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80-613: The Hell Creek Formation is an intensively studied division of mostly Upper Cretaceous and some lower Paleocene rocks in North America, named for exposures studied along Hell Creek , near Jordan, Montana . The formation stretches over portions of Montana , North Dakota , South Dakota , and Wyoming . In Montana, the Hell Creek Formation overlies the Fox Hills Formation . The site of Pompeys Pillar National Monument

160-467: A stratigraphic range from the lower to the upper Hell Creek Formation. 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. For example, the HCIa zone is dominated by "Dryophyllum" subfalcatum , Leepierceia preartocarpoides , "Vitis" stantonii , and "Celastrus" taurenensis , and

240-501: 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

320-459: A bottom layer about 0.5m thick ("unit 1"), and a top layer about 0.8m thick (unit 2), capped by a 1 – 2 cm layer of impactite tonstein that is indistinguishable from other dual layered KPg impact ejection materials found in Hells Creek, and finally a layer around 6 cm thick of plant remains. The excavated pointbar and event deposits show that the point bar had been exposed to the air for

400-434: A considerable time, with evidence of habitation and filled burrows, before an abrupt, turbulent, high energy event filled these burrows and laid down the deposits. The event included waves with at least 10 meters run-up height (the vertical distance a wave travels after it reaches land). A BBC documentary on Tanis, titled Dinosaurs: The Final Day , with Sir David Attenborough , was broadcast on 15 April 2022. This program

480-464: A flat, forested floodplain with a relatively subtropical climate that supported a variety of plants ranging from angiosperm trees to gymnosperms such as the conifers , cycadeoids and ginkgos to ferns and moss . The Hell Creek Formation was laid down by streams, on a coastal plain along the edge of the Western Interior Seaway . The presence of crocodylians suggests climate

560-507: 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

640-507: A large inland extension to the Gulf of Mexico . The Hell Creek Formation was at this time very low-lying or partly submerged land at the northern end of the seaway, and the Chicxulub impact occurred in the shallow seas at the southern end, approximately 3,050 km (1,900 miles) from the site. Although Tanis and Chicxulub were connected by the remaining Interior Seaway, the massive water waves from

720-612: A layer of glass tektites with associated impact impressions—deposited minutes to hours after the impact. The remains of many animals including dinosaurs were found in the Hell Creek Formation. Its location at the changing conjunction of the eastern coast of Laramidia and the adjacent western shallows of the Western Interior Seaway led to the preservation of fossils of both marine and terrestrial creatures. Vertebrates include dinosaurs, pterosaurs , crocodiles , champsosaurs , lizards , snakes , turtles , frogs and salamanders . Remains of fishes and mammals have also been found in

800-631: A northward course towards Asia. In the Southern Hemisphere, Australia and Antarctica seem to have remained connected and began to drift away from Africa and South America. Europe was an island chain. Populating some of these islands were endemic dwarf dinosaur species. In the Late Cretaceous, the hadrosaurs , ankylosaurs , and ceratopsians experienced success in Asiamerica (Western North America and eastern Asia). Tyrannosaurs dominated

880-513: A possible Tyrannosaurus rex footprint, dating from 2007 and described a year later. The largest Triceratops skull ever discovered, nicknamed 'Dragon King', was found in Glendive, Montana, which is in the Hell Creek Formation. Historically, numerous teeth have been attributed to various Dromaeosaurid and Troodontid taxa with known body fossils from only older formations, including Dromaeosaurus , Saurornitholestes , and Troodon . However, in

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960-515: A single brief event. The exceptional nature of the findings and conclusions led some scientists to await further scrutiny before agreeing that the discoveries at Tanis had been correctly understood, further exacerbated by concerns over the reliability of data with researchers racing to claim credit for findings. The Cretaceous–Paleogene ("K-Pg" or "K-T") extinction event around 66 million years ago wiped out all non-avian dinosaurs and many other species. Proposed by Luis and Walter Alvarez , it

1040-499: 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

1120-561: A variety of small theropods , pachycephalosaurs , ankylosaurs , crocodylomorphs and squamates , including various animal fossils unearthed in the Hell Creek Formation. The most complete hadrosaurid dinosaur ever found, an Edmontosaurus , was retrieved in 2000 from the Hell Creek Formation and widely publicized in a National Geographic documentary aired in December 2007. A few bird , mammal, and pterosaur fossils have also been found. The teeth of sharks and rays are sometimes found in

1200-536: A very different dinosaurian fauna, with most predators being abelisaurids and carcharodontosaurids ; and titanosaurs being among the dominant herbivores. Spinosaurids were also present during this time. Birds became increasingly common, diversifying in a variety of enantiornithe and ornithurine forms. Early Neornithes such as Vegavis co-existed with forms as bizarre as Yungavolucris and Avisaurus . Though mostly small, marine Hesperornithes became relatively large and flightless, adapted to life in

1280-574: Is prairie , then Hell Creek's flora was hardwood forest mixed with deciduous and evergreen forest. 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 . There appears to be floral transitions visible on

1360-655: Is a paleontological site in southwestern North Dakota, United States. It is part of the heavily studied Hell Creek Formation , a geological region renowned for many significant fossil discoveries from the Upper Cretaceous and lower Paleocene . Uniquely, Tanis appears to record in detail, extensive evidence of the direct effects of the giant Chicxulub asteroid impact which struck the Gulf of Mexico 66.043 million years ago, and wiped out all non-avian dinosaurs and many other species (the so-called "K–Pg" or "K–T" extinction ). The extinction event caused by this impact began

1440-790: Is a small isolated section of the Hell Creek Formation. In 1966, the Hell Creek Fossil Area was designated as a National Natural Landmark by the National Park Service . It is a series of fresh and brackish-water clays , mudstones , and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous period and the beginning of the Paleogene ) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along

1520-475: Is believed to have triggered earthquakes estimated at magnitude 10 – 11.5 , releasing up to 4000 times the energy of the Tohoku quake. Co-author Mark Richards, a professor of earth sciences focusing on dynamic earth crust processes, suggests that the resulting seiche waves would have been approximately 10–100 m (33–328 ft) high in the Western Interior Seaway near Tanis and credibly, could have created

1600-411: 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

1680-765: Is considerable detail for times greater than hundreds of thousands of years either side of the event, and for certain kinds of change on either side of the K-Pg boundary layer. But relatively little fossil evidence is available from times nearer the crucial event, a difficulty known as the Signor–Lipps effect . The Hell Creek Formation is a well-known and much-studied fossil-bearing formation (geological region) of mostly Upper Cretaceous and some lower Paleocene rock that stretches across portions of Montana , North Dakota , South Dakota , and Wyoming in North America . The formation

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1760-444: Is different from any modern plant community. There are numerous types of leaves, seeds, flowers and other structures from Angiosperms , or flowering plants. 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. Compared to today Hell Creek's flora which

1840-491: Is evidenced in Hells Creek, the Tanis deposit does not appear to relate to any other known marine transgression (inland shoreline movement). Characteristics of the site include: The deposit itself is about 1.3m thick, sharply overlaying the point bar, in a drape-like manner. It comprises two layers with sand and silt grading (coarse sands at the bottom, finer silt/clay particles at the top). It can be divided into two layers,

1920-486: Is for Smithsonian National Museum of Natural History ; and so on. The majority of Hell Creek megafloral specimens are collected at the Denver Museum of Nature & Science . Overview (from Johnson, 2002) : 302 plant morphotypes based on leaf only, including : Gymnosperms Ginkgos Angiosperms Upper Cretaceous During the Late Cretaceous, the climate was warmer than present, although throughout

2000-474: Is it particularly evident that a true systematic decline was ever in place, especially with the discovery of smaller pterosaur species. Several old mammal groups began to disappear, with the last eutriconodonts occurring in the Campanian of North America . In the northern hemisphere, cimolodont , multituberculates , metatherians and eutherians were the dominant mammals, with the former two groups being

2080-511: 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 . 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

2160-488: Is named for early studies at Hell Creek, located near Jordan, Montana , and it was designated as a National Natural Landmark in 1966. The formation contains a series of fresh and brackish-water clays , mudstones , and sandstones deposited during the Maastrichtian and Danian (respectively, the end of the Cretaceous and the beginning of the Paleogene periods) by fluvial activity in fluctuating river channels and deltas and very occasional peaty swamp deposits along

2240-522: Is now widely accepted that the extinction was caused by a huge asteroid or bolide that impacted Earth in the shallow seas of the Gulf of Mexico , leaving behind the Chicxulub crater . The impactor tore through the Earth's crust , creating huge earthquakes , giant waves , and a crater 180 kilometers (112 mi) wide, and blasted aloft trillions of tons of dust, debris, and climate-changing sulfates from

2320-448: Is rare in any case for animals and plants to be fossilized, the fossil record leaves some major questions unanswered. One of these is whether dinosaurs were already declining at the time of the event due to ongoing volcanic climate change . Also, there is little evidence on the detailed effects of the event on Earth and its biosphere . No fossil beds were yet known that could clearly show the details that might resolve these questions. There

2400-455: The 10 – 11 m (33 – 36 feet) high water movements evidenced inland at the site; the time taken by the seismic waves to reach the region and cause earthquakes almost exactly matched the flight time of the microtektites found at the site. This would resolve conflicting evidence that huge water movements had occurred in the Hell Creek region near Tanis much less than an hour after impact, although

2480-557: The 1950 Assam-Tibet earthquake ( India / China ) (seiches in England and Norway ), the 2010 Chile earthquake (seiches in Louisiana ). Notably, the powerful magnitude 9.0 – 9.1 Tōhoku earthquake in 2011, slower secondary waves traveled over 8,000 km (5,000 miles) in less than 30 minutes to cause seiches around 1.5–1.8 m (4.9–5.9 ft) high in Norway . The Chicxulub impact

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2560-450: The Cenozoic , in which mammals —including humans—eventually came to dominate life on Earth. The site was originally discovered in 2008 by University of North Georgia Professor Steve Nicklas and field paleontologist Rob Sula. Their team successfully removed fossil field jackets that contained articulated sturgeons , paddlefish , and bowfins . These fossils were delivered for research to

2640-453: The Deccan Traps , both of which have been firmly dated to the time of the extinction event. In theory, these events reduced sunlight and hindered photosynthesis , leading to a massive disruption in Earth's ecology . A much smaller number of researchers believe the extinction was more gradual, resulting from slower changes in sea level or climate . Tanis (fossil site) Tanis

2720-657: The Field Museum of Natural History in Chicago. Recognizing the unique nature of the site, Nicklas and Sula brought in Robert DePalma, a University of Kansas graduate student, to perform additional excavations. The site was systematically excavated by Robert DePalma over several years beginning in 2012, working in near-total secrecy. Key findings were presented in two conference papers in October 2017. The full paper introducing Tanis

2800-499: 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 . Johnson claims that there are no grasses , oaks , maples , beeches , figs , or willows in

2880-491: The Maastrichtian and Danian (respectively, the end of the Cretaceous period and the beginning of the Paleogene ) by fluvial activity in fluctuating channels and deltas and very occasional peaty swamp deposits along the low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway . The Hell Creek Formation, as typified by exposures in the Fort Peck area of Montana, has been interpreted as

2960-482: The University of North Dakota ; and the University of North Carolina which began in 1998. The Hell Creek Formation is an intensively studied geological formation of mostly Upper Cretaceous and some Early Paleocene rocks in North America, named for exposures studied along Hell Creek, near Jordan, Montana . The formation stretches over portions of Montana , North Dakota , South Dakota , and Wyoming . In Montana,

3040-513: The gypsum seabed, and it may have created firestorms worldwide. With the exception of some ectothermic species such as the ancestors of the modern leatherback sea turtle and crocodiles , no tetrapods weighing more than 25 kg (55 lb) survived. It marked the end of the Cretaceous period and the Mesozoic Era, opening the Cenozoic Era that continues today. However, because it

3120-497: The primary waves travelling through rock at about 5 km/s (11,000 mph), probably reached Hell Creek within six minutes, and quickly caused massive water surges known as seiches in the shallow waters close to Tanis. Seiche waves often occur shortly after significant earthquakes, even thousands of miles away, and can be sudden and violent. Some recent examples include the 1964 Alaskan earthquake (seiches in Puerto Rico ),

3200-654: The Cretaceous Period derived from the German name Kreidezeit , and T is the abbreviation for the Tertiary Period (a historical term for the period of time now covered by the Paleogene and Neogene periods). The event marks the end of the Mesozoic Era and the beginning of the Cenozoic Era. "Tertiary" being no longer recognized as a formal time or rock unit by the International Commission on Stratigraphy ,

3280-637: The Cretaceous period and has been dated to 66 ± 0.07 Ma old. The 90-metre (300 ft) thickness of the formation is estimated to have been deposited in about 2 million years. Lancian fauna characteristic of Hell Creek are found as high as a few meters below the boundary. 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)

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3360-519: The Hell Creek Formation overlies the Fox Hills Formation . In 1966, the Hell Creek Fossil Area was designated as a National Natural Landmark by the National Park Service . The Hell Creek Formation in Montana overlies the Fox Hills Formation and underlies the Fort Union Formation , and the boundary with the latter occurs near the Cretaceous–Paleogene boundary (K–Pg), which defines the end of

3440-437: The Hell Creek Formation. The formation has produced impressive assemblages of invertebrates (including ammonites ), plants , mammals, fish, reptiles (including the lizard Obamadon ), marine reptiles (including the marine reptiles like mosasaurs , plesiosaurs and sea turtles ), and amphibians . Notable dinosaur finds include Tyrannosaurus and Triceratops , ornithomimids as well, caenagnathids like Anzu ,

3520-439: The Hell Creek Formation. There is no evidence of fern prairie either. 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

3600-403: The Hell Creek Formation; the best known examples include Paronychodon , Pectinodon and Richardoestesia , respectively. 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. A great diversity of herbaceous flowering plants , ferns and moss grew in

3680-436: 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 the other hand, there is a great increase in the abundance of fossil fern spores in the two centimeters of rock that directly overlies

3760-847: The K-T event is now called the Cretaceous—Paleogene (or K-Pg) extinction event by many researchers. Non- avian dinosaur fossils are found only below the Cretaceous–Paleogene boundary and became extinct immediately before or during the event. A very small number of dinosaur fossils have been found above the Cretaceous–Paleogene boundary, but they have been explained as reworked fossils , that is, fossils that have been eroded from their original locations then preserved in later sedimentary layers. Mosasaurs , plesiosaurs , pterosaurs and many species of plants and invertebrates also became extinct. Mammalian and bird clades passed through

3840-406: The analyses, insufficient methods for accurate replication and problematic isotopic graphs with irregular data and error bars. The hundreds of fish remains are distributed by size, and generally show evidence of tetany (a body posture related to suffocation in fish), suggesting the suffocation of an entire population. Fragile remains spanning the layers of debris show the site was laid down in

3920-568: The area was largely forested by small- to medium-sized trees. 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) Outcrops sampled by the Hell Creek Project were divided into three sections: lower, middle and upper slices. The top and bottom sections were

4000-608: The boundary is well within the Ludlow member, 3 metres (9.8 ft) above the boundary with the Hell Creek. 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. The Tanis site in North Dakota contains possible evidence of the Chicxulub meteorite impact—such as the chaotic mixing of fossil carcasses and

4080-468: The boundary with few extinctions, and evolutionary radiation from those Maastrichtian clades occurred well past the boundary. Rates of extinction and radiation varied across different clades of organisms. Many scientists hypothesize that the Cretaceous–Paleogene extinctions were caused by catastrophic events such as the massive asteroid impact that caused the Chicxulub crater , in combination with increased volcanic activity , such as that recorded in

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4160-442: The breakdown, but were relatively rare. Other dinosaurs, such as Sphaerotholus , Denversaurus , Torosaurus , Struthiomimus , Acheroraptor , Dakotaraptor , Pectinodon , a possible Parasaurolophus walkeri , 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

4240-435: The direct effects of the impact on plants and animals alive at the time of the large impact some 3,000 km (1,900 mi) distant. The events at Tanis occurred too soon after impact to be caused by the megatsunamis expected from any large impact near large bodies of water. Instead, much faster seismic waves from the magnitude 10 – 11.5 earthquakes probably reached the Hell Creek area as soon as ten minutes after

4320-418: The first megatsunamis from the impact zone could not have arrived at the site for almost a full day. Site details are as follows: The site was originally a point bar —a gently sloped crescent-shaped area of deposit that accumulates on the inside bend of streams and rivers below the slip-off slope . Point bars are common in mature or meandering streams. Both the site and the river are called Tanis. From

4400-610: 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 Thescelosaurus , Ornithomimus , Pachycephalosaurus and Ankylosaurus were also included in

4480-456: The forest understory. On the exposed point bars of large river systems , there were shrubs and vines . The evidence of the forested environment is overwhelmingly supported by petrified wood , rooted gley paleosols , and ubiquitous tree leaves. The presence of the simple and lobed leaves, combined with an extremely high dicot diversity, extinct cycadeoid Nilssoniocladus , Ginkgo , many types of monocots , and several types of conifers

4560-567: The formation, near its boundary with the overlying Fort Union Formation . The world's largest collection of Hell Creek fossils is housed and exhibited at the Museum of the Rockies in Bozeman, Montana . The specimens displayed are the result of the museum's Hell Creek Project, a joint effort between the museum; Montana State University ; the University of Washington ; the University of California, Berkeley ;

4640-703: 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 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 . Among

4720-462: The impact area were probably not responsible for the deposits at Tanis. Any water-borne waves would have arrived between 18 and 26 hours later, long after the microtektites had already fallen back to earth, and far too late to leave the geological record found at the site. It is not even clear whether the massive waves were able to traverse the entire Interior Seaway. Instead, the initial papers on Tanis conclude that much faster earthquake waves ,

4800-402: The impact fallout 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

4880-532: The impact were arriving on ballistic trajectories from their source in what is now the Yucatán Peninsula . Reported findings include: Analysis of the fish skeletons found them to be in the spring phase of their annual cyclical changes, implying that the impact had occurred in spring. Depalma et al. (2021) opted for a spring-summer range, but During et al. (2024) reevaluated and criticized this study based on its lack of primary data, unidentified laboratory for

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4960-464: The impact, creating seiche waves between 10–100 m (33–328 ft) high in the Western Interior Seaway . The site formed part of a bend in an ancient river on the westward shore of the seaway, and was flooded with great force by these waves, which carried sea, land, freshwater animals and plants, and other debris several miles inland. The seiche waves exposed and covered the site twice, as millions of tiny microtektite droplets and debris from

5040-590: The large predator niche in North America. They were also present in Asia, although were usually smaller and more primitive than the North American varieties. Pachycephalosaurs were also present in both North America and Asia. Dromaeosaurids shared the same geographical distribution, and are well documented in both Mongolia and Western North America. Additionally therizinosaurs (known previously as segnosaurs) appear to have been in North America and Asia. Gondwana held

5120-614: 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. "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; "DMNH" is for the Denver Museum of Nature & Science ; "USNM"

5200-427: The low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway . The iridium -enriched Cretaceous–Paleogene boundary, which separates the Cretaceous from the Cenozoic , is distinctly visible as a discontinuous thin marker above and occasionally within the formation. Numerous famous fossils of plants and animals, including many types of dinosaur fossils, have been discovered there. At

5280-451: The low-lying eastern continental margin fronting the late Cretaceous Western Interior Seaway . The climate was mild; the presence of crocodilians along with palm trees suggests a subtropical and temperate climate with no prolonged annual cold. The famous iridium -enriched Cretaceous–Paleogene boundary, which separates the Cretaceous from the Cenozoic , occurs as a discontinuous but distinct thin marker bedding above and occasionally within

5360-636: The most common mammals in North America. In the southern hemisphere there was instead a more complex fauna of dryolestoids , gondwanatheres and other multituberculates and basal eutherians ; monotremes were presumably present, as was the last of the haramiyidans , Avashishta . Mammals, though generally small, ranged into a variety of ecological niches, from carnivores ( Deltatheroida ), to mollusc-eater ( Stagodontidae ), to herbivores (multituberculates, Schowalteria , Zhelestidae and Mesungulatidae ) to highly atypical cursorial forms ( Zalambdalestidae , Brandoniidae ). True placentals evolved only at

5440-531: The numerous teleost fishes, which in turn evolved into new advanced and modern forms ( Neoteleostei ). Ichthyosaurs and pliosaurs , on the other hand, became extinct during the Cenomanian-Turonian anoxic event . Near the end of the Cretaceous Period, flowering plants diversified. In temperate regions, familiar plants like magnolias , sassafras , roses , redwoods , and willows could be found in abundance. The Cretaceous–Paleogene extinction event

5520-407: The open sea. Though primarily represented by azhdarchids , other forms like pteranodontids , tapejarids ( Caiuajara and Bakonydraco ), nyctosaurids and uncertain forms ( Piksi , Navajodactylus ) are also present. Historically, it has been assumed that pterosaurs were in decline due to competition with birds, but it appears that neither group overlapped significantly ecologically, nor

5600-449: 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

5680-527: The period a cooling trend is evident. The tropics became restricted to equatorial regions and northern latitudes experienced markedly more seasonal climatic conditions. Due to plate tectonics , the Americas were gradually moving westward, causing the Atlantic Ocean to expand. The Western Interior Seaway divided North America into eastern and western halves; Appalachia and Laramidia . India maintained

5760-423: The riverine Hell Creek Formation, suggesting that some of these taxa were then, as now, tolerant of fresh water. The "Lancian" fauna is more similar overall phylogenetically to East Asian and Canadian/Alaskan faunas than most Campanian North American faunas. Fossil insects from inclusions found within amber are known. It is a series of fresh and brackish-water clays , mudstones , and sandstones deposited during

5840-452: The seaway to the newly formed Rocky Mountains . These formations are composed largely of sandstone and mudstone which have been attributed to floodplain , fluvial , lacustrine , swamp, estuarine and coastal plain environments. Hell Creek is the best studied of these ancient environments. At the time, this region had a subtropical, warm and moist climate. The climate was humid, with flowering plants, conifers, palm trees , and ferns in

5920-570: The size of the deposits beneath the flood debris, the Tanis River was a "deep and large" river with a point bar that was towards the larger size found in Hell's Creek, suggesting a river tens or hundreds of meters wide. The river flowed eastward from an inland area to the west, and the site itself was in an ancient river valley close to the western shore of the Interior Seaway. Although other flooding

6000-515: The swamps, and conifers, canopy , understory plants, high diversity of angiosperm trees and shrubs in the forests. In northwestern South Dakota , strips of black layers deposited in the wetland environment are rich in coal, and a bright band-like layer of sand and mud from the river floodplain accumulated. Many plant species were supported, primarily angiosperms, and less commonly conifers , ferns and cycadeoids . An abundance of fossil leaves are found at dozens of different sites indicating that

6080-555: The time of the Chicxulub impact, the present-day North American continent was still forming. Most of central North America had recently been a large shallow seaway, called the Western Interior Seaway (also known as the North American Sea or the Western Interior Sea), and parts were still submerged. This had initially been a seaway between separate continents , but it had narrowed in the late Cretaceous to become, in effect,

6160-433: The very end of the epoch; the same can be said for true marsupials . Instead, nearly all known eutherian and metatherian fossils belong to other groups. In the seas, mosasaurs suddenly appeared and underwent a spectacular evolutionary radiation. Modern sharks also appeared and penguin-like polycotylid plesiosaurs (3 meters long) and huge long-necked elasmosaurs (13 meters long) also diversified. These predators fed on

6240-475: Was a large-scale mass extinction of animal and plant species in a geologically short period of time, approximately 66  million years ago (Ma). It is widely known as the K–T extinction event and is associated with a geological signature, usually a thin band dated to that time and found in various parts of the world, known as the Cretaceous–Paleogene boundary (K–T boundary). K is the traditional abbreviation for

6320-410: Was subtropical; there was no cold season and probably ample precipitation . The Hell Creek Formation, Lance Formation and Scollard Formation represent different sections of the western shore of the shallow sea that divided western and eastern North America during the Cretaceous. Swampy lowlands were the habitat of various animals, including dinosaurs. A broad coastal plain extended westward from

6400-449: Was widely covered in worldwide media on 29 March 2019, in advance of its official publication three days later. The co-authors included Walter Alvarez and Jan Smit , both renowned experts on the K–Pg impact and extinction . At Tanis, unlike any other known Lagerstätte site, it appears specific circumstances allowed for the preservation of moment-by-moment details caused by the impact event. These include finds which allow examination of

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