Journal of Paleontology, 90(1), 2016, p. 133–146 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2016.42
The ‘duck-billed’ dinosaurs of Careless Creek (Upper Cretaceous of Montana, USA), with comments on hadrosaurid ontogeny
Albert Prieto-Márquez1 and Susana Gutarra2
1School of Earth Sciences, University of Bristol, Bristol BS8 1TQ, UK ⟨
a.prietomarquez@
bristol.ac.uk⟩ 2Cell Biology Group, Department of Experimental and Health Sciences, Pompeu Fabra University, CIBER on Neurodegenerative diseases, E-08003 Barcelona, Spain ⟨
susana.gutarra@
upf.edu⟩
Abstract.—The Careless Creek Quarry (CCQ) is a multitaxic bonebed in the Campanian Judith River Formation of south-central Montana (USA) that produced a diverse assemblage of vertebrates, including several dinosaurian clades. We describe the morphology of the CCQ hadrosaurid material and reevaluate its taxonomic affinities. Our osteological comparative observations, coupled with maximum parsimony phylogenetic analyses, indicate that the majority of the hadrosaurid material is referable to kritosaurin saurolophines. Only an ischium is unambiguously referable to Lambeosaurinae. Most of the kritosaurin specimens likely represent a taxon that forms a polytomy with species of Gryposaurus and Rhinorex condrupus Gates and Sheetz, 2015. This form may represent individuals of either G. latidens Horner, 1992 or G. notabilis Lambe, 1914, or a new species cogeneric or not with Gryposaurus. The juvenile material exemplifies several patterns of mandibular and appendicular osteological variation previously observed in other hadrosaurids. However, it also shows some departures from the common trends, supporting the fact that not all skeletal growth changes can be generalized to all hadrosaurids.
Introduction
The Careless Creek Quarry (CCQ) is a multispecific bonebed occurring in strata of the Judith River Formation of eastern Wheatland County, central Montana, USA (Fiorillo, 1991; Fig. 1). Excavations conducted in the 1980s recovered approximately 1,500 vertebrate remains. These remains con- sisted of chondrychthian and teleost fishes, amphibians, lizards, crocodylians, pterosaurs, and a diverse assemblage of dinosaur clades that included theropods, ankylosaurs, pachycephalo- saurs, ceratopsians, and hadrosaurids (Dodson, 1986; Fiorillo, 1989). The CCQ is an important fossil site because of the rich- ness and diversity of vertebrate taxa preserved; it is becoming one of the most productive localities of the Judith River Formation (Fiorillo, 1991). In addition, it documented the co-occurrence of lambeosaurine and saurolophine hadrosaurids, as well as one of the earlier findings of juvenile material for this clade of ornithopods (Fiorillo, 1987, 1989). The Judith River Formation is 170–180m thick in
northwestern Montana (Rogers, 1998), and about 130m of the lower-most part of the formation is exposed at Careless Creek Quarry (Fiorillo, 1991). CCQ is found in the lower third of the local section, approximately 30m above the Parkman Sandstone of the Pierre Shale (Fiorillo, 1991, fig. 2). This author reports radiometric ages approximately 78–79Ma for bentonite layers that are low in the Judith River Formation cropping out in northern Montana. Congruent with those dates, recently Longrich (2013, fig. 8) suggested a lower-middle Campanian age for the CCQ ceratopsian Avaceratops.
The CCQ was deposited in a river channel within the
context of an aggrading coastal plain, and the bone assemblage was interpreted to have accumulated as a logjam (Fiorillo, 1991). We focus on the hadrosaurid remains from CCQ. The fossil bones were recovered as associated skeletal remains, possibly representing a group death event and subsequent transport of the carcasses (Fiorillo, 1991). A scenario such as a herd crossing a river in flood conditions has been pointed out as a possible explanation for this event (Fiorillo, 1987, 1991). Hadrosaurids were among the most diverse and abundant
large terrestrial herbivores of the Late Cretaceous (Lull and Wright, 1942; Horner et al., 2004), with a fossil record spanning the Santonian through the late Maastrichtian of Eurasia, the Americas, and Antarctica (Prieto-Márquez, 2010a). Hadrosaurids are notorious for having complex dental batteries, expanded rostra, and hypertrophied nasal passages often associated with supracranial crests (Evans, 2006; Prieto-Márquez, 2010b). Fiorillo (1989) reported the presence of Corythosaurus and Kritosaurus/Hadrosaurus in the CCQ on the basis of post- cranial comparisons with data presented in Brett-Surman (1975; erroneously referenced as 1972). The monospecific genus Hadrosaurus is a pre-saurolophid hadrosaurid restricted to the Campanian of the east coast of North America (Leidy, 1858; Prieto-Márquez et al., 2006; Prieto-Márquez, 2011a), whereas the two known species of Kritosaurus come from upper Campanian rocks of southern North America (Brown, 1910; Lucas et al., 2006; Prieto-Márquez, 2014a). It is therefore unlikely that Hadrosaurus or Kritosaurus are present in the CCQ sample. This and the fact that no mention was given of
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