Tyrannosaurid theropods are characterized by a generalized body plan, and all well-known taxa possess deep and strong skulls that are optimized for exerting powerful bite forces. deep within the megapredatory Tyrannosauridae, and within the tyrannosaurine subclade that also includes and is characterized by a similar sequence of ontogenetic changes as the megapredatory and is undoubtedly the most recognizable dinosaur. and several close relatives are grouped together within Tyrannosauridae, a derived clade of large-bodied theropods that were the apex predators in most terminal Cretaceous ecosystems in North America and Asia (1C3). Tyrannosaurids are characterized by a general body plan: All well-known species possess deep skulls, peg-like teeth, 649735-63-7 supplier and strong lower jaws as adults, features thought to relate to powerful bite causes and an unusual puncture-pull feeding style in which the teeth penetrate through bone (4C6). As such, these animals can be referred to as megapredators. One enigmatic tyrannosaurid, from the Late Cretaceous of Mongolia, appears to deviate from this trend (7). The fragmentary holotype and only known specimen seems to represent a long-snouted and gracile predator with an extreme degree of cranial ornamentation, but further details of its anatomy, biology, and phylogenetic relationships have proven controversial. Some phylogenetic analyses place deep within the radiation of Asian tyrannosaurids (8), whereas other studies find it as a basal taxon outside of Tyrannosauridae (1, 2; phylogenetic definitions following ref. 9). Furthermore, some authors have suggested that the holotype of is a juvenile and possibly an immature (3). These debates are difficult to resolve with the fragmentary holotype material. We report the discovery of a peculiar new tyrannosaurid from the Late Cretaceous of Mongolia that is known from a remarkably complete and well-preserved skeleton, including a disarticulated skull that allows for meticulous observation of tyrannosaurid cranial anatomy. This new tyrannosaur is small-bodied and possesses a bizarre long-snouted and gracile skull with eight discrete horns, an unusual cranial form for a Late Cretaceous tyrannosaurid. It is extremely similar to, and shares many unique characters with, the holotype of is a small, sleek, longirostrine tyrannosaurid, which lived alongside the megapredatory Kurzanov, 1976; (IGM 100/1844). (and 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) (10). 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 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 1.9); a laterally projecting horn on the jugal; a thick ridge on the dorsal surface of the ectopterygoid; a palatine pneumatic recess extending 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 (see is distinguished from the holotype of also possesses three differences with that are size-related 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 can be distinguished from juveniles of the contemporary by numerous characters. Namely, 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 (11, 12). Description and Comparisons. Until the discovery of was poorly understood because of a number of factors, including the fragmentary nature of the holotype, the fact that it has Rabbit Polyclonal to GPR174 649735-63-7 supplier never been completely described, and difficulties in gaining access to the specimen. The skull of is remarkably long and low compared with those of other tyrannosaurids, and the snout comprises 2/3 of the skull length (Fig. 1). Individual bones of the snout, most notably the maxilla, nasal, jugal, and dentary, are elongate, whereas those of the posterior skull roof, braincase, and circumtemporal series are similar in proportion to other 649735-63-7 supplier tyrannosaurids. Other than its overall shape, the most conspicuous features of the skull are several ornamental projections. These include, on each side of the skull, a lacrimal horn, a postorbital cornual process, and a ventrally projecting jugal cornual process, as is usual for tyrannosaurids. In addition, possesses an.