![]() ![]() Until now, no one has been able to compute population numbers for long-extinct animals, and George Gaylord Simpson, one of the most influential paleontologists of the last century, felt that it couldn’t be done. What few paleontologists had fully grasped, he said, including himself, is that this means that some 2.5 billion lived and died over the approximately 2 1/2 million years the dinosaur walked the earth. Probably lived at any one time, give or take a factor of 10, which is in the ballpark of what most of his colleagues guessed. What the team found, to be published this week in the journal Pestered his paleontologist colleagues with for years until he finally teamed up with his students to find an answer. Roamed North America during the Cretaceous period? ![]() Other tyrannosaurs also have extreme growth patterns, "hatching out not much heavier than a house cat, and growing to the size of an elephant over 15 years or so," Schroeder said.Over approximately 2.5 million years, North America likely hosted 2.5 billion Tyrannosaurus rexes, a minuscule proportion of which have been dug up and studied by paleontologists, according to a UC Berkeley study. rex's growth from tiny tot to hulking adult. More is known about tyrannosaurs older than 2 years - for instance, for a study published in June in the journal PeerJ, a paleontologist exhaustively analyzed T. Related: Photos: Fossilized dino embryo is new oviraptorosaur species Embryonic fossils are rare, she told Live Science in an email, because "even before they were born, dinosaurs would have been under threat of predation from egg-stealing mammals, and had this baby tyrannosaur hatched, it likely would have had to avoid being eaten by dromaeosaurs ( Velociraptor-like dinosaurs), older tyrannosaurs, crocodilians and possibly even giant pterosaurs." But even without knowing the genus, "finding the remains of extremely young tyrannosaurs is very exciting," said Kat Schroeder, a doctoral student of biology at the University of New Mexico, who wasn't involved with the research but attended the conference presentation. It's a mystery which genus of tyrannosaur these fossils are from, but a few well-known predators from this group include Tyrannosaurus rex, Gorgosaurus and Albertosaurus. (Image credit: Gregory Funston 2020) Embryonic mysteries The newly studied tyrannosaur jawbone (third from top, but magnified at the bottom) is tiny compared with the jaws of a young Gorgosaurus tyrannosaur (top) and a juvenile Daspletosaurus (second from top). (The toe-claw fossil might also be from an embryo because one surface wasn't fully formed, Funston noted.) This style of replacement has recently been found in the first generation of teeth in reptile embryos." "In our case, the replacement tooth is beside the older tooth, and there's no evidence of root disintegration. ![]() "In one of the tooth sockets, a replacement tooth is being developed, but in an unusual way: Typically, replacement teeth lie directly below the older tooth, and they eat away at the root to release the older tooth," Funston said. The teeth on the jawbone aren't fully developed, and one tooth in particular offers clues that this fossil might belong to an embryonic tyrannosaur, meaning the tiny tyrannosaur would have died before it had hatched. A hypothetical cranial reconstruction based on the newly analyzed jawbone (blue), next to other hypothetical reconstructions from other datasets (left and right), indicates that embryonic tyrannosaurs had heads about the size of a modern mouse.
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