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551 36 Historical Collecting Bias and the Fossil Record of Triceratops in Montana MARK B. GOODWIN AND JOHN R. HORNER Triceratopsisoneofthemostfamiliarandrecognizable LateCretaceousdinosaursfromNorthAmericaduetoits prominentpostorbitalandnasalhornsandsolidfrillcomposedoftheparietalandpairedsquamosals .Complete, undistortedadultTriceratopsskullsarefoundprimarilyin sandstonesandsiltstones,whichcontributegenerouslyto thesedimentarycompositionoftheHellCreekFormation inMontana,NorthandSouthDakota,theequivalent LanceandEvanstonformations,Wyoming,theLaramie FormationofColorado,andtheScollardandFrenchman formationsofAlbertaandSaskatchewan,Canada.Disarticulatedandoccasionallyhighlyconcretedadultskulls mayoccurinmudstones;however,nonadultTriceratops areseldomfoundinsandstonesandsiltstones,exceptas isolatedcranialelementsinchannellagdeposits.InMontana ,relativelycompletebaby(post-neonate),juvenile, andsubadultTriceratopsskullsarefoundalmostexclusivelyinmudstonefaciesoftheHellCreekFormation .We hypothesizethatahistoricalcollectingbiasandfaciesand taphonomicfactorsareresponsibleforthelimitednumberofnon -adultTriceratopsskullsandskullelements knownpriortothepublicationofthefirstTriceratopscranialgrowthseriesbyHornerandGoodwinin2006 .Baby, juvenile,andsubadultTriceratopsarenotasrareintheHell CreekFormationaspreviouslyreported,andpermita morecompleteassessmentofTriceratopssystematics, ontogeny,morphology,andvariation. Introduction By anyone’s measure, Triceratops, or ‘‘three-horned face,’’ is one of the most familiar genera of Late Cretaceous dinosaurs since it was named by O. C. Marsh in 1889. Early expeditions near the turn of the twentieth century by Barnum Brown, John Bell Hatcher, the team of Charles H. Sternberg, and others resulted in the discovery and collection of dozens of adult skulls from Upper Cretaceous sediments of the Western Interior of North America, particularly from Montana and adjacent states. Only a limited number of what we now recognize as subadult Triceratops were collected (Schlaikjer 1935; Dodson et al. 2004). Hatcher collected for O. C. Marsh and Yale University in the Upper Cretaceous of east central Wyoming in the Laramie ‘‘Ceratops’’ beds, now formally referred to as the Lance Formation of Maastrichtian age. Hatcher discovered over 30 partial to complete skulls, most of which were assigned to the genus Triceratops, from a relatively restricted area near Lusk, in Niobrara County (previously a part of Converse County), Wyoming. The result of Hatcher’s early work in the Lance Formation, supplemented by collections from the Hell Creek Formation of Montana and equivalent beds in North and South Dakota, Colorado, and Alberta and Saskatchewan , Canada, was the description of 16 species of Triceratops (Dodson et al. 2004). Most of these taxa are not diagnosable and instead reflect individual cranial variation and ontogeny (Ostrom and Wellenhofer 1986; Forster 1996a, b; Dod- 552 goodwin & horner FIGURE 36.1. Index map of Montana showing the general location of the Hell Creek Project and area where the Triceratops described in this study occur near Jordan, Montana, at the black arrow. The CretaceousPaleocene boundary is delineated by the lowest laterally extensive coal bed (‘‘Z’’ coal). son 1996). Today, most students of Triceratops recognize one taxon, Triceratops horridus (Ostrom and Wellenhofer 1986; Dodson 1996; Lehman 1998), or alternatively two species, T. horridus and T. prorsus (Forster 1996b). Regardless of your opinion on these matters, adult Triceratops skulls are well represented in museum collections across North America and Europe (Ostrom and Wellnhoffer 1986; Dodson et al. 2004; Goussard 2006). We hypothesize that a historical collecting bias, controlled in part by facies and taphonomic factors, influenced the known fossil record of Triceratops for many decades. We propose that these factors are likely responsible for the limited number of non-adult Triceratops skulls and skull elements known prior to the publication of the first cranial growth series by Horner and Goodwin (2006). Historically, this collecting bias also played a considerable role in the evolution of Triceratops systematics, based primarily on horn and frill morphology , and until recently, limited the evaluation of behavioral hypotheses and sexual dimorphism. This study is based largely on an assemblage of Triceratops from the Hell Creek Formation of Garfield and McCone Counties, Montana, in the collections of the Museum of the Rockies, Montana State University and the University of California Museum of Paleontology. Future work will test this hypothesis of a facies-related collecting bias in contemporaneous beds that yield Triceratops in neighboring states and Canada. Institutional Abbreviations. AMNH: American Museum of Natural History, New York; MOR: Museum of the Rockies, Bozeman; UCMP: University of California Museum of Paleontology , Berkeley. Geology The Upper Cretaceous Hell Creek Formation of eastern Montana (Fig. 36.1) was deposited as the continental facies of the last major regression of the Western Interior Eperic Sea (Lofgren 1995). The Hell Creek Formation in Garfield and McCone Counties is a predominantly fluvial deposit dominated by sandstones and siltstones that encroached eastward as a prograding wedge of clastic sediment across a low, broad coastal plain as the seaway regressed (Archibald 1982). The base of the Z coal complex that defines the upper formational boundary near Hell Creek has a weighted mean 40Ar/39Ar age determination of 65.0 Ma (Swisher et al. 1993). Beginning in the summer field season of 2000, J. Horner initiated the Hell Creek Project, at the time a five-year paleontological and geological investigation of the entire Hell Creek Formation exposed in Garfield and McCone Counties, eastern Montana...

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