Conceived and designed the experiments: JH NM. Performed the experiments: JH MG. Analyzed the data: JH MG. Contributed reagents/materials/analysis tools: JH MG. Wrote the paper: JH MG.
Nathan Myhrvold contributed financially to the Hell Creek Project and intellectually to the design of the study.
A dinosaur census recorded during the Hell Creek Project (1999–2009) incorporates multiple lines of evidence from geography, taphohistory, stratigraphy, phylogeny and ontogeny to investigate the relative abundance of large dinosaurs preserved in the Upper Cretaceous Hell Creek Formation of northeastern Montana, USA. Overall, the dinosaur skeletal assemblages in the Hell Creek Formation (excluding lag-influenced records) consist primarily of subadult or small adult size individuals. Small juveniles and large adults are both extremely rare, whereas subadult individuals are relatively common. We propose that mature individuals of at least some dinosaur taxa either lived in a separate geographic locale analogous to younger individuals inhabiting an upland environment where sedimentation rates were relatively less, or these taxa experienced high mortality before reaching terminal size where late stage and often extreme cranial morphology is expressed.
The Hell Creek Project (1999–2009), a collaborative, multi-institutional field study of the Upper Cretaceous Hell Creek Formation, northeastern Montana, produced this dinosaur census from a well-documented collection of all taxa. The overall goal of the project was to create a comprehensive biotic foundation from which paleobiological and geological hypotheses could be tested. One of the many projects included the collection of dinosaur specimens to test hypotheses focusing on the relative abundances and the presence or absence of various dinosaurian ontogenetic stages. Previous Hell Creek Formation surveys
In this current survey, we focused on the Upper Cretaceous Hell Creek Formation exposed around Fort Peck Reservoir in northeastern Montana. Here, contiguous outcrops are traceable over an area of about 1000 sq. km (
Contiguous outcrops are traceable over an area of about 1000 square km adjacent to Fort Peck Lake shown in blue. Dark orange represents the lower Hell Creek Formation and light orange represents the middle and upper Hell Creek Formation. The enlarged view of the study area is indicated by the rectangle in the northeast quarter of the map of Montana.
Here we present the relative abundance of large dinosaur taxa, their general ontogenetic stage within a stratigraphic and sedimentological context and offer generalized hypotheses to explain several of the more interesting patterns that have emerged. This dinosaur census sets the stage for future statistical analysis and evaluation of the end Cretaceous faunal record and will add to recent paleoecological studies of diverse dinosaur faunas and clades that focused on body size, habitat partitioning and living space requirements
The Hell Creek Project spanned 11 field seasons from 1999–2009. The first five years focused on the collection of specimens from the lower third of the formation (See
In order to evaluate possible changes in dinosaur taxa and their relative abundance through time, the Hell Creek Formation was divided into three stratigraphic units
Pie charts I – IV reflect the relative abundance of dinosaur genera based on skeletons (charts I and IV) and individual bones (charts II and III).
In this study, the L3 and U3 are subdivided based on sedimentological and stratigraphic details confirmed in the study area (
The U3 is comprised of two sandstone units: the Apex sandstone (U3.AS) and the 10-meter sandstone (U3.10mS). An interbedded variegated mudstone (U3.ibMS) separates these two sandstones and an upper variegated mudstone (U3.uMS) overlies the 10-meter sandstone. U3 is capped by an overlying uppermost coal zone (U3.CZ).
High-resolution digital video (remote control camera mounted on a Bell 407 helicopter) was utilized to track stratigraphic horizons and facies over large distances to assure stratigraphic resolution and supplement standard stratigraphic sections and mapping within the study area.
Census figures for the dinosaurs were determined using two different methods: (1) recording numbers of articulated or associated skeletons and (2) recording numbers of individuals based on isolated elements collected from lag deposits in the L3 ( = Jen-rex sand) and U3 ( = Apex sandstone). Although more than 150 microsites were recorded and sampled during the course of the Hell Creek Project, microsite census data are not included here because we think these sites impart biased dinosaur census data (contra
Countable dinosaur specimens are herein defined as any group of three or more bones most likely belonging to one individual based on size, association and taphonomic details (color, wear, etc.). We assume that three bones represent the minimum number of elements of a skeleton or skull that has not been transported and redeposited after initial burial. As for bones collected from lag deposits, it is assumed that these sites contain a potentially time-averaged, cross section of the dinosaur fauna, as long as the skeletal elements vary in size from small teeth to large limb elements (≥0.50 m). Census results from two such deposits are included in this study: (1) the Jen-rex sand (3B-1 Lag: MOR locality no. HC-312) and (2) the U3 unit at the base of the Apex sandstone (Doldrum's Lag: MOR locality no. HC-530). Lag specimens of dinosaurs were collected and logged only if they could be reasonably identified to genus. Dinosaur genera recognized for this study include
In addition to the census of skeletons and lag specimens (
With the exception of one specimen of
In addition to producing a census of individuals of each taxon in the L3 and U3 units, we also determined the relative ontogenetic stages of the counted individuals. These general ontogenetic stages were determined by morphological ontogenetic markers in some taxa, such as
Six ontogenetic stages identified with a letter ascending in size from “A” to “F” are employed for the
The dinosaur census results are summarized in
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Stratigraphic level |
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Upper Hell Creek Fm (U3) skeletons | n = % | 2369% | 516% | 516% | |||||
Pie chart I, |
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"Doldrum’s" lag deposit at base of Apex sandstone (MOR locality HC-530) | n = % | 1641% | 923% | 718% | 410% | 25% | 13% | ||
Pie chart II, |
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"3B-1" lag deposit at base of Jen-rex sand (MOR locality HC-312) | n = % | 2333% | 1927% | 1826% | 710% | 23% | 11% | ||
Pie chart III, |
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Lower Hell Creek Fm (L3) skeletons | n = % | 1128% | 1128% | 615% | 410% | 513% | 25% | ||
Pie chart IV, |
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Totals for the entire Hell Creek Formation(see |
n = % | 7340% | 4424% | 3620% | 158% | 95% | 21% | 21% |
Values determined from the dinosaur census tables (
The most interesting census result in the L3 is the high number of
Thirty-two skeletons were collected from the U3 unit, four of which were collected prior to the Hell Creek Project by the Museum of the Rockies (MOR 009,
Teeth were not collected or annotated because of the difficulties in using them for ontogenetic assessment with the exception of two large
The
In this census, growth stages at either end of the dinosaurian ontogenetic spectrum are least represented. Specimens of both the smallest and presumably youngest juveniles and the largest, and presumably oldest adults are the most rare dinosaurs recorded. The smallest specimen of
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
Even though
When ontogenetic stages are considered, we observe a low number of both “A” and “F” class (see ontogeny column in
The uncommonness of apparently fully mature adults is more mysterious and not easily explained. What is now apparent, however, is this pattern contributed to an historical increase in the naming of new dinosaur species from the Hell Creek Formation. For example, over many decades it was presumed that the taxon “
Reproductive maturity in some dinosaurs was achieved during subadulthood (e.g.,
The abundance of
The Hell Creek Project generated eleven years (1999–2009) of collecting and field studies in the Upper Cretaceous Hell Creek Formation, eastern Montana, and resulted in the discovery of a wide variety of new dinosaur specimens, many of which revealed for the first time the ontogeny of well-known dinosaur taxa such as
The relatively high abundance of
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We are grateful to Judd and Jay Twitchell for allowing us to camp on their land, and to the Twitchell families and Clay Taylor family for access to federal and state lands. We also thank Terry and Mary Kohler and the Windway Foundation for summer donations of their helicopter (with fuel, mechanic and pilot). Bill Berg and the staff of the Charles M. Russell Wildlife Refuge (USFW) in Lewistown, Gary Smith and Doug Melton and other staff of the Bureau of Land Management in Billings and Miles City, and Patrick Renne and staff at Montana State Lands (DNRC) in Helena facilitated access to land under their management and we are grateful for their cooperation and support. We also thank the staff of Hell Creek State Park and campground for hosting our field camp in the early years of the project. We thank Bob Harmon and his staff of field crew chiefs, and all the volunteers on the Hell Creek Project for their discoveries, and Carrie Ancell and Jamie Jette for the preparation of the fossil specimens. Thanks also to Denver Fowler, John Scannella, Jennifer Flight, Greg Wilson, Laura Wilson, and Ben Shoup for discussions concerning their thesis and dissertation data. David Polly and an anonymous reviewer provided thoughtful comments and reviews of an earlier draft of this manuscript. We thank Peter Roopnarine, PLoS One Academic Editor, for his editorial guidance and attention, and William Clemens and Patricia Holroyd for their comments and interest in this study. David Smith provided expert graphics assistance of the final figures. Pat Leiggi and Shelley McKamey were generous hosts on repeated visits to the Museum of the Rockies and Bozeman, Montana by MBG, and their support and friendship facilitated this research. This is University of California Museum of Paleontology contribution no. 2022.