Juvenile Gorgosaurus Skulls Shed Light on Tyrannosaurid Growth Patterns

By | April 15th, 2022|Adobe CS5, Dinosaur and Prehistoric Animal News Stories, Dinosaur Fans, Main Page, Palaeontological articles, Photos/Pictures of Fossils|0 Comments

Researchers have described two beautifully preserved skulls of juvenile Gorgosaurus dinosaurs (G. libratus). The articulated specimens have enabled the scientists to build up a comprehensive picture of how these tyrannosaurids changed as they grew. Gorgosaurus and the much bigger, later tyrannosaurid T. rex exhibit similar changes in their skulls as they grow. This study will help palaeontologists to decipher tyrannosaur material and to determine the identity of previously misidentified specimens. It should also provide more evidence to help resolve the Nanotyrannus/T. rex debate.

Lateral views of juvenile Gorgosaurus skulls.
Skulls of the two new juvenile Gorgosaurus libratus specimens in lateral view. A, TMP 2009.12.14; B, TMP 2016.14.1. Note scale bar equals 10 cm. Picture credit: Voris et al.

Gorgosaurus libratus

Named and described in 1914 (Lambe), Gorgosaurus is known from dozens of fossil specimens found in the upper Campanian Dinosaur Park Formation of Alberta and Judith River Formation of Montana. It is one of the best sampled and researched of all the Late Cretaceous tyrannosaurs, but juvenile material is rare. The recent discovery of additional juvenile Gorgosaurus libratus specimens from the Dinosaur Park Formation, including two well-preserved skeletons with articulated skulls, provided researchers which include Jared Voris and Darla Zelenitsky (University of Calgary), along with collaborators from the University of Ohio, the University of Alberta and the Royal Tyrrell Museum, an opportunity to develop a map outlining how this dinosaur changed as it grew and matured.

Juvenile and adult Gorgosaurus skulls compared.
Illustrations of juvenile (left) and adult (right) skulls of Gorgosaurus in lateral (top) and dorsal views (bottom). Arrows and numbers indicate ontogenetically invariant autapomorphies of Albertosaurinae and Gorgosaurus as per emended diagnosis. Juvenile illustration based on TMP 2016.14.1 (lateral) and TMP 2009.12.14 (dorsal), adult illustration based on UALVP 10. Picture credit: Voris et al.

Sorting out Daspletosaurus Specimens

The research team, which also included Professor Phil Currie (University of Alberta), found that although the skulls of tyrannosaurs changed dramatically as they grew, several taxonomically informative traits remain present regardless of the age of the animal. This means that palaeontologists can use this information to determine which taxon is represented by juvenile fossil material.

Thanks to this research, two specimens previously identified as examples of immature Daspletosaurus individuals (coeval with Gorgosaurus) are instead confirmed as Gorgosaurus.

Gorgosaurus dentaries compared
Left dentaries in lateral view of A, small juvenile (TMP 1994.12.155); B, juvenile (TMP 2016.14.1); C, subadult (TMP 1991.36.500); D, young adult (ROM 1247); and E, adult (TMP 1967.9.164) specimens of Gorgosaurus libratus. Note the development of the autapomorphic dorsoventral expansion in the posterior region of the bone through ontogeny. Scale bar equals 10 cm. Picture credit: Voris et al.

Comparisons with Tyrannosaurus rex

The team also found that both Gorgosaurus and T. rex underwent similar anatomical changes over their lifespans, but at different times. The changes started later in Tyrannosaurus rex and occurred over a longer time interval, resulting in a larger size and longer lifespan for T. rex when compared to Gorgosaurus.

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

Implications for Nanotyrannus

Having identified a series of anatomical traits that can be relied upon to permit palaeontologists to confidently assign juvenile tyrannosaur skull fossils to a specific taxon, this allows some specimens considered small or “dwarf” forms such as Nanotyrannus (N. lancensis) to be revisited. Some of these specimens may have been misidentified, since key characteristics may not have developed in young individuals before death, but this new data set would allow closer scrutiny of the fossil material.

Nanotyrannus lancensis skull replica.
A cast of CMNH 7541 skull of Nanotyrannus lancensis (lateral view). The shallow and proportionately narrower skull assigned to N. lancensis may represent a juvenile T. rex. This new study will help scientists to determine the taxon of represented by juvenile fossil material. Picture credit: S. Anselmo.

The scientific paper: “Two exceptionally preserved juvenile specimens of Gorgosaurus libratus (Tyrannosauridae, Albertosaurinae) provide new insight into the timing of ontogenetic changes in tyrannosaurids” by Jared T. Voris, Darla K. Zelenitsky, François Therrien, Ryan C. Ridgely, Philip J. Currie and Lawrence M. Witmer published in the Journal of Vertebrate Paleontology.