Young Tyrannosaurs Had More Slender Feet than Adults, Study Finds
An international team of paleontologists has examined a collection of 72.5-million-year-old tyrannosaur — probably Albertosaurus — footprints from the Wapiti Formation of Alberta, Canada, to learn more about the way these predatory dinosaurs aged and how they moved.
Fossil footprints have long been used to infer aspects of the biology of non-avian dinosaurs that are not readily apparent from skeletal remains, including estimated speeds and inferred social behaviors.
The footprints can be ambiguous and hard to interpret correctly — the shape of a footprint may be influenced by the type of ground surface that is stepped on and the motions of the animal making the footprints. In addition, the exact identity of the animal may not always be clear.
These challenges have previously limited the use of fossil footprints in understanding dinosaur growth.
The answer lay in the Grande Prairie region of Alberta in Canada, where University of New England Ph.D. student Nathan Enriquez and his colleagues worked with well-preserved samples of footprints of different sizes that are suggested to belong to the same type of animal.
“We explored a remote dinosaur footprint site where we discovered a new set of large carnivorous dinosaur footprints within very similar rocks to those which have produced tyrannosaur tracks in the past,” Enriquez said.
“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.”
“We were also careful to assess the quality of preservation in each footprint, and only considered specimens which were likely to reflect the shape of the actual feet that produced them.”
Once the paleontologists had a suitable sample, they analyzed the outlines of each specimen using a method called geometric morphometrics.
This process removes the effect of overall size differences between each footprint and shows what the most important differences in track shape are.
“The greatest difference in shape was found to be the relative width and surface area of the heel impression, which significantly increased in size between smaller and larger footprints,” Enriquez said.
“The smaller tracks are comparably slender, while the biggest tyrannosaur tracks are relatively broader and had much larger heel areas.”
“This makes sense for an animal that is becoming larger and needs to support its rapidly increasing body weight. It also suggests the relative speed of these animals decreased with age.”
“Increasingly bulky feet in the adults align with previous suggestions that juvenile tyrannosaurs would have been faster and more agile for their body size in comparison to their parents, and means that we can add footprints as another line of evidence in the debate over tyrannosaur growth.”
“Lastly, it demonstrates the usefulness of footprints for investigating a potentially wider range of ideas about the lives of extinct species than has been considered previously.”
The findings were published in the Journal of Vertebrate Paleontology.
Nathan J. Enriquez et al. Exploring possible ontogenetic trajectories in tyrannosaurids using tracks from the Wapiti Formation (upper Campanian) of Alberta, Canada. Journal of Vertebrate Paleontology, published online April 21, 2021; doi: 10.1080/02724634.2021.1878201