New research by an international team including paleontologists at the University of Alberta and the Philip J. Currie Dinosaur Museum has revealed evidence that juvenile tyrannosaurs were more trim and slender-bodied than their multi-tonne elders, a difference that may have helped them pursue fast-moving prey.
“The results suggest that as some tyrannosaurs grew older and heavier, their feet also became comparably more bulky,” said Nathan Enriquez, lead author on the study and a PhD student at the University of New England (UNE) in Armidale, Australia.
The researchers analyzed a collection of fossil tyrannosaur footprints to learn more about how the huge carnivorous dinosaurs moved and how their feet were shaped. The study was a collaboration between UNE, the University of Bologna, Alberta-based Cutbank Palaeontological Consulting, the Philip J. Currie Dinosaur Museum and the University of Alberta, including co-author Corwin Sullivan, associate professor in the Department of Biological Sciences. The findings confirm and build on previous research that has looked at tyrannosaur growth using bone anatomy and computer models of muscle mass.
“Fully grown tyrannosaurs were believed to be more robust than younger individuals based on their relatively shorter hind limbs and more massive skulls,” said Enriquez. “But until now, no research had explored this growth pattern using fossil footprints—which are unique in that they can provide a snapshot of the feet as they appeared in life, with outlines of the soft, fleshy parts of the foot that are rarely preserved as fossils.”
Over multiple summers of fieldwork in the Grande Prairie area, supported logistically by Grande Prairie Regional College, the research team documented multiple well-preserved tyrannosaur footprints. These proved similar to tracks previously discovered by paleontologists working in the same part of Alberta, as well as just across the border in British Columbia, leading the scientists to conclude that they likely belonged to the same type of dinosaur.
“Based on the relatively close proximity between these discoveries and their nearly equivalent ages—about 72.5 million years old—we suggest they may indeed belong to the same species,” said Enriquez.
Once the team had a suitable sample, they analyzed the outline of each specimen using a method called geometric morphometrics. This process uses computer imaging to scale the footprints to the same size, clarifying the most important variations in track shape. Crucially, the new tracks included a trackway made by a small, almost certainly immature individual, with a footprint about 40 centimetres long. The other tracks varied from nearly 50 cm to more than 60 cm long, and were probably made by adult or nearly adult tyrannosaurs.
“We can’t say exactly what kind of tyrannosaur produced these tracks,” said Sullivan, who is also Philip J. Currie Professor of Vertebrate Palaeontology and curator of the Philip J. Currie Dinosaur Museum. “However, a good guess would be Albertosaurus, a smaller cousin of the infamous Tyrannosaurus rex. Albertosaurus is about the right size and is known from rocks of about the same age in southern Alberta, so it’s not a huge stretch to imagine it in Grande Prairie.”
The team found that the width and area of the “heel” impression of each track was proportionally greater in larger footprints than smaller ones.
"The smaller tracks are comparably slender, while the biggest tyrannosaur tracks are relatively broader and had much larger heel areas,” said Enriquez. “This makes sense for an animal that is becoming larger and needs to support its rapidly increasing body weight.”
The findings also suggest that the running ability of these animals declined somewhat with age. Younger, smaller individuals were likely able to move faster, relative to their own body size, and with greater agility. Although previous studies of tyrannosaur bones and muscles have reached similar conclusions, the researchers said their new evidence from footprints reinforces and fleshes out the case that tyrannosaurs became somewhat bulkier and less athletic as they matured.
The study, “Exploring possible ontogenetic trajectories in tyrannosaurids using tracks from the Wapiti Formation (Upper Campanian) of Alberta, Canada,” was published in the Journal of Vertebrate Paleontology.