Journal of Paleontology, 90(1), 2016, p. 102–132 Copyright © 2016, The Paleontological Society 0022-3360/16/0088-0906 doi: 10.1017/jpa.2015.67


Anatomy, systematics, paleoenvironment, growth, and age of the sauropod dinosaur Sonorasaurus thompsoni from the Cretaceous of Arizona, USA


Michael D. D’Emic,1 Brady Z. Foreman,2 and Nathan A. Jud3


1Department of Biology, Adelphi University, Garden City, NY 11530-0701, USA ⟨mdemic@adelphi.edu⟩ 2Western Washington University, Department of Geology, Bellingham, WA 98225-9080, USA ⟨brady.foreman@wwu.edu⟩ 3Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA ⟨najud@flmnh.ufl.edu⟩


firsthand observation, bone histology, and fieldwork in the holotypic quarry and environs. The validity of S. thompsoni is substantiated by autapomorphies. Cladistic analysis firmly places it within the Brachiosauridae, in contrast to results of some recent analyses. Bone histology suggests that the only known exemplar of Sonorasaurus grew slowly and sporadically compared to other sauropods and was approaching its adult size. In contrast with previous assessments of a coastal/estuarine paleoenvironment for the Turney Ranch Formation, our sedimentological and plant macrofossil data indicate that Sonorasaurus lived in a semiarid, low relief evergreen woodland that received highly variable (perhaps seasonal) precipitation. We obtained detrital zircons from the holotypic quarry for U-Pb dating, which only yielded Barremian-aged and older grains, whereas other radiometric and biostratigraphic data suggest that the sediments at the quarry were deposited near the Albian-Cenomanian boundary. Sonorasaurus is taxonomically valid, represents one of the geologically youngest brachiosaurid sauropods, and


Abstract.—Sauropod dinosaurs are rare in the Cretaceous North American fossil record in general and are absent from that record for most of the Late Cretaceous. Sonorasaurus thompsoni from the Turney Ranch Formation of the Bisbee Group of Arizona, USA, potentially represents one of the youngest sauropods before their ca. 30-million-year- long hiatus from the record. The anatomy of Sonorasaurus has only been briefly described, its taxonomic validity has been questioned, several hypotheses have been proposed regarding its phylogenetic relationships, and its life history, geologic age, and reported paleoenvironment are ambiguous. Herein we assess the systematics, paleoenvironment, life history, and geologic age of Sonorasaurus based on


inhabited a harsh inland evergreen-dominated woodland environment that limited its growth. A review of other Bisbee Group dinosaurs suggests that its fauna, although poorly sampled, exhibits broad similarity to those from coeval North American horizons, reinforcing the apparent faunal homogeneity at the time.


Introduction


Cretaceous-aged vertebrates from Arizona, including dinosaurs, are rare. Despite the paucity of data, early reports (e.g., Miller, 1964) recognized two main Cretaceous faunas in the state: one ‘middle’ Cretaceous (?Aptian-?Cenomanian) and one Late Cretaceous (Campanian) in age, a distinction that still holds today (see Lucas and Heckert, 2005). Whereas known Late Cretaceous diversity is high, known ‘middle’ Cretaceous diversity is low, with only a few fragmentarily represented dinosaur taxa, including nodosaurids, sauropods, iguano- dontians, dromaeosaurids, and large theropod dinosaurs (‘car- nosaurs,’ Miller, 1964; Galton and Jensen, 1989; McCord and Gillette, 2005; Lucas and Heckert, 2005). Most of these remains are taxonomically ambiguous, housed in private collections, or only substantiated by brief mention in the literature (e.g., Rat- kevitch, 1997a; Hodnett, 2003; Lucas and Heckert, 2005). Fragmentary sauropod remains are known from at least three published localities (Ratkevitch, 1998; McCord and Gillette, 2005). Among dinosaur remains, only the sauropod


Sonorasaurus is known from somewhat complete material, including about one-third of a partially articulated skeleton. The holotype and only known specimen of Sonorasaurus


thompsoni was discovered by Dr. Richard Thompson of Tucson, Arizona in the fall of 1994 in the Whetstone Mountains of southeastern Arizona (Fig. 1). It was collected from 1995 to 1999 by crews from the Arizona-Sonora Desert Museum. Earliest reports (e.g., Thayer and Ratkevitch, 1995; Ratkevitch, 1997a, b) informally used the name ‘Sonorasaurus’ for the dinosaur from this site, originally tentatively identifying it as a sauropod (Thayer and Ratkevitch, 1995) or hadrosaur (E. Colbert personal commu- nication in Hill and Ratkevitch, 1996). The discovery of Sonorasaurus generated significant attention because of the pur- ported presence of a skull (Ratkevitch, 1997a:fig. 2), which was later shown to be a crushed dorsal vertebra (Curtice, 2000). Later, the holotype specimen, ASDM 500, was described and identified as a new genus and species along with identification as a brachiosaurid sauropod (Ratkevitch, 1998). At the time of its original description, ASDM 500 was the most complete ‘middle’ Cretaceous NorthAmerican sauropod specimen (Ratkevitch, 1998).


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