Friday 8 May 2015

Saurornitholestes sullivani: A new species of Dromaeosaurid from the Late Cretaceous of New Mexico.

The Dromaeosaurid (a type of small Theropod Dinosaur closely related to Birds and Troodontids) Saurornitholestes langstoni was first described from the Late Cretaceous Judith River Formation in Dinosaur Provincial Park in Alberta in 1978, and has subsequently also been found in the Two Medicine Formation of northwestern Montana. In 2000 a frontal bone (part of the skull) from the De-na-zin Member of the Kirtland Formation from the Late Cretaceous ofthe San Juan Basin in New Mexico was assigned to Saurornitholestes langstoni (the frontal bones of Theropod Dinosaurs are considered to be highly distinctive, and can often be assigned to species level). In 2006 a second frontal was found in the De-na-zin Member, and described as a second species of Saurornitholestes, Saurornitholestes robustus. However subsequent studies have called into doubt the validity of Saurornitholestes robustus, and suggested that it cannot be confidently assigned to species or even genus level due to poor preservation, and may not even be a Dromaeosaurid.

In a paper published in the New Mexico Museum of Natural History andScience Bulletin in 2015, Stephen Jasinski of the Section of Paleontology and Geology at the State Museum of Pennsylvania, the Department of Earth and Environmental Science at the University of Pennsylvania and the DonSundquist Center of Excellence in Paleontology, re-examines the original New Mexico Saurornitholestes specimen and concludes that while it does belong in the same genus as the Canadian material it does not belong to the same species.

Jasinski redescribes the specimen as Saurornitholeste ssullivani, in honour of palaeontologist RobertSullivan, who discovered the specimen, for his work on the Late Cretaceous deposits of New Mexico. The frontal of Saurornitholestes sullivani is similar to Saurornitholestes langstoni in being triangular and lacking a basin between the median suture and the orbital rim (a feature seen in the closely related Bambiraptor), and having a well-rounded, slightly inflated posterior, lacking a frontoparietal crest. However it differs from Saurornitholestes langstoni in being constricted at its anterior end, having less prominent nasal sutures, and a less prominent anterior projection between the nasal and lacrimal facets, having a deeper but less strongly demarked orbital rim, having a more pronounces and longer ventrally-directed ridge between the olfactory bulb surface and the cerebral hemisphere surface, having a smaller and more robust frontal-frontal suture, and generally being smaller and more robust. Saurornitholestes sullivani is also slightly younger than Saurornitholestes langstoni, being 73 million years old, or possibly a little younger, while Saurornitholestes langstoni is dated to around 75 million years ago.

Saurornitholestessullivani (SMP VP-1270, holotype), nearly complete left frontal; photographs to the left and illustrations to the right.(A) dorsal view; (B) ventral view; (C) medial view; (D) anterior view; (E) left lateral view; and (F) posterior view. (A)and(B) anterior is up; (C) anterior to right; and (E) anterior to left. Abbreviations: ch, cerebral hemisphere surface; cs, convex surface between olfactory bulb surface and cerebral hemisphere surface; laf, lacrimal facet; naf, nasal facet; nf , nutrient foramen (or foramina); obs, olfactory bulb surface; or, orbital rim ; pop, postorbital process (peduncle); pp , parietal process (peduncle); and sas, sagittal articular surface. Scale Bar is 1 cm. Jasinski (2015).

Jasinski also notes that an isolated tooth and an incomplete left second pedal ungula were also referred to ‘Saurornitholestes robustus’. However these were not collected with the specimen and are not in themselves considered diagnostic. A survey of isolated teeth from the Kirtland Formation of New Mexico suggests that more than one Dromaeosaurid species was present in the area at the time, so these specimens should not be assigned to any particular species.

See also…

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