Allometric growth and intraspecific variation of the craniomandibular bones of <i>Tarbosaurus bataar</i> (Theropoda, Tyrannosauridae): a geometric morphometric approach

Since it was first described in 1955, many fossils of the tyrannosaurid theropod Tarbosaurus bataar have been recovered from the Upper Cretaceous Nemegt Formation of Mongolia and its contemporaneous units in east Asia. Among these, there were individuals of different sizes and stages of maturity, but not much research has been done on the changes that occurred during the growth of this dinosaur. In this work, growth trajectories in the shape of various, individual craniomandibular bones of Tarbosaurus bataar are examined through geometric morphometrics. Several major changes in craniofacial anatomy are observed through the growth series, including increases of the relative heights of the dentary, jugal, maxilla, and nasal; the transition of the lacrimal from a T-shape to a 7-shape; negative allometric growth in the anteroposterior length of the orbit; increased sizes of the cornual processes of the postorbital and the ventral flange of the jugal; broadening of the frontal accompanied by an enlargement of the dorsotemporal fossa; and widening and thickening of the nuchal crest so that the midlength of the parietal appears relatively narrower. Such results indicate the main allometric shape change patterns in craniomandibular anatomy of Tarbosaurus bataar were broadly congruent with those of other tyrannosaurids, particularly with Tyrannosaurus rex. It is assumed that many of these changes were related to disproportionate increase of the bite forces and strengthening the skull structure during growth. Additionally, a significant amount of variation appears to be uncorrelated with size, suggesting that Tarbosaurus bataar, like other theropod dinosaurs, had significant intraspecific variation in craniomandibular anatomy.