Albertosaurus libratus, Lambe, 1914

Carr, T. D., 1999, Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Coelurosauria), Journal of Vertebrate Paleontology 19 (3), pp. 497-520 : 508

Ontogenetic Variation in A. libratus

The growth series of A. libratus is divided into three stages; based on the optimal distribution of ontogenetic character states, the growth stages may be characterized as follows. Stage 1 specimens are the least mature, and display nascent ontogenetic states. Stage 2 specimens are typified by the presence of large marginal maxillary neurovascular foramina, a depressed interfenestral strut, an oblique caudal lacrimal suture of the jugal, a postorbital situated dorsal to the orbit floor, a spheroid occipital condyle, a deep surangular, a large and asymmetrical caudal surangular foramen, a lacrimal cornual process with one apex, a dorsolateral lamina of the lacrimal that is as deep as the antorbital fossa, a ventrally oriented and wide oval scar of the basisphenoid, and a deep scar ventrolateral to the glenoid of the surangular. Also, Stage 2 specimens may exhibit nascent Stage 1 features.

Stage 3 specimens are characterized by an antorbital fenestra in which the height approaches the length, an expansive rostrolateral surface of the maxilla, a convex rostral margin of the rostroventral lamina of the lacrimal, a deep maxillary process of the jugal, and elongate rostral processes of the dorsal ramus of the lacrimal. In Stage 3, all small Stage 1 features are transformed but large Stage 1 and Stage 2 features may be unmodified.