The Hand of Tenontosaurus Tilletti (Dinosauria, Ornithopoda)

Ornithopods are among the most common members of North America’s Cretaceous dinosaurian faunas, yet despite their excellent representation in existing collections, the anatomy of the manus remains poorly known or inadequately described for many species. In general, descriptions of new species lack treatment of the hand altogether; in most other cases, the autopodium is described based on incomplete and/or disarticulated manus elements (e.g., Galton, 1974; Ostrom, 1970; Sereno, 1991), even among relatively well-represented taxa. Among these is Tenontosaurus tilletti Ostrom, 1970, a medium-sized early iguanodontian first described by Ostrom (1970) from the Lower Cretaceous Cloverly Formation of Montana. At the time of Ostrom’s (1970) work, 26 partial or nearly complete skeletons were known. Of these, only four examples included material sufficient for description of the manus: OMNH 10321 (previously listed as “OU 11”), BB 1, AMNH 3031 and YPM 5459. Unsurprisingly, the lack of complete, articulated hands led to confusion about the placement of some elements. In a redescription of the postcranial skeleton of T. tilletti, Forster (1990) noted that Ostrom had erroneously placed phalanx V-1 as the distal phalanx of digit IV and the radiale was identified as the terminal phalanx of digit V. Based on these corrections, Forster (1990) inferred the phalangeal formula of T. tilletti to be 2-3-3-1? -1? rather than the 2-3-3-2-2 as originally suggested by Ostrom (1970). Until now, this arrangement has gone unchallenged. The discovery of new specimens of T. tilletti with articulated manus, from the Aptian-Albian Antlers Formation of southeastern Oklahoma, prompts us to revise what is known of the manus in Tenontosaurus, and to place the revised morphology in the context of hand morphology known for other ornithopods. The Antlers Formation of Oklahoma has yielded 16 partial or nearly complete skeletons of T. tilletti, belonging to multiple ontogenetic classes (Werning, 2012). Most noteworthy is OMNH 58340, a nearly complete, articulated, subadult or possibly early adult T. tilletti skeleton with multiple elements retaining three-dimensional articulation. Elements such as the skull (see Thomas, 2015 for description of cranial elements), pelvis, right foot, and left manus (Werning, 2012) are most notable for their preservation. This specimen includes rarely preserved bones such as the ceratobranchial (i.e., ‘hyoid’) elements, atlantal ribs, and distal-most phalanges. The skeleton lacks only a few vertebrae from the middle of the tail, lost to weathering prior to collection. The articulation of this individual was preserved during preparation so that all elements remain in their in situ articulation, leaving no doubts as to the positioning of the bones with respect to each other. In addition to its exceptional completeness and preservation, this individual has five pathological bones indicative of trauma and subsequent infection (Hunt et al., 2019). Our description of the manus of T. tilletti is based on OMNH 58340 and two other intact manus (OMNH 62990, 63525). We make comparisons to several other taxa known by relatively complete manus and relevant to placement of Tenontosaurus among basally branching Iguanodontia. These include the early ornithischian Lesothosaurus diagnosticus Galton, 1978, the early ornithopodsHypsilophodon foxiiHuxley, 1869 (more fully described in Huxley, 1870) and Thescelosaurus neglectus Gilmore, 1913, Convolosaurus marri Andrzejewski, Winkler, and Jacobs, 2019, the early ankylopollexians Uteodon aphonocetes (described by Carpenter and Wilson, 2008, as a species of Camptosaurus and later assigned to its own genus, Uteodon, by McDonald, 2011) and Camptosaurus dispar Marsh, 1879, the hadrosauriform Iguanodon bernissartensis Boulenger, 1881, and the derived hadrosaurine Edmontosaurus annectens Marsh, 1892. As one of few specimens known to preserve the entire wrist and hand in life position, OMNH 58340 promises to be a benchmark for understanding the evolution of the hand of ornithopods. Institutional Abbreviations—AMNH, American Museum of Natural History, New York City, New York, U.S.A. ; BB, Buffalo Bill Center of the West, Cody, Wyoming, U.S.A. ; FWMSH, Fort Worth Museum of Science and History, Fort Worth, Texas, U.S.A. ; MOR, Museum of the Rockies, Montana State University, Bozeman, Montana, U.S.A. ; OMNH, Sam Noble Museum of Natural History, Norman, Oklahoma, U.S.A. ; YPM, Yale Peabody Museum of Natural History, New Haven, Connecticut, U.S.A.