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55 3 Preliminary Analysis of a Sub-adult Tyrannosaurid Skeleton from the Judith River Formation of Petroleum County, Montana Walter W. Stein and Michael Triebold In 2002 we discovered an enigmatic theropod skeleton approximately 15 miles northwest of the town of Melstone, Montana, along the Petroleum/ Musselshell county line. Based upon the size, robustness, and interpreted stratigraphic position (the lower third of the Hell Creek Formation) of the exposed elements, the skeleton, at the time, was thought to be a sub-adult Tyrannosaurus rex. Recent, detailed geologic mapping in the area, however, places this site within the lower third of the Judith River Formation, and analysis of the recovered skeletal elements leave more questions than answers. Study continues to present. To date, approximately 20–25 percent of the skeleton has been recovered , including approximately 20–30 percent of the skull. Major elements of the skeleton consist of the left femur, both ischia, several cervical and dorsal vertebrae, thoracic and cervical ribs, and many gastralia. Important skull elements recovered include both dentaries, the right squamosal, left lachrymal, left postorbital, left ectopterygoid, left pterygoid, left quadratojugal , and a portion of the right jugal. The elements were completely disarticulated, poorly preserved, and encased in sideritic ironstone concretions , and they show strong evidence for both pre-depositional weathering and possible dispersal from scavenging or predation (tooth marks and shed tyrannosaur teeth). Recent analysis of the recovered elements shows many similarities to both Tyrannosaurus and Daspletosaurus, but neither is a perfect match. As a result, assignment to a specific generic taxon is not conclusive at this time. Three possibilities, however, exist: 1) The specimen represents a large-bodied new genus and species of Late Cretaceous tyrannosaur more closely aligned with Tyrannosaurus and Daspletosaurus than to the albertosaurines (Carr 2005) Albertosaurus, Gorgosaurus and Nanotyrannus. 2) The specimen represents a new, larger, more advanced species of Daspletosaurus 3) The specimen represents a very large individual, or robust sexual morphotype of Daspletosaurus torosus. Abstract Institutional abbreviations AMNH, American Museum of Natural History, New York; BHI, Black Hills Institute of Geological Research, Hill City, South Dakota; BMR, Burpee Museum of Natural History, Rockford, Illinois; FMNH, Field Museum of Natural History, Chicago; MOR, Museum of the Rockies, Bozeman, Montana; RMDRC, Rocky Mountain Dinosaur Resource Center, Woodland Park, Colorado; RTMP, Royal Tyrrell Museum of Palaeontology , Drumheller, Alberta; TCM, The Children’s Museum, Indianapolis, Indiana. Stein and Triebold 56 Further study of this specimen by skilled anatomists is necessary for final taxonomic classification. Enigmatic large theropod skeletons from the Late Cretaceous of North America are nothing new (Currie 2003; Carr and Williamson 2004). Paleontologists have been wrestling with the questions of taxonomic classification, sexual dimorphism, ontogenetic variation, and individual variation of tyrannosaurs ever since the first Tyrannosaurus rex tooth was discovered near Denver, Colorado, in 1874 (Carpenter and Young 2002). Since that lone tooth discovery, many skeletons, skulls, and isolated elements of these amazing creatures have been collected, described, studied, and fought over. The majority of these remains are often fragmentary and open to much debate. Whether it is the validity of genera such as Nanotyrannus (Bakker et al. 1988; Carr 1999; Larson 2013), Dinotyrannus (Olshevsky and Ford 1995), Aublysodon (Leidy 1868; Molnar and Carpenter 1989), or Tarbosaurus (Maleev 1955; Paul 1988; Currie et al. 2003) or whether to split Tyrannosaurus rex into two separate species or sexual morphotypes (Bakker, pers. comm., 2005; Larson 2008), the theoretical battles will continue to rage for years to come. That is the nature of the beast and the nature of dinosaur paleontology. Each year, however, additional material is discovered, and with this new material we get a better understanding of the taxonomy, evolutionary relationships, and variations of one of the most amazing groups of carnivores to ever walk the planet. Since we cannot travel back in time to answer the questions of variation, it is only through additional discoveries (such as this one) and large datasets that we get closer to the truth. In 2002, we discovered a new enigmatic tyrannosaurid skeleton in Petroleum County, Montana, and entered it into the pantheon of tyrannosaurid skeletons. This specimen, informally known as “Sir William” (RMDRC 2002.MT-001; also known as AMNH 30564, a single bone histology thin section), displays many interesting characters that make it quite unusual . Like many tyrannosaur specimens it inspires more questions than answers. This paper is an attempt to document the discovery, collection, geology, taphonomy, and preliminary taxonomic classification of the Sir William discovery so that others may become aware of its...

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