Hell Creek Formation

Hell Creek Formation
Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Approximate paleocoordinates		52°36′N 74°24′W / 52.6°N 74.4°W / 52.6; -74.4
Region	Montana, North Dakota, South Dakota, Wyoming
Country	United States
Extent	Williston Basin
Type section
Named for	Hell Creek, Jordan, Montana
Named by	Barnum Brown
Year defined	1907
Hell Creek Formation (the United States) Show map of the United States Hell Creek Formation (Montana) Show map of Montana

The Hell Creek Formation is an intensively studied division of mostly

In 1966, the Hell Creek Fossil Area was designated as a National Natural Landmark by the National Park Service.^[5]

Geology

The Hell Creek Formation in Montana overlies the

The K-Pg boundary is generally situated near the contact between the upper Hell Creek and the lower Ludlow member of the Fort Union Formation, though in some areas (e.g. in North Dakota) the boundary is well within the Ludlow Member, 3 metres (9.8 ft) above the boundary with the Hell Creek in some areas.^[8] On the other hand, in some small regions of Montana, the Hell Creek Formation contains the K-Pg boundary, and extends slightly into the Paleogene.^[9]

The

The remains of many animals including

It is a series of fresh and brackish-water

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 Horner, Goodwin, and Myhrvold (2011)^[18]

• Ceratopsidae 40%
• Tyrannosauridae 24%
• Hadrosauridae 20%
• Hypsilophodontidae 8%
• Ornithomimidae 5%
• Ankylosauridae 1%
• Pachycephalosauridae
  1%
• Troodontidae 1% (represented only by teeth)
• Dromaeosauridae 1% (represented only by teeth)

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 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

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.[19] The largest Triceratops skull ever discovered, nicknamed 'Dragon King', was found in Glendive, Montana, which is in the Hell Creek Formation.^[20][21]

Eumaniraptorans

Historically, numerous teeth have been attributed to various

The Hell Creek Formation was a low

The plants of the Hell Creek Formation generally represent angiosperm-dominated

There is evidence of transitional floras in the middle of the Hell Creek Formation as shown by HCII and HCIII zones. The HCII flora represents a transitional period where taxa from the lower Hell Creek are replaced by the HCIII flora. The diversity of the HCIII zone is very high, and its composition is more uniform than that of HCII, many of which were rare or absent from the zones below, and some others that used to be common below became rarer in the HCIII zone. These forms include Elatides longifolia, "Dryophyllum" tennessensis, Liriodendrites bradacii, and many members of the Laurales including Bisonia niemii, "Ficus" planicostata, and Marmarthia trivialis, while "Celastrus" taurenensis, Leepierceia preartocarpoides, and many cupressaceous conifers became rarer. This phenomenon suggests that the global temperature was warming during the last 300,000-500,000 years of the Cretaceous period.^{[25][26][27][28]}

Johnson claims that there are no grasses, oaks, maples, beeches, figs, or willows in the Hell Creek Formation. There is no evidence of fern prairie either.^[29] However, there was an extremely high angiosperm diversity — common plane trees, "Dryophyllum" subfalcatum, Leepierceia preartocarpoides, and palm trees — along with extinct cycadeoid Nilssoniocladus, Ginkgo, araucariaceous, podocarpaceous, and cupressaceous conifers. This represents the mixed deciduous and evergreen broad-leaved forest as the Hell Creek landscape. The nature of these forests is uncertain because Johnson found that the majority of the angiosperm and conifer genera are now extinct. He also believes that, very roughly 80% of the terrestrial plant taxa died out in what is now Great Plains at the K-Pg boundary. On other hand, 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-Pg boundary layer). This increase in fern spore abundance is commonly referred 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).

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 Ficus or Juglans, as two examples, actually lived in the Late Cretaceous.

Compared to the rich Hell Creek Formation fossil plant localities of the

• 
  Ferns
• 
  Cycads
• 
  Ginkgo (uncommon)
• 
  Various

  angiosperms

• 
  Monkey-puzzle leaves
• 
  Palm trees

  indicate a hotter paleoclimate
• 
  Redwood

  seed cones
• 
  Laurus
• 
  Cercidiphyllum
• 
  Magnolia (common)

Plants of the Hell Creek Formation

• various
  ferns and cycadeoids
• Equisetum (Equisetaceae)

Gymnosperms

Ginkgos

• Ginkgo adiantoides