A new specimen of Torvosaurus tanneri originally collected by Elmer Riggs

Abstract

A new specimen of the theropod dinosaur Torvosaurus tanneri discovered by Elmer Riggs in 1899 in the Freezeout Hills of Wyoming and held in the Field Museum of Natural History is described. This specimen demonstrates that unreported material of this species has been present in museum collections 80 years prior to the species' scientific description and it likely represents the first non-dental material of this species to be collected. This material comprises parts of the left foot and right hand, including phalanges which were previously unknown for Torvosaurus, and substantiates the possibility that other Torvosaurus material may be undiscovered in museum collections. Its occurrence in a multitaxon quarry is consistent with other skeletal finds of Torvosaurus, all or most of which occur in association with other, more common Morrison dinosaur taxa.

Keywords::

• dinosaur
• Morrison Formation
• Late Jurassic
• Theropoda
• Megalosauridae

Acknowledgements

The authors thank William Simpson for finding this specimen during an inventory of unopened field jackets, for access to the records and field notes of Elmer Riggs, and for cataloguing the specimen; Akiko Shinya for preparation assistance and thin sectioning; Nathan Smith for constructive discussion; John Foster for input on Morrison Fauna; Paul Brinkman for input regarding Riggs' expeditions; Kenneth Carpenter for information on the Peterson Quarry Torvosaurus material; Matthew Carrano for populating the Paleobiology Database with data and two anonymous reviewers for their constructive criticism and comments.

Notes

^1.[email protected]

^2. Allain et al. (2012) referred this exceptionally large ungual (BYUVP 2020) to an indeterminate spinosaurid, reasoning that enlarged first-digit manual unguals are only known in spinosaurids and the specimen was not found in association with any other skeletal material that could point to its origin. They also suggested that the specimen should not be used to diagnose Spinosauroidea sensu Sereno et al. 1998 ( = Megalosauroidea). We do not follow this position and consider the ungual to belong to Torvosaurus for the following reasons: Allain et al. (2012) recovered Megalosauridae and Spinosauridae as sister taxa in their phylogenetic analysis in spite of their position on this specimen and in agreement with other studies (e.g. Benson 2010; Carrano et al. 2012). Furthermore, as discussed in this work, megalosaurid manual anatomy remains poorly understood. It is possible that the spinosaurid ungual morphology may have been more widely distributed in Megalosauria. Both Allain et al. (2012) and Carrano et al. (2012) recover Torvosaurus as either basal or sister to the clade containing Afrovenator and Dubreuillosaurus, which lacks the enlarged first-digit ungual. Given the current state of knowledge, it is still reasonable to view the spinosaurid condition a possible basal synapomorphy for Megalosauria, and its loss in other Megalosaurids as a derived condition. Finally, Allain et al. (2012) acknowledge that, in the absence of this ungual, no distinctly spinosaurid dental or skeletal material has been recorded from the Morrison Formation. Pending the discovery of such material, it is most parsimonious to regard the specimen as belonging to a taxon already known from the Morrison Formation's Brushy Basin Member, and Torvosaurus remains the best candidate.

^3. The holotype humerus of Torvosaurus tanneri belongs to a local accumulation of cranial, vertebral, pelvic and limb material from several individuals of the same species in the Dry Mesa Quarry. Galton and Jensen (1979) considered a set of left and right humeri, a right radius, and left and right ulnae (BYUVP 2002) to belong to a single individual and designated the assemblage as the holotype. Britt (1991) saw no reason to presume that the elements belonged to the same individual and designated the left humerus as the holotype, regarding the other bones as part of the paratype series.