Hell Creek Formation

The "Tooth Draw Deposit" is an extensive, high diversity, multitaxic, dinosaur-rich, bone bed in the latest Cretaceous, Hell Creek Formation. The mapped portions of the deposit show a complex channel lag formed by a combination of both abiotic and biotic processes, at least 40 meters wide and well over 180 meters long. It is composed of interbedded sandstones and conglomerates representing fluvial, riverine deposition, a portion of which shows evidence of hyper-concentrated or debris flow rates. This lag has been sampled in different locations including the main "Tooth Draw Quarry (TD)", "Tooth Draw West (TDW)", "Tooth Draw South (TDS)" and "Becca's Tooth Draw (BTD)". To date, several thousand vertebrate specimens have been recovered over the last 15 years of work. These elements include both dinosaur and non-dinosaur remains, showing a diverse fauna of at least 21 dinosaur and over 48 vertebrate genera. Taphonomic markers indicate that there are at least two distinct communities represented by the deposit. One that represents a more localized fauna, caught up in the initial formation of the lag and another that represents reworked, hydraulically emplaced elements, likely accumulating from further upstream (parautochthonous). Many of the reworked elements show pre-depositional breaks, spiral fractures, bite marks, insect borings, and evidence of sub-aerial exposure. Tyrannosaurid teeth make up over 30% of the logged specimens, over 700 teeth, with more added each year. Dromaeosaurid teeth are also fairly common with over 180 specimens to date. Given the high numbers of shed theropod teeth, and the broken and bite marked bones, the most parsimonious conclusion is that portions of the Tooth Draw assemblage represent the remains of multiple seasons of feeding activity a short distance upstream along the ancient river system.

A census of Hell Creek and Lance Formation dinosaur remains was conducted from April, 2017 through February of 2018. Online databases were reviewed and curators and collections managers interviewed in an effort to determine how much material had been collected over the past 130+ years of exploration. The results of this new census has led to numerous observations regarding the quantity, quality, and locations of the total collection, as well as ancillary data on the faunal diversity and density of Late Cretaceous dinosaur populations. By reviewing the available data, it was also possible to make general observations regarding the current state of certain exploration programs, the nature of collection bias present in those collections and the availability of today's online databases.

A total of 653 distinct, associated and/or articulated remains (skulls and partial skeletons) were located. Ceratopsid skulls and partial skeletons (mostly identified as Triceratops) were the most numerous, tallying over 335+ specimens. Hadrosaurids (Edmontosaurus) were second with at least 149 associated and/or articulated remains. Tyrannosaurids (Tyrannosaurus and Nanotyrannus) were third with a total of 71 associated and/or articulated specimens currently known to exist. Basal ornithopods (Thescelosaurus) were also well represented by at least 42 known associated and/or articulated remains. The remaining associated and/or articulated specimens, included pachycephalosaurids (18), ankylosaurids (6) nodosaurids (6), ornithomimids (13), oviraptorosaurids (9), dromaeosaurids (1) and troodontids (1).
Over 41,800 isolated bones and teeth, were also located. This number represents only a small fraction of the actual total collection as many of the museums and institutions surveyed were unable to provide complete numbers on isolated elements. Over 46% of these isolated bones and teeth were identified as hadrosauridae, usually identified as Edmontosaurus. Isolated elements identified as ceratopsids made up just over 21% of the total. These were generally identified as Triceratops. Isolated bones and teeth of tyrannosaurids were significantly less, at only 4.6%. The large difference between the associated and/or articulated remains and the isolated bones and teeth of tyrannosaurids (10.9% down to 4.6%) and ceratopsids (51.3% to 21.5%) is likely due to both a preservational and collection bias towards the larger, more likely to be fossilized, and more likely to be collected, Tyrannosaurus and Triceratops skeletons and skulls.

Even though small theropods accounted for less than 0.6% of the total recovered associated and/or articulated remains, their teeth and isolated elements were encountered quite frequently. Isolated bones and teeth of dromaeosaurids, troodontids and “unidentified small theropods” accounted for as much as 16% of the total number of isolated remains. This data suggests that there is a tremendous level of collection and preservational bias in the current Hell Creek and Lance sample set Actual small theropod diversity and populations in the Late Cretaceous were most likely much higher than previously considered.
It is highly likely that the fluvial and geochemical environment that dominated the Late Cretaceous of this region was simply too rough and tumble for bones of most genera under the 400 kg live weight

The Journal of Paleontological Sciences: JPS.C.2019.01 2
threshold to be preserved. Both preservational bias and collection bias appear to play significant roles in how we currently view the diversity of the Hell Creek and the Lance. This has directly influenced our perception of the end Cretaceous extinction event. It is entirely possible that many genera will never be known from more than fragmentary remains.

Modern debate regarding the extinction of non-avian dinosaurs was ignited by the publication of the Cretaceous–Tertiary (K–T) asteroid impact theory and has seen 30 years of dispute over the position of the stratigraphically youngest in situ dinosaur. A zone devoid of dinosaur fossils reported from the last 3 m of the Upper Cretaceous, coined the ‘3 m gap’, has helped drive controversy. Here, we report the discovery of the stratigraphically youngest in situ dinosaur specimen: a ceratopsian brow horn found in a poorly rooted, silty, mudstone floodplain deposit located no more than 13 cm below the palynologically defined boundary. The K–T boundary is identified using three criteria: (i) decrease in Cretaceous palynomorphs without subsequent recovery, (ii) the existence of a ‘fern spike’, and (iii) correlation to a nearby stratigraphic section where primary extraterrestrial impact markers are present (e.g. iridium anomaly, spherules, shocked quartz). The in situ specimen demonstrates that a gap devoid of non-avian dinosaur fossils does not exist and is inconsistent with the hypothesis that non-avian dinosaurs were extinct prior to the K–T boundary impact event.

The Cretaceous/Tertiary (K/T) boundary corresponds to one of the very few unique events in the geological record discovered to date, representing a single traceable timeline across the world. This timeline, coincident with the geochemical and mineralogical singularities caused by the impact of a large extraterrestrial body, is also coincident with the end-Cretaceous extinction event in North America. This precise timeline gives an ideal context for comparing the trends recorded by the different groups of the terrestrial fossil record during the K/T ...

Though the dinosaur Thescelosaurus neglectus was first described in 1913 and is known from the relatively fossiliferous Lance and Hell Creek formations in the Western Interior Basin of North America, the cranial anatomy of this species remains poorly understood. The only cranial material confidently referred to this species are three fragmentary bones preserved with the paratype, hindering attempts to understand the systematic relationships of this taxon within Neornithischia. Here the cranial anatomy of T. neglectus is fully described for the first time based on two specimens that include well-preserved cranial material (NCSM 15728 and TLAM.BA.2014.027.0001). Visual inspection of exposed cranial elements of these specimens is supplemented by detailed CT data from NCSM 15728 that enabled the examination of otherwise unexposed surfaces, facilitating a complete description of the cranial anatomy of this species. The skull of T. neglectus displays a unique combination of plesiomorphic and apomorphic traits. The premaxillary and ‘cheek’ tooth morphologies are relatively derived, though less so than the condition seen in basal iguanodontians, suggesting that the high tooth count present in the premaxillae, maxillae, and dentaries may be related to the extreme elongation of the skull of this species rather than a retention of the plesiomorphic condition. The morphology of the braincase most closely resembles the iguanodontians Dryosaurus and Dysalotosaurus, especially with regard to the morphology of the prootic. One autapomorphic feature is recognized for the first time, along with several additional cranial features that differentiate this species from the closely related and contemporaneous Thescelosaurus assiniboiensis. Published phylogenetic hypotheses of neornithischian dinosaur relationships often differ in the placement of the North American taxon Parksosaurus, with some recovering a close relationship with Thescelosaurus and others with the South American taxon Gasparinisaura, but never both at the same time. The new morphological observations presented herein, combined with re-examination of the holotype of Parksosaurus, suggest that Parksosaurus shares a closer relationship with Thescelosaurus than with Gasparinisaura, and that many of the features previously cited to support a relationship with the latter taxon are either also present in Thescelosaurus, are artifacts of preservation, or are the result of incomplete preparation and inaccurate interpretation of specimens. Additionally, the overall morphology of the skull and lower jaws of both Thescelosaurus and Parksosaurus also closely resemble the Asian taxa Changchunsaurus and Haya, though the interrelationships of these taxa have yet to be tested in a phylogenetic analysis that includes these new morphological data for T. neglectus.

In this study, we examine surface collected teeth of Myledaphus pustulosus from the Upper Cretaceous (Maastrichtian) Hell Creek Formation of Garfield County, Montana. Macroscopic analysis of teeth shows that 54% of the teeth have pronounced attritional wear due to a durophagous diet. A higher percentage of unworn teeth than worn teeth (64% vs 27%) have acid damage due to predation or scavenging. Ultrastructural analysis of whole and sectioned teeth show that the enameloid of the teeth is 100-125 µm thick and is composed of a single crystallite enameloid in which apatite crystallites show no preferred orientation. Orthodentine in the tooth crown is composed of dense material with numerous canals for odontoblast processes which extend nearly to the tooth surface. Dentinal tubules surround odontoblasts most of the way through the enameloid. The enameloid microstructure of M. pustulosus is consistent with the pattern seen in other batoids.

The Cretaceous/Tertiary (K/T) boundary corresponds to one of the very few unique events in the geological record discovered to date, representing a single traceable timeline across the world. This timeline, coincident with the geochemical and mineralogical singularities caused by the impact of a large extraterrestrial body, is also coincident with the end-Cretaceous extinction event in North America. This precise timeline gives an ideal context for comparing the trends recorded by the different groups of the terrestrial fossil record during the K/T extinction event. However, in southwestern North Dakota, numerous studies conducted on excellent exposures of the K/T boundary showed that the geochemical and mineralogical criteria associated with the impact are rarely preserved. For that reason, palynology is preferred as a simple and efficient way for identifying the K/T boundary. In this context, a previously undescribed outcrop section from southwestern North Dakota was found to preserve an extensive record of the continental K/T boundary with miospores and plant megafossils, as well as microvertebrates and large vertebrate elements. Preliminary studies on the fossil associations recovered from this site showed an inconsistency between the Paleocene age given by the vertebrate and megafloral component and the Cretaceous age given by the palynological record. This issue is investigated in this paper, with a major emphasis on the description and analysis of the palynological record associated with sedimentological and paleoenvironmental data. Results shows that palynologically, the K/T boundary is placed more accurately using relative abundance variations of selected taxa.

Choristoderes are freshwater, diapsid reptiles that are commonly represented by champsosaurs in the Hell Creek Formation (Upper Cretaceous; Maastrichtian). Analysis of 115 vertebrae reveals four major findings: 1) all vertebrae were missing neural arches and any processes (ribs) leaving only the centra. Fluvial transportation explains the missing processes and why the Hell Creek champsosaurs are typically represented by scarce fragmentary bones. Randomly collected vertebrae have no signs of sampling bias based on size or type, indicating no preferential sorting by fluvial transportation or predation. The exception is the scarcity of posterior-caudal bones suggesting these longer bones are susceptible to breakage or predation. The vertebra types represented were 21.7% Cervical; 29.6% Dorsal; 6.9% Sacral; 40% Caudal which correlates well with the known distribution in complete specimens. 2) The average vertebrae length was 17.4mm, while juveniles were not distinguished from adults; it is still further evidence that Cretaceous champsosaurs are smaller than Paleocene specimens. 3) Ontogeny data reveals isometric growth for the population. Comparing the measurements gave R-squared values consistently ranging from 0.70 - 0.92, with the best correlation represented by Anterior end Height: Midline Height. 4) Variation in vertebrae measurements proved beneficial in identification. Those providing the most separation to aid identification were ratios for Length:Midline Height and Length:Anterior diameter. The consistency of all these measurements confirm that taphonomic processes, such as diagenetic alteration, are not deforming the shapes of the vertebrae making them valid tools for other studies such as analysis of growth lines as paleoclimate indicators.

Background. Freshwater mussels of superfamily Unionoidea are a diverse group with an evolutionary history of at least 360 million years. Many fossil and modern species exhibit a generally unsculptured, roughly elliptical shell outline. Such morphology results in difficulties when attempting to identify or classify fossil material, as only hard parts are preserved. Several latest-Cretaceous fossil localities over a small geographic area in southwestern North Dakota contain poorly preserved edentulous freshwater mussels that effectively lack all but shell outlines. This paper discusses methods that were used to attempt to determine how many fossil taxa were present in one of these assemblages. Methods. Elliptical Fourier Analysis was performed on two-dimensional shell outlines of both edentulous fossils and modern taxa to create Fourier scores representing outline shape. Morphospace occupation, or the amount of variation within a sample of shell outlines, was calculated using two meth...

The Hell Creek-Ludlow formational contact in northwestern South Dakota has received less research attention than correlative units in the northern Great Plains region. The Hell Creek Formation and lower Fort Union Group have been more extensively studied in eastern Montana and southwestern North Dakota. The goal of this study is to extend that research and characterize the formational contact and the beds directly above and below the contact in northwestern South Dakota. Although a disconformable K-Pg boundary has tentatively been identified above marine units in the White River Badlands of southwestern South Dakota, its terrestrial equivalent is yet poorly described in South Dakota. This research is a first step in examining the Hell Creek and Ludlow formations and the boundary between, to better describe the lithological changes and look for evidence to associate the formational contact with the K-Pg chronostratigraphic boundary. The formational contact was characterized based on changes in lithology and, to a lesser extent, a semi-persistent lignite unit. By examining thin sections of rock units above and below the formational contact it was found that the Hell Creek Formation contains a high percentage of clay, whereas the Ludlow Formation contains a more sand and lithic clasts. When the thin sections were examined using the scanning electron microscope, both formations were found to contain numerous iron-rich grains. No iridium of other elements that can be traced to the K-Pg bolide impact were identified, however the Hell Creek Formation samples were found to contain rare-earth element rich grains, whereas those of the Ludlow Formation contain grains of barite, albite and zeolites. Although no conclusive evidence was found to link the formational contact with the K-Pg boundary, a lithostratigraphic framework was established upon which future studies can be built.

... 1989: Nichols et al.. 2000), and is further supported by magnetostratigraphy (Hicks et al.. this vol-ume). ... Those data are summarized here for purposes of comparison with data on the palynoflora of the Ludlow Member of the Fort Union Formation and to provide a basis for ...