Analysis of juvenile T.rex fossils has given researchers a better understanding of the life history of the world’s most famous dinosaur, including the fact that—like humans—they didn’t reach adulthood until around 18 years of age. Although drastic differences occurred as juvenile T.rex matured, the late adulthood did not stop the species from dominating the food chain thanks to their ability to niche partition.
Studying juvenile T.rex fossils collected from Montana in the early 2000s, Holly Woodward, professor of anatomy at Oklahoma State University Center for Health Sciences, catalogued the differences between juvenile and adult T.rex. The juvenile fossils, nicknamed Jane and Petey, were between 13 and 15 years old, with a much lighter build than an adult T.rex. Jane and Petey’s jaws were not able to crush bone, like their adult counterparts could. Instead, their skull was better adapted to cutting flesh and precision bites, which implies a different method of feeding than from adult T. rex—who could eat bone and flesh in one bite.
In fact, juvenile T.Rex bones present so differently from adult ones that paleontologists, including Woodward, think there are more in museum collections but they have been overlooked or assigned to a different species because they are so small and not immediately recognized.
King at all ages
Woodward was able to determine the ages of Jane and Petey by counting the annual growth rings in their leg bones, not unlike counting tree rings. When doing this, she noticed something interesting—the rings were inconsistently spaced, meaning some years Jane and Petey did a lot of growing, while other years they did not.
“We think the amount of growing, and the space between the rings, has to do with food resource abundance,” Woodward explained to Laboratory Equipment. “During years with low abundance, the juvenile T. rex grew for fewer months than in times of abundance food resources. In this way, juvenile T. rex could maintain a certain body size for a period of time and take advantage of the resources in that niche.”
This phenomenon suggests T.rex were efficient at niche partitioning, or the process by which natural selection drives competing species into different patterns of resource use. Successful niche partitioning allowed T.rex, regardless of age or size, to rule all ecological niches in its environment.
Further research
Due to their small size, there had been speculation that Jane and Petey weren’t T.rex at all, rather the “nanotyrannus” species, a proposed pygmy relative. Supporters of nanotyrannus say the bones are different enough from T. rex that it represents a different species, whereas Woodward and most T. rex researchers argue that the differences in morphology are just due to juvenile features. By examining the microstructure of bone tissue—leg bones in this case—using a polarized petrographic microscope, Ballad and her team were able to confirm Jane and Petey’s T.rex origin.
“Bone fibers are birefringent in polarized light, and the degree of birefringence can tell us about bone tissue organization,” explained Woodward. “The more organized the tissue, the slower the growth. We found the tissue of the small tyrannosaurs to be disorganized and less birefringent, implying the fast growth associated with juvenile animals.”
Published in Science Advances, Woodward et. all’s paper is the first to use bone histology to assess juvenile T.rex. It is the first paper published out of a larger project to examine the entire life history of T.rex.
“Specifically, we are trying to histologically sample as many T. rex specimens as possible, from as many growth stages as possible, to better understand T. rex growth throughout its entire life, and also to understand how much individual variation in growth was present,” concluded Woodward.