Describer

Norell, Makovicky, Bever, Balanoff, Clark, Barsbold & Rowe 2009

Time

Cretaceous Late Maastrichtian

Classification

Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Troodontidae

Diet

Carnivore

Fossilsite

Nemegt Formation, Bugiin Tsav, Omnogov Aimag, Mongolia

Length

>2 meter | skull length 272 mm

Info

Zanabazar junior (Norell, Makovicky, Bever, Balanoff, Clark, Barsbold & Rowe 2009) > Saurornithoides junior (Barsbold, 1974)

Abstract

We review the morphology, taxonomy, and phylogenetic relationships of the upper Cretaceous Mongolian troodontid Saurornithoides. Saurornithoides mongoliensis is known only by the holotype from Bayan Zag, Djadochta Formation. This specimen includes a nearly complete, but weathered, skull and mandibles, a series of dorsal, sacral, and caudal vertebrae, and a partial pelvic girdle and hind limb. Saurornithoides junior, here referred to Zanabazar, also is known only by the holotype from Bugiin Tsav, Nemegt Formation.

This specimen consists of a skull and partial mandible, a series of sacral and caudal vertebrae, a partial pelvic girdle, and the distal part of the right hind limb. Saurornithoides + Zanabazar is one of the few Mongolian taxa known from both the Djadokhta and Nemegt formations. The monophyly of Saurornithoides + Zanabazar has not been seriously questioned historically, yet empirical support for this clade is currently tenuous. A privileged phylogenetic relationship between Saurornithoides, Zanabazar, and the North American troodontid Troodon formosus is supported by numerous characters including the presence of a subotic recess, lateroventrally projecting and hollow basipterygoid processes, a lacrimal whose anterior process is significantly longer than its posterior process, a highly pneumatized parasphenoid rostrum, a constricted neck of the occipital condyle, a symphyseal region of the dentary that is slightly recurved medially, and an obturator process located near the middle of the ischiadic shaft. CT data for the skulls of both species facilitated a description of the endocranial anatomy of Saurornithoides mongoliensis and Zanabazar junior, including a reconstruction of the endocranial space of Zanabazar junior. Despite being the largest of the known troodontid species, the endocranial volume of Zanabazar junior is considerably smaller than that estimated for Troodon formosus, suggesting that the extremely high encephalization quotient of Troodon formosus may be autapomorphic among troodontids.

Holotype

IGM 100/1, a skull and partial mandible, six sacral and 11 caudal vertebrae, and the distal part of the right hind limb. The skull is largely complete except for the left and right quadrate, quadratojugal, ventral part of the lacrimal, palatine, pterygoid, and ectopterygoid, and the left squamosal, postorbital, and jugal, posterior part of the right jugal, and the postdentary bones other than the splenials. The posterior part of the left nasal is reconstructed in plaster, as are small parts of the left maxilla anterior to the maxillary fenestra and dorsal to the interfenestral bar. Plaster covers the ventral surface of the bones roofing the antorbital fenestra. The mandible comprises only the dentaries and splenials. The right hind limb includes the distal end of the tibia with the coossified astragalus and calcaneum, a distal tarsal, and the proximal ends of MT II–IV.

Referred specimens

No new material of this species has been described, but two other troodontids have been described from the Nemegt Formation of Mongolia. Borogovia gracilicrus Osmolska, 1987, is known only from a partial hind limb from Altan Uul IV, and Tochisaurus nemegtensis Kurzanov and Osmolska, 1991, is known only from a metatarsus from the Nemegt area. Because the postcranial elements of Zanabazar junior do not include those parts diagnostic of the other two species, it is possible that one of the other two Nemegt Formation taxa is a junior synonym.

Etymology

In honor of Zanabazar (1635–1723), the first Bogd Gegen of Mongolia.

Locality and age

Nemegt Formation at Bugiin Tsav, Omnogov Aimag, Mongolia. The Nemegt Formation overlies the Djadokhta Formation and is considered to be Maestrichtian based primarily upon the vertebrate fauna (Jerzykiewicz and Russell, 1991).

Diagnosis

Differs from Saurornithoides mongoliensis and most other troodontids (save Troodon formosus) in lacking a small depression on the lateral wall of the braincase just dorsal to the trigeminal foramen. The holotype of Zanabazar junior is the largest published troodontid specimen (midline length of the skull 272 mm), and is substantially larger than most other troodontid species. Only Troodon formosus appears to approach it in size. Differs from Troodon formosus in having a more ovoid-shaped foramen magnum, a posttemporal fenestra that is enclosed by an almost equal proportion of the exoccipital/ opisthotic and squamosal (this same feature in Troodon formosus is formed almost entirely by the exoccipital/opisthotic), a deeper paroccipital process resulting in a different positon of the posttemporal fossa relative to the foramen magnum, and the lack of an osseous signature of the ophthalmic branch of the trigeminal nerve (CN V) in the laterosphenoid.

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