New evidence that Nanotyrannus was a juvenile T. rex

Skeleton of the probable T. rex nicknamed Jane (Photo courtesy Zissoudisctrucker)
Skeleton of the probable T. rex nicknamed Jane (Photo courtesy Zissoudisctrucker)

An article published in the journal “Science Advances” reports a study that offers an analysis of the fossils of what was classified as Nanotyrannus offering evidence that actually they’re young specimens of Tyrannosaurus rex. A team of researchers led by Dr. Holly Woodward of the Oklahoma State University Center for Health Science conducted a paleohistological examination on bone fragments of the specimens nicknamed Jane and Petey concluding that fibers and other microstructures were growing. According to their estimates, Jane was around 13 years old at the time of death while Petey was around 15 years old.

The history of the fossils at the center of a series of studies that led to various changes in their classification and which could now be considered T. rex juveniles began in 1942, with the discovery of a skull that was described in 1946 and later again in 1988, leading to classification as Nanotyrannus lancensis. In 2001, a much more complete skeleton was discovered cataloged as BMRP 2002.4.1 and nicknamed Jane with similar characteristics, which was attributed to the same species. In subsequent years, various studies tried to establish whether it was really a valid species or the two specimens were T. rex juveniles, also thanks to the discovery of another skeleton, cataloged as BMRP 2006.4.4 and nicknamed Petey.

To search for new information that offered answers on the nature of the skeletons nicknamed Jane and Petey, Dr. Holly Woodward and her collaborators conducted a paleohistological examination, meaning a histological examination on fossils and not on live tissues, to study the two specimens’ bone microstructures. The examination is based on the fact that bones, like trees, grow forming annual rings that allow to estimate the age of the individual, in this case a dinosaur. Studies on T. rex indicate that they reached their adult size at around 20 years while Jane and Petey’s ages at the time of their death were estimated at least 13 and 15 years respectively.

An interesting discovery is that Jane and Petey’s bone rings show a different growth rate year by year. This suggests that T. rex had a growth rate directly proportional to the amount of food they available to them.

The examination also showed characteristics of the bone fibers typical of animals during their growth phase, which in the case of Jane and Petey seems to be a rapid phase. In contrast, some typical bone characteristics of adult individuals are absent.

This study suggests that T. rex grew at a rate similar to that of modern warm-blooded animals. Jane and Petey’s characteristics suggest that when they were young they were agile and fast predators with sharp knife-like teeth that could cut their prey’s flesh. As adults, T. rex based their hunting more on the power of their bite, which according to recent studies could pulverize their prey’s bones.

The results of this study could offer a final answer to the doubts that already existed on Nanotyrannus. Dr. Holly Woodward and her collaborators intend to continue their studies to better understand the growth of T. rex. The classification of some fossils such as Nanotyrannus was due precisely to the lack of knowledge of how T. rex changed during its growth but now perhaps paleontologists will be able to better understand this iconic dinosaur.

Artist's concept of juvenile T. rex (Image courtesy Julius T. Csotonyi)
Artist’s concept of juvenile T. rex (Image courtesy Julius T. Csotonyi)

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