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How to know if a fossil belongs to a hatchling or an adult? The answer is not simple if we take into account that the shape of many animals changes as they grow. The discovery of a strange little tyrannosaurus skull in 1942 led some paleontologists to think that they were dealing with a pygmy species, which was named Nanotyrannus. A study published today in the journal Science Advances ensures that these specimens were nothing more than Tyrannosaurus rex teenagers.
Suspicions about Nanotyrannus existence they are not new. "As early as 1965 it was proposed that the skull belonged to a juvenile T. rex," explains Holly Woodward, a researcher at Oklahoma State University and co-author of the work, to Sinc. Further studies in 2004 led to the pygmy tyrannosaurus being 'promoted' to king.
"Today, most experts agree that Nanotyrannus is a T. rexjuvenil," Woodward clarifies, "but some defenders [of the existence of a dwarf species] have continued to publish arguments based on the shape of the bones."
To settle the matter, the paleontologist examined the fossils of two supposed specimens of Nanotyrannus under a microscope, which were called Jane and Petey, found in the State of Montana at the beginning of the century.
“Bones keep track of an animal's growth and can tell us how fast it grew throughout its life and its age when it died.”.
Microscopic examination revealed that Jane and Petey - 13 and 15 years old, respectively - they were growing fast when they died, so they had not yet reached their adult size. "This suggests that they were juvenile specimens, and since T. rex is the only tyrannosaurus known in the Hell Creerk Formation sediments, it is most likely that they belonged to that species ”.
The 'T. rex 'needed about 20 years to reach their adult size. With a 1.5 meter skull and powerful jaws, they were capable of crushing bones with one bite. A small specimen would be unable to carry out this feat, hence its skull was somewhat different.
"Many animals today are very different when they are young than when they are adults, and it may be the same with dinosaurs." Woodward's study suggests that young T. rex occupied and dominated an ecological niche different from that of their parents.
“It is something we see with alligators today: hatchlings eat insects and small fish; the adults, cows, deer and whatever they want, ”says Woodward. The researcher considers that something similar would happen with young tyrannosaurs, which, therefore, “would occupy different roles in the ecosystem”.
The study also determined, thanks to the bones of Jane and Petey, that young tyrannosaurs grew a lot when food was abundant, but that they were able to stunt their growth if it was scarce.
End point to decades of doubt
“The debate on Nanotyrannus has persisted because we had a very poor record of young and juveniles of T. rex”Woodward explains. "For a long time only the largest fossils were selected to be displayed in museums, so we had no idea what the young ones looked like."
To this factor, the paleontologist adds the fact that in the past dinosaur species were named, among other factors, "by their size." In other words, the small specimens were considered different species from the large ones.
"Our research supports the already abundant bone evidence that Nanotyrannus is a simple juvenile T. rex." The researcher highlights that her work "independently supports" this hypothesis by being based on microscopic evidence and not on the shape of the bones.
Does this mean that we should forget about Nanotyrannus? "Right now the available evidence rules out the hypothesis that there was a small tyrannosaurus living next to T. rexBut in science there is never a last word. This could change if one day a small specimen with adult bones is found, ”concludes Woodward.
Holly N. Woodward, Katie Tremaine, Scott A. Williams, Lindsay E. Zanno, John R. Horner, and Nathan Myhrvold. "Growing up Tyrannosaurus rex: Osteohistology refutes the pygmy" Nanotyrannus "and supports ontogenetic niche partitioning in juvenile Tyrannosaurus." Science Advances (January 1, 2020). DOI: 10.1126 / sciadv.aax6250
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