Cranial ontogenetic variations in Mapusaurus roseae (Dinosauria, Theropoda) and the probable role of hetereochrony in carcharodontosaurid evolution

CANALE, JUAN IGNACIO; NOVAS, FERNANDO EMILIO; SALGADO LEONARDO; CORIA, RODOLFO

Ituiutaba

Simposio; 1º Brazilian Dinosaur Symposium; 2013

Universidade Federal de Uberlandia

Mapusaurus roseae is one of the better-known carcharodontosaurid theropods, represented by disarticulated bones corresponding to at least seven individuals of different ontogenetic stages. It comes from Huincul Formation (Cenomanian) from Neuquén Province, Argentina. Some repeated skull elements of Mapusaurus (i.e., maxillae, lacrimals, jugals, dentaries and teeth) are amenable for allometric studies. Available postcranial bones do not show substantial differences and are not considered in this study. A suite of morphological differences is verified when comparing smaller with larger specimens (assumed as representative of younger and older specimens, respectively). We hypothesize that the skull of Mapusaurus manifested the following changes during ontogeny: 1) increase in ornamentation of lateral surfaces of maxilla, lacrimal, and dentary; 2) obliteration of the maxillary fenestra; 3) ventral sector of antorbital fossa becoming not visible in lateral view; 4) anteroposterior widening of the postorbital process of jugal; and 5) decrease in number of tooth serrations. Not fully-grown specimens of Mapusaurus resemble basal allosauroids (e.g., Allosaurus, Concavenator, Acrocanthosaurus). On the contrary, bigger specimens of Mapusaurus exhibit derived features (e.g., strongly ornamented facial bones) that are only documented in more derived carcharodontosaurids among Allosauroidea; such as Tyrannotitan and Giganotosaurus. This morphological pattern is consistent with a peramorphic ontogenetic process. A heterochronic peramorphic trend is proposed for the evolution of Allosauroidea, which is reliable with the extremely large body sizes recorded in derived members of the group. Gigantic body sizes have been usually observed in clades where peramorphosis predominates over paedomorphosis, such as in tyrannosaurid coelurosaurs, or outside Therpoda, among diplodocine sauropods: in these "peramorphoclades" usually occurs through hypermorphosis, acceleration, or a combination of both processes. We conclude that heterochrony is the general trend in the evolution of skull morphology in Allosauroidea, which is revealed by the reduction of craniofacial pneumaticity (by loss of maxillary fenestra) and an increase in bone ornamentation.