Describer

Brusatte, Carr, Erickson, Bever & Norell 2009

Time

Cretaceous Late Maastrichtian

Classification

Saurischia Theropoda Tyrannosauria Tyrannosauridae Tyrannosaurinae

Diet

Carnivore

Fossilsite

Nemegt Formation - Nemegtskaya Svita, Tsagaan Khushuu (originally called Tsagaan Uul), Mongolia

Fall Under

Alioramus remotus

Typespecies

Alioramus remotus

Info

Alioramus remotus (Kurzanov, 1976) > Alioramus altai (Brusatte, Carr, Erickson, Bever & Norell 2009)

Abstract

Tyrannosaurid theropods are characterized by a generalized body plan, and all well-known taxa possess deep and robust skulls that are optimized for exerting powerful bite forces. The fragmentary Late Cretaceous Alioramus appears to deviate from this trend, but its holotype and only known specimen is incomplete and poorly described. A remarkable new tyrannosaurid specimen from the Maastrichtian (Late Cretaceous) of Mongolia, including a nearly complete and well-preserved skull and an extensive postcranium, represents a new species of Alioramus, Alioramus altai. This specimen conclusively demonstrates that Alioramus is a small, gracile, long-snouted carnivore that deviates from other tyrannosaurids in its body plan and presumably its ecological habits. As such, it increases the range of morphological diversity in one of the most familiar extinct clades. Phylogenetic analysis places Alioramus deep within the megapredatory Tyrannosauridae, and within the tyrannosaurine subclade that also includes Tarbosaurus and Tyrannosaurus. Both pneumatization and ornamentation are extreme compared with other tyrannosaurids, and the skull contains eight discrete horns. The new specimen is histologically aged at nine years old but is smaller than other tyrannosaurids of similar age. Despite its divergent cranial form, Alioramus is characterized by a similar sequence of ontogenetic changes as the megapredatory Tyrannosaurus and Albertosaurus, indicating that ontogenetic change is conservative in tyrannosaurids.

Holotype

Institute of Geology, Mongolia (IGM), Ulaan Baatar 100/1844 is a substantially complete skeleton found associated and belonging to a single individual that includes a nearly complete and disarticulated skull and is missing the forelimbs, regions of the hindlimbs, the pubes, and much of the dorsal and caudal vertebral series.

Etymology

Altai is in reference to the Altai mountain range, a prominent topographic feature of southern Mongolia.

Horizon and Locality

The specimen was collected in 2001 at Tsagaan Khushuu (originally called Tsagaan Uul). These beds are part of the Maastrichtian Nemegt Formation, which crops out extensively at a number of localities in the area. The type locality for A. remotus, Nogon-Tsav, is often referred to as a Nemegt equivalent, but detailed correlations have yet to be undertaken, and faunal differences suggest that Tsaagan Khushuu and Nogon-Tsav may be different in age.

Diagnosis

A. altai is a tyrannosaurid theropod possessing the following autapomorphies: an accessory pneumatic fenestra posterodorsal to promaxillary fenestra of maxilla; enlarged and elongated maxillary fenestra (length:depth ratio of 1.9); a laterally projecting horn on the jugal; a thick ridge on the dorsal surface of the ectopterygoid; a palatine pneumatic recess ex-tending posteriorly beyond the posterior margin of the vomeropterygoid process; 20 dentary alveoli; an anteroposteriorly elongate anterior mylohyoid foramen of the splenial; a thin epipophysis on the atlantal neurapophysis that terminates at a sharp point; a pneumatic pocket on the anterior surface of the cervical transverse processes; an external pneumatic foramina on the dorsal ribs; and an anterodorsally inclined midline ridge on the lateral surface of the ilium. Many of these features are present on elements not preserved in the holotype of A. remotus. A. altai is distinguished from the holotype of A. remotus, which is at approximately the same ontogenetic stage judging by the slight 3% difference in reconstructed skull length between the two specimens, by: s.c. flange on maxilla (the lateral surface of the maxilla extends dorsally to form a narrow slot between itself and the antorbital fossa below the ventral margin of the antorbital fenestra, which is absent in A. remotus); three lessdeveloped rugosities on the nasal (as opposed to six moreprominent rugosities in A. remotus); an anterior process of quadratojugal terminates posterior to the anterior margin of the lateral temporal fenestra; a squamosal anterior process that extends anterior to the anterior margin of the lateral temporal fenestra; and an epipterygoid not bifurcated ventrally. A. altai also possesses three differences with A. remotus that are sizerelated in other tyrannosaurids but may be significant given the similar size of the holotypes: 17 maxillary and 20 dentary alveoli (16 and 18, respectively, in A. remotus); a single dorsoventral groove between the basal tubera (groove bifurcated by ridge in A. remotus); and a tapering anterior process of the parietals overlapping frontals on the midline (larger, rectangular process in A. remotus). Although representing a juvenile animal (see Histological and Ontogenetic Analysis), the holotype of A. altai can be distinguished from juveniles of the contemporary Tarbosaurus by numerous characters. Namely, Tarbosaurus subadults have a deeper maxilla, a deeper tooth-bearing region of the maxilla, fewer teeth anterior to the antorbital fossa, a rounder maxillary fenestra, more closely spaced maxillary and promaxillary fenestrae, low and indistinct lacrimal horns, low nasal rugosities, a larger postorbital horn, and a considerably lower tooth count in the maxilla and dentary .

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