The Cenomanian (Earliest Late Cretaceous) Mussentuchit Member of the Cedar Mountain Formation, which outcrops in central Utah, is one of the most abundant Late Cretaceous bonebeds known from anywhere in the world, with more than 100 species described to date, including Tyrannosaurids, Dromaeosaurs, Velociraptors, Troodontids, Therizinosaurs, Titanosauromorphs, Thescelosaurids, Pachycephalosaurids, Neoceratopsians, Nodosaurids, Hadrosaurids, and non-Hadrosaurid Iguanodontians, as well as a number of less well defined specimens.
In a paper published in the journal PLoS One on 7 June 2023, Lindsay Zanno, Terry Gates, and Haviv Avrahami of Paleontology Research at the North Carolina Museum of Natural Sciences, and the Department of Biological Sciences at North Carolina State University, Ryan Tucker of the Department of Earth Sciences at Stellenbosch University, and Peter Makovicky of the Department of Earth and Environmental Sciences at the University of Minnesota, describe a new species of Rhabdodontomorph Iguanodontian Dinosaur from the Cedar Mountain Formation.
The new species is described upon the basis of a single partial skeleton (specimen NCSM 29373) comprising a largely complete, disarticulated skull; cervical, dorsal, sacral, and caudal vertebrae; associated ribs and haemal arches; and portions of the right and left pectoral girdle, left pelvic girdle, right forelimb, and right hindlimb. The elements of the briancase are incompletely fused, leading Zanno et al. to conclude that the specimen is immature. The specimen was recovered from an exposure of the Mussentuchit Member in Emery County, Utah, 1 m above the boundary with the Ruby Ranch Member, and 7 m below Mussentuchit Ash Zones 1, which has been dated to 99.490 million years before the present. The new species is named Iani smithi, where 'Iani' refers to the Roman god Ianus (or Janus), who presides over boundaries, in reference to the fact that the mid-Cretaceous was a time of biological transition in western North America, and 'smithi' honours Joshua Aaron Smith for his contributions to the discovery and conservation of paleontological resources in the region, particularly early explorations by the North Carolina Museum of Natural Sciences.
Most of the skull is present, but the missing elements include the nasal and maxilla bones, so that the length and shape of the face of Iani smithi can only be estimated from the length of the mandible. This mandible comprises fragmentary predentary, complete pair of dentaries, right surangular, partial splenial, and several isolated dentary teeth. The rear part of the premaxila is thickened and shows the alveoli (sockets) of three premaxilary teeth. Premaxilary teeth are unusual in Ornithopod Dinosaurs, with most groups having lost them early in their history, but they are known in several other Rhabdodontomorphs.
The dentition of Iani smithi appears similar the other Rhabdodontomorphs, and non-Rhabdodontomorph early-branching Ornithopods such as Tenontosaurus and Qantassaurus. All of the teeth on the left dentary are still present, although the crowns of the middle teeth are damaged. Five teeth are still present in the left dentary, and thirteen loose teeth were found with the skeleton, interpretted as eight maxillary teeth, five dentary teeth, and a tooth of indeterminate origin. A single predentary tooth is present.
Portions of at least seven cervical (neck) vertebrae are present, although the atlas vertebra (which connects the spine to the skull) is absent. The axis (second vertebra) is present, and is compressed dorsoventrally, somewhat eroded ventrally, and approximately 150% longer than wide. At least eight unfused neural arches from the dorsal part of the spine are preserved, along with two partially fused dorsal vertebrae are preserved in variable states of damage and distortion. The dorsal vertabrae appear to be shorter and more gracile towards the head. Six sacral neural arches and four isolated sacral centra are present, showing the sacrum to have been unfused. The first seven caudal (tail) vertebrae are also present.
At least fourteen dorsal ribs are present, these being more robust and strongly bowed than in other Rhabdodontomorphs. Both scapulae are preserved, these being stout and robust, but within the range of other Rhabdodontomorph species. An incomplete right humerus is present, with most of the proximal portion preserved, with the exception of most of the deltopectoral crest, but the distal portion is missing. The right ulna is well preserved and undistorted. A portion of a radius is also present, though this is highly eroded and lacks both ends, making it unclear which side it came from. Four elements of a manus (hand) are present; these are poorly preserved and somewhat compressed, making identification difficult, although they may be two metacarpals and penultimate and ultimate (ungual) phalanges of digits 1, 2, or 3. Few early-branching Ornithopods have well preserved manus, making comparison between these elements difficult. The shaft and distal end of the left ischium are preserved; the shaft is generally straight, but has a twist towards the distal end. The right femur is preserved, generally in good condition, but with some erosion in the region between the greater and lesser trochanters.
Zanno et al. carried out several phylogenetic analyses, using a number of different matrices, which consistantly found that Iani smithi is closely related to the Rhabdodontomorphs and the non-Rhabdodontomorph Tenontosaurus, with all analyses bar one placing Iani smithi within the Rhabdodontomorphs.
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