Gorgosaurus libratus: Two new juvenile specimens shed light on the ontogeny of a Late Cretaceous Tyrannosaurid Dinosaur.

Gorgosaurus libratus is one of the best known Tyrannosaurids, with dozens of skeletons known from the Late Cretaceous Dinosaur Park Formation of Alberta and Judith River Formation of Montana. This has enabled scientists to develop a good understanding of the Ontogeny (developmental changes during growth) of this Dinosaur. The species was first described in 1914 from a mature specimen from the Dinosaur Park Formation. A second species of Gorgosaurus, Gorgosaurus sternbergi, was subsequently described on the basis of a smaller, more slender skeleton from the same formation, though this was later recognised as a juvenile, rather than a separate species, the beginning of a long process of discovery about the life history of this Tyrannosaurid.

In a paper published in the Journal of Vertebrate Paleontology on 13 April 2022, Jared Voris and Darla Zelenitski of the Department of Geoscience at the University of Calgary, François Therrien of the Royal Tyrrell Museum of Palaeontology, Ryan Ridgely of the Department of Biomedical Sciences at Ohio University, Philip Currie of Biological Sciences at the University of Alberta, and Lawrence Witmer, again of the Department of Biomedical Sciences at Ohio University, describe two new juvenile specimens of Gorgosaurus libratus from the Dinosaur Park Formation, and the implications of these for our understanding of ontogeny in the species.

The first specimen described is TMP 2009.12.14, a juvenile Gorgosaurus libratus skeleton with an articulated skull, a partial vertebral columns with ribs, a pelvic girdle, and an articulated left pectoral girdle. The skull of this specimen is almost complete on the left side, but lacks the articular, epipterygoid, jugal, lacrimal, prearticular, quadrate, quadratojugal, squamosal, and surangular bones on the right side, as well as the unpaired ethmoid and orbitosphenoid bones, while the right angular, ectopterygoid, and post-orbital are preserved, but disarticulated from the skull.

Skull of MP 2009.12.14 in lateral view. Voris et al. (2022).

The second specimen, TMP 2016.14.1, is a partial skeleton with an  articulated skull, partial vertebral column with ribs, and pelvic girdle. In this case the skull is largely intact, although theright quadratojugal is disarticulated.

Skull of TMP 2016.14.1. in lateral view. Scale bar equals 10 cm. Voreis et al. (2022).

Together, TMP 2009.12.14 and TMP 2016.14.1 represent two of the most complete juvenile Gorgosaurus libratus specimens known, and considerably to our understanding of the ontogeny of this species. Tyrannosauroids in general are known to have undergone dramatic morphological changes as they grew. 

In Gorgosaurus libratus juveniles had narrow, shallow skulls with large circular orbits (eye sockets) and ziphodont teeth (flat, sharp teeth with serrated edges), as well as uninflated sinuses and little cranial ornamentation, while adults had wide, deep skulls with incrassate (thickened) teeth, p-shaped orbits, inflated sinuses, and prominent cranial ornamentation. 

The additional data provided by the new specimens enables a better understanding of when these changes took place. The proportions of juvenile Gorgosaurus libratus individuals seems to have remained fairly constant until they reached about 50% of their maximum size, when they began to grow much more rapidly, with their skulls becoming deeper, wider, and generally more robust. Once the skulls reached about 60% of their maximum size, other adult features, including changes to the shape of the bones around the orbits, thickening of the teeth, and the development of ornamentation. When the skull had reached 80% of its maximum size the relative increases in depth and width plateaued, with the skull maintaining the same approximate proportions for the rest of its growth, and the sutures of the braincase had all closed, while the sinuses expanded and the bones around the orbit began to reach their final structures. At 90% of maximum size the transformation appeared to be complete, with the final stages including the resorbtion of the extremities of the antorbital fossa and the expansion of the flange on the posterior of the dentary.

Comparison of the growth series of Gorgosaurus libratus and Tyrannosaurus rex, demonstrating similar ontogenetic stages (and morphologies) occurring at similar relative size (percent of largest specimen skull length) but different body sizes and biological ages. Voris et al. (2022).

The development of the cranial morphology during ontogeny has also been studied Tyrannosaurus rex, enabling direct comparison of these two closely related species. Both species entered a period of accelerated growth when they reached about 50% of their maximum size. In both species this period of accelerated growth was accompanied by a significant increase in the relative width and depth of the skull, as well as a general increase in robustness. Furthermore, both species developed incrassate teeth, nasal ornamentation and changes to the shape of the eye socket when they reached about 50% of their maximum size, with the eye socket shape continuing to change until the animal reached about 80% of its maximum size and these bones began to permanently fuse. However, while the general growth trajectory of the two species was generally similar, in Tyrannosaurus this occurred when the Dinosaur was both older and larger. A Gorgosaurus with a skull length of about 720 mm would have been about 14 years old, and have had features consistent with a young adult developmental stage, whereas a Tyrannosaurus the same age would have been 11-13 years old, with completely juvenile features. This suggests that the onset of accelerated growth in Tyrannosaurids was associated with maturity rather than absolute size, and that the larger size achieved by Tyrannosaurus was linked to both the delayed occurrence and increased length of this developmental stage.

Simplified comparison of ontogenetic trajectories of Gorgosaurus and Tyrannosaurus. Relative to the more basal morphology of Gorgosaurus, the delayed onset of similar ontogenetic changes in Tyrannosaurus coupled with its more hypermorphic features may suggest sequential hypermorphosis to have played a role in the evolution of this taxon. Voris et al. (2022).

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