Paleobiota of the Hell Creek Formation
This is an overview of the fossil flora and fauna of the Maastrichtian-Danian Hell Creek Formation. Invertebrates. Insects like "Diptera", "Zygopteran", and possibly "hemiphlebiid damselflies" have been unearthed in Hell Creek in amber. Fossils found in the Hell Creek Formation and the Fort Union Formation of these insects went extinct during the K-T Event. Dinosaurs. A paleo-population study is one of the most difficult of analyses to conduct in field paleontology. 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 (1998). Outcrops sampled by the Hell Creek Project were divided into three sections: lower, middle and upper slices. 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. "Triceratops" was the most common in each section, but, surprisingly, "Tyrannosaurus" was just as common, if not slightly more common, than the hadrosaur "Edmontosaurus". In the upper Hell Creek section, for example, the census included twenty two "Triceratops", five "Tyrannosaurus", and five "Edmontosaurus". The dinosaurs  Thescelosaurus, "Ornithomimus", "Pachycephalosaurus" and "Ankylosaurus" were also included in the breakdown, but were relatively rare. Other dinosaurs, such as "Sphaerotholus", "Denversaurus", "Torosaurus", "Struthiomimus", "Acheroraptor", "Dakotaraptor", "Pectinodon," "Richardoestesia", "Paronychodon", "Anzu", "Leptorhynchos" and "Troodon (more likely Pectinodon)", were reported as being rare and are not included in the breakdown. The dinosaur collections made over the past decade during the Hell Creek Project yielded new information from an improved genus-level collecting schema and robust data set that revealed relative dinosaur abundances that were unexpected, and ontogenetic age classes previously considered rare. We recognize a much higher percentage of "Tyrannosaurus" than previous surveys . "Tyrannosaurus" equals "Edmontosaurus" in U3 and in L3 comprises a greater percentage of the large dinosaur fauna as the second-most abundant taxon after "Triceratops", followed by "Edmontosaurus". This is surprisingly consistent in (1) the two major lag deposits (MOR loc. 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. "Triceratops" is by far the most common dinosaur at 40% (n = 72), "Tyrannosaurus" is second at 24% (n = 44), "Edmontosaurus" is third at  20% (n = 36), followed by "Thescelosaurus" at 8% (n = 15), "Ornithomimus" at 5% (n = 9), and "Pachycephalosaurus" and "Ankylosaurus" both at 1% (n = 2) are relatively rare. Fossil footprints of dinosaurs from the Hell Creek Formation are very rare. As of 2017, there is only one find of a possible "Tyrannosaurus rex" footprint, dating from 2007 and described a year later. 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". Ornithischians. Pachycephalosaurs. An undescribed Pachycephalosaur is present  in North Dakota. Pachycephalosaur remains have been unearthed in Montana as in the case of the now invalid genus "Stenotholus kohleri", which is now a junior synonym of "Pachycephalosaurus". Ceratopsians. Indeterminate ceratopsid teeth and some identifiable bones from "Triceratops" can be extremely common. 8.31% of all vertebrate remains from the Hell Creek Formation are unassigned ceratopsids. 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. Ornithopods and relatives. Indeterminate hadrosaurid remains are very common in the Hell Creek Formation. Theropods. Theropod tracks have  been found in South Dakota. A trackway from South Dakota, named "Wakinyantanka", was made by a mid-sized theropod with three slender toes, possibly a small tyrannosaurid. 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. This second track measures long, shorter than the track described by Lockley and Hunt. Whether or not the track was made by "Tyrannosaurus" is unclear, though "Tyrannosaurus" is the only large theropod known to have existed in the Hell Creek Formation, though in  past albertosaurine remains have described here but its most likely that they are the remains of "Tyrannosaurus rex". Theropod remains are very common in Hell Creek, some of which belong to indeterminate species on maniraptorans. Ornithomimosaurs. Ornithomimid remains are not uncommon in the Hell Creek Formation. Fifteen specimens from the Hell Creek Formation are undetermined ornithomimids Oviraptorosaurs. Oviraptorosaur fossils have been found at the Hell Creek Formation for many years, most notably from isolated elements until the discovery of "Anzu". In the past, oviraptorosaur fossils found were thought to have belonged to "Caenagnathus", "Chirostenotes", and "Elmisaurus". In 2016, an undescribed  large-bodied caenagnathid was unearthed in Montana. Eumaniraptorans. 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. This was disproved in a 2015 study, DePalma "et al.", when they described the new genus "Dakotaraptor", a large species of dromaeosaur. 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 belonging to possible intermediate species of "Dromaeosaurus" and "Saurornitholestes" have been unearthed at the Hell Creek Formation and the nearby Lance Formation. Pterosaurs. Indeterminate azhdarchid remains, most likely belong to "Quetzalcoatlus" or an unidentified genus, have been found in Hell Creek as well as the Javelina Formation and the Ojo Alamo Formation. Indeterminate pteranodontid remains have also been found here as well. Squamata. Indeterminate mosasaur remains have been unearthed in North Dakota. Flora. The Hell Creek Formation was a low floodplain  at the time before the sea retreated, and in the wet ground of the dense woodland, laurels, sycamores, beech, magnolias, and palm trees grew. Ferns and moss grew in the forest understory. Plant fossils from the upper early Paleocene of the Hell Creek Formation include the ferns "Botrychium", "Woodwardia", "Osmunda", "Onoclea" and "Azolla"; conifers "Metasequoia", "Glyptostrobus" and cupressaceous conifers; the monocot "Limnobiophyllum" (a relative of duckweeds); and the dicots "Cercidiphyllum" and "Platanus". There are numerous types of leaves, seeds, flowers and other structures from Angiosperms, or flowering plants. The Hell Creek Formation of this layer contains 300 tablets or more  of plants. Angiosperms are by far the most diverse and dominant flora of the entire population, about 90 percent. However, the evergreens included conifers, ginkgo, bald cypress, and cycads. Flowering plants included a variety of trees that no longer exist. Today Hell Creek's flora is hardwood forest mixed with deciduous and evergreen forest and apparently similar to then, but with a closer look, the current plant community is distinct. In sharp contrast to Montana today, the presence of palm trees meant the climate was warmer then. Dr. Kirk Johnson claims that there are no grasses, oaks, maples, or willows  in the Hell Creek Formation. Ferns are uncommon in the majority of the formation, however there is a great increase in the abundance of fossil fern spores in the two centimeters of rock that directly overlies the impact fallout layer (the famous K-T boundary layer). This increase in fern spore abundance is commonly referred to as "the fern spike" (meaning that if the abundance of spores as a function of stratigraphic position were plotted out, the graph would show a spike just above the impact fallout layer). Johnson also found that the majority of the angiosperm genera in the  Hell Creek Formation are now extinct. He also believes that, very roughly, 80% of the terrestrial plant taxa died out in what is now Montana and the Dakotas at the K/T boundary. Many of the modern plant affinities in the Hell Creek Formation (e.g., those with the prefix "aff." or with quotes around the genus name) may not in reality belong to these genera; instead they could be entirely different plants that resemble modern genera. Therefore, there is some question regarding whether the modern "Populus" or "Juglans", as two examples, actually lived in the late Cretaceous. Compared to the  rich Hell Creek Formation fossil plant localities of the Dakotas, relatively few plant specimens have been collected from Montana. A few taxa were collected at Brownie Butte Montana by Shoemaker, but most plants were collected from North Dakota (Slope County) and from South Dakota. "TYPE" after the binomial means that it is represented by a type specimen found in the Yale-Peabody Museum collections. "YPM" is the prefix for the Yale-Peabody Museum specimen number. Overview (from Johnson, 1997): 190 plant morphotypes, including: Gymnosperms Ginkgos Angiosperms