GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 A NEW ATOPOSAURID CROCODYLOMORPH FROM THE MORRISON FORMATION (UPPER JURASSIC) OF WYOMING, USA John R. Foster Theme Issue An Ecosystem We Thought We Knew— The Emerging Complexities of the Morrison Formation SOCIETY OF VERTEBRATE PALEONTOLOGY Annual Meeting, October 26 – 29, 2016 Grand America Hotel Salt Lake City, Utah, USA © 2018 Utah Geological Association. All rights reserved. For permission to copy and distribute, see the following page or visit the UGA website at www.utahgeology.org for information. Email inquiries to [email protected]. GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association ISSN 2380-7601 Volume 5 2018 Editors UGA Board Douglas A. Sprinkel Thomas C. 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Trujillo — University of Wyoming 2017–2020 Term Tom Chidsey [email protected] 801.537.3364 John Foster — Utah Field House of Natural History State Park Museum Cary Woodruff — University of Toronto State Mapping Advisory Committe Octavio Mateus — Universidade Nova de Lisboa UGA Representative Jason Blake [email protected] 435.658.3423 Production Earthquake Safety Committe Cover Design and Desktop Publishing Chair Grant Willis [email protected] 801.537.3355 Douglas A. Sprinkel UGA Website Cover The Little Houston Quarry (Mammal Pit), www.utahgeology.org Webmasters Paul Inkenbrandt [email protected] 801.537.3361 in the lower middle right of the photograph, is in the Morrison Formation of the north- UGA Newsletter western Black Hills, Crook County, Wyo- Newsletter Editor Bill Lund [email protected] 435.590.1338 ming, which yielded the crocodyliform jaw described here. 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Utah Geological Association formed in 1970 from a merger of the Utah Geological Society, founded in 1946, and the Intermountain Association of Geologists, founded in 1949. Affiliated with the American Association of Petroleum Geologists. i GEOLOGY OF THE INTERMOUNTAIN WEST an open-access journal of the Utah Geological Association Volume 5 2018 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA John R. Foster Utah Field House of Natural History State Park Museum, 496 East Main St., Vernal, UT 84078; [email protected] ABSTRACT A left mandible of a small crocodyliform found in the Upper Jurassic Morrsion Formation of north- eastern Wyoming represents the first occurrence of the atoposaurid Theriosuchus in North America. The specimen demonstrates lower jaw morphology, including heterodonty (as indicated by alveolus shape), similar to Theriosuchus and Knoetschkesuchus, but autapomorphies and a unique combination of charac- ters among these taxa indicate that it is a distinct, new species of Theriosuchus. INTRODUCTION Fossil crocodylomorphs are diverse in the Upper Ju- rassic Morrison Formation of western North America, with seven species of terrestrial and semi-aquatic forms occurring, with collective widespread distribution and high abundance (e.g., Clark, 2011; Pritchard and oth- ers, 2013; Foster and McMullen, 2017). In 2004, the left Sheridan mandible of a small crocodyliform was collected from I-90 the Little Houston Quarry in the Morrison Formation of Jackson Wyoming the Black Hills, northeastern Wyoming (figure 1). This Casper specimen was found along with a diverse assemblage I-25 of dinosaurs and microvertebrates, the latter including Laramie mammals, fish, amphibians, a lizard, sphenodontians, Devils Tower NM the choristodere Cteniogenys, and turtles, among oth- ers (Foster, 2001). The dinosaurs included particularly Gillette I-90 Sundance WYOMING LHQ Site abundant basal neornithischians, theropods, and di- SOUTH DAKOTA plodocine and camarasaurid sauropods. The crocody- Newcastle 50 km liform specimen was initially described as a juvenile goniopholidid that would have demonstrated strong Figure 1. Location of Little Houston Quarry (LHQ) in allometric growth in the relative elongation of the lower Crook County, northeastern Wyoming, USA. Citation for this article. Foster, J.R., 2018, A new atoposaurid crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA: Geology of the Intermountain West, v. 5, p. 287–295. © 2018 Utah Geological Association. All rights reserved. For permission to use, copy, or distribute see the preceeding page or the UGA website, www.utahgeology.org, for information. Email inquiries to [email protected]. 287 A New Atoposaurid Crocodylomorph from the Morrison Formation (Upper Jurassic) of Wyoming, USA Foster, J.R. jaw through ontogeny (Foster, 2006). Re-examination Diagnosis of the specimen (MWC 5625) indicates heterodonty in Theriosuchus species with the following unique the lower jaw and strong similarities to Theriosuchus, combination of characters (*denote autapomorphic for and a redescription is provided here. species relative to Theriosuchus and Knoetschkesuchus): greatly enlarged D2 and D3 alveoli*; dramatic reduc- INSTITUTIONAL ABBREVIATIONS tion in mesiodistal diameter of alveoli from D3 to D4*; MWC – Museums of Western Colorado, Dinosaur overall structure of mandible similar to T. pusillus in Journey Museum, Fruita, Colorado; BMNH – The Nat- depth:length ratio, lack of external mandibular fenestra, ural History Museum, London, England. retroarticular process angle, and dentary dorsal profile in two “waves” in lateral view; however, dentary nutri- SYSTEMATIC PALEONTOLOGY ent foramina row and symphysis orientation relative to tooth row both more similar to K. guimarotae. Crocodylomorpha Crocodyliformes Revised Description (modified from Foster, 2006) Neosuchia Mesoeucrocoylia Specimen MWC 5625 is 141 mm long, with a pit- Atoposauridae ted to rugose lateral surface and a relatively deep dorso- Theriosuchus Owen, 1879 ventral dimension (figure 2). The lingual side of the jaw Type species – T. pusillus (particularly along the ventral margin) is not well pre- Theriosuchus morrisonensis sp. nov. served in the central area, but the articular region and Figures 2 and 3 the rostral third are in good condition. The tooth row is relatively short. There are alveoli for 16 teeth, although LSID. urn:lsid:zoobank.org:act:DAB8542A-7444-4026- all teeth are missing, and the lateromedial widths of the 9460-7735F0143C82 alveoli for the caudal seven of those teeth have been reduced somewhat by postmortem crushing. There is Type Specimen no external mandibular fenestra. In lateral view, the retroarticular process is short, and its dorsal surface is MWC 5625, left mandible, nearly complete but steeply inclined in a caudoventral direction. Although missing teeth. sutures of the bones of the lower jaw are difficult to dis- tinguish in many areas, the splenial is involved in the Type Locality caudal part of the oval symphysis. A deep foramen in- Little Houston Quarry (Mammal Pit), Crook Coun- termandibularis oralis occurs just caudal to the sym- ty, Wyoming (Foster and Martin, 1994; Foster, 2001). physis. A row of foramina extends along the occlusal surface of the dentary just lingual to the alveoli of D2 Type Horizon to D9 (figure 3). The edges of the alveoli are vertically festooned around D2 through D5 and are flat posterior Morrison Formation undifferentiated; thin local to that. Morrison section of only ~23 m (Mapel and Pillmore, There