It’s settled! Jane was a juvenile, not another species

Story by Greg Liggett, Montana State Paleontologist

A 70 year-old debate has recently been resolved--the dinosaur called "Jane" is a juvenile Tyrannosaurus rex and not a different species.

Jane was collected from BLM land in Carter County, Montana, by the Burpee Museum of Natural History in Rockford, Illinois. Histological work, looking at the microscopic growth patterns in Jane's leg bones, clearly shows that Jane was still growing when she died.

BLM research partner Holly Woodward from Oklahoma State University and her colleagues published these findings on New Year’s Day 2020.

The controversy began way back in 1942 when paleontologists from the Cleveland Museum of Natural History were exploring in southeastern Montana. Among their collection that summer were an unusual skull and jaw that clearly belonged to a meat-eating dinosaur, but it was far smaller than T. rex, the only other large meat-eater from that area. The new skull was given the name “Nanotyrannus,” or the “very small tyrannosaur.”

Did the small skull come from a different species, or could it be a juvenile T. rex? Researchers pointed out differences between known adult T. rex skulls and the little one, like differences in the number of teeth and skull proportions. So, the issue remained an open question.

Fast forward to 2002, when researchers found the best skeleton of “Nanotyrannus” on BLM land in Montana. Not long after, another specimen was found, and each of these specimens had more than just skulls—they had limb bones, which turns out to be very fortunate. If it could be demonstrated that the “Nanotyrannus” specimens are juveniles, that would be strong support for them actually being juvenile T. rex. But how do you age a dinosaur?

Turns out that in the last 15 years scientists have figured that out. By cutting a slice out of the limb bone of a dinosaur and grinding it down until it is so thin that light can pass through it, amazing details of the bone’s structure can be observed, including tell-tail signs of growth. Very much like a tree-ring, rings of growth observed in the bone are laid down in an annual cycle, so you can count the rings to see how old the animal was when it died.

The results just published show that one of the specimens of “Nanotyrannus” died at about 13 years of age, and the other at 15, and both were only half the size of an adult. From studying the “big guys,” it is known that T. rex reaches full size after about 20 years. This and other lines of evidence have convinced most paleontologists that “Nanotyrannus” specimens are actually juvenile T. rex specimens. And the considerable size difference between juveniles and adults points to very different lifestyles between the two groups. Juveniles could not deliver the tremendous bone-crushing bites that their elders could provide, indicating that they likely fed differently over their lifetimes.

However, the young specimens contain even more exciting potential for telling us about the life of this iconic dinosaur. One of the limb bones appears to contain a tissue type that is typical of egg-laying modern birds. If that is the case then we can confidently say that the specimen is a female, and at what age T. rex reached sexually mature.

BLM works closely with our partner researchers working on BLM fossils, and the contribution to science of fossils from public lands cannot be overstated.