuce, previously assigned to the genus Pistia.

L. wardiana M. pulchra Occurs in southern the Wyoming flora, Leo Hickey claims it is found further north in Montana and the Dakotas.

Spinifructus S. antiquus A fruit seed that may belong to the palm family (Arecaceae), closely related to the genus Astrocaryum.