Tiny T.Rex Fossils Are Teenagers Not "Dwarf" Species, Study Concludes

Young T. rex fossils are a rare find, possibly because juvenile mortality rates were low thanks to no one messing with Mama. Herschel Hoffmeyer/Shutterstock

A decades-long debate about whether two tiny T.rex fossils indicated a couple of teenage tyrannosaurs or an entirely new pygmy species appears to have come to an end with a new study that claims they were definitely juveniles and the elusive dwarf T. rex never existed.

Tiny of course is relative. When a full-grown adult reaches around 12.3 meters (40 feet) in length and stands up to 6 meters (20 feet) tall, a 6-meter-long (20-foot) version can arguably be considered small. This is what confused palaeontologists when two diminutive dinos were found in the Hell Creek Formation that spans Montana, Wyoming, and the Dakotas in the early 2000s.

Jane, a nearly complete fossil, and Petey, as they have been named by the Burpee Museum of Natural History in Illinois where they are housed, are about the size of a draft horse and twice as long; half the size of an adult Tyrannosaurus rex.

Many specialists assumed this meant they were adolescents and hadn’t experienced the growth spurt yet that would turn them into fully-fledged adults. However, some argued they could be examples of a proposed pygmy species of T. rex put forward in the late 1980s after palaeontologist Robert Bakker and colleagues from the Cleveland Museum of Natural History reclassified one of their specimens, declaring it a new species and naming it Nanotyrannus – "tiny tyrant".

However, a team led by Holly Woodward from Oklahoma State University have performed a new microanalysis on the bones of both, confirming in a paper published in Science Advances that they were adolescents not adults, and that by clarifying this, it is unlikely Nanotyrannus existed as Bakker’s specimen was most likely an adolescent too.

Finding young T.rex fossils is rare, possibly because they were so successful their juvenile mortality rate was low. Only five relatively complete ones have been found in the last 100 years. The last, in 2018, was described as a “1-in-a-million” find. Some of them are in private collections (having been sold on eBay) so are unavailable to study.

"Everyone loves T. rex, but we don't really know much about how it grew up," Woodward told AFP. "It's probably the most famous dinosaur in the world, and we mostly just have really large skeletons of it."

By slicing into the leg bones of the two specimens and studying them under microscopes, they were able to glean by the size of the blood vessel openings that the two dinosaurs were still in a period of rapid growth when they died.

They could also work out the animals’ ages by counting the growth rings in the bones. All vertebrates have a period every year when bone growth pauses, leaving a handy circle in the bone that we can count, just like using tree rings to work out the age of a tree.

They dated Jane to be 13 years old, and Petey 15, when they died, weighing around a ton each. It's thought T. rexes reached adulthood at 20 and could live for up to 30 years. 

The study lends more evidence to the hypothesis T. rex went through a phase of rapid growth that these two hadn't reached yet, which would take them from 1 ton to 10 tons in just a few years. This may also explain why so few specimens are found of this age.

Alas, this means the existence of the elusive tiny tyrant is unlikely.

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