Could T. rex keep up with its kids? New research says ‘no’.

Researchers at the University of New England’s (UNE) Paleoscience Research Centre found that young tyrannosaurs were much faster than their parents, suggesting that the adults could have had actual trouble keeping up with their young. It likely all came down to the significant difference in body size between adults and juveniles, the team explains.

The findings are based on a collection of fossilized tyrannosaur footprints which helped record how these animals moved throughout their different developmental stages.

Bigger, harder, stronger, slower

“Fully grown tyrannosaurs were believed to be more robust than younger individuals based on their relatively shorter hind limbs and more massive skulls, but nobody 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,” said UNE PhD student and lead author of the paper, Nathan Enriquez.

“The results suggest that as some tyrannosaurs grew older and heavier, their feet also became comparably more bulky,” he adds, which would reduce their top speed.

There are a lot of elements that influence the final shape of a footprint. Things like soil composition and properties, the exact position of the animal as the print was made, the geography of the surface (and a lot of others) will influence the final shape that is imparted to a surface. Unsurprisingly, this makes interpreting footprints, especially fossilized ones, a very difficult process that’s fraught with pitfalls. Due to this, fossilized tracks haven’t been used extensively to understand dinosaur growth.

This set of footprints, however, from the Grande Prairie region of Northern Alberta, Canada, were found in very good condition and sported prints that belonged to individuals of the same species but different sizes.

“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,” says Enriquez.

“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.”

After establishing which of the prints were suitable for their research, the team analyzed their outline using an approach called geometric morphometrics. This was meant to look past the differences in overall size between the tracks, and spot the key differences in shape between these tracks.

The most important difference in shape they found was the width and surface area of the heel relative to the overall imprint size. This ratio was significantly lower in the smaller prints. The team explains that the smaller tracks were “slender”, while the larger ones were “broader” and had larger heel areas. This increase was needed as the animal increased in size as it aged, as their legs needed to be able to physically support their bulk, but it also suggests that older individuals weren’t able to reach the same speeds as their young.

“Increasingly bulky feet in the adults aligns 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,” Enriquez notes.

One of the most exciting parts of science, for me personally, is that if you understand how the different parts of a picture fit together, you can then draw conclusions from seemingly unrelated elements — such as judging how fast an animal was able to go based on its pawprints.

Since we don’t have many reliable sources of data regarding long-extinct species such as T. rex, any sliver of information we can get is priceless. The current paper offers up one such tidbit which will help us better understand how the dinosaur’s abilities and ways of life changed as it aged. Hopefully, such an approach will be refined in the future to make it more reliable and widely-applicable, and that it will be used on prints from other dinosaur species as well.

The paper “Exploring possible ontogenetic trajectories in tyrannosaurids using tracks from the Wapiti Formation (upper Campanian) of Alberta, Canada” has been published in the Journal of Vertebrate Paleontology.