Tyrannosaurus rexlived from 67 to 66 million years ago; ancestrally, it may have originated in Asia and then dispersed into the island-continent of Laramidia, what is now western North America. T. rex went through such extreme growth changes that juveniles and adults look superficially like different species. T. rex was at the top of the food chain, and feasted on herbivorous dinosaurs such as Triceratops, Edmontosaurus, and Ankylosaurus. Among meat-eating dinosaurs, its only rival for size and power was Spinosaurus.
T. rexwas the largest (12 meters) and most massive (9 tonnes) of the tyrannosaurids; however, a recent study has found that size does not reflect age among adults. In fact, the largest and most massive specimen, nicknamed Scotty, is a relatively young adult, and so it is not the oldest and most mature as previously thought. T. rex lived to at least 28 years old, and had a growth spurt between the ages of 13 and 15, where the skull transformed from sleek to deep, a transformation that occurred over no more than two years. As such, the deep skull and thick teeth that we associate with T. rex grew in halfway through its lifespan, which made subadult T. rex as terrifying as their adult counterparts.
Another notable change during T. rex growth is the initial increase in tooth count in the upper and lower jaws, which is also seen in other reptiles, but it is followed by a steady decrease in tooth count, which is also seen in some other tyrannosaurids. The oldest T. rex had as few as 54 teeth in their jaws, in contrast to the 74 seen in juveniles – a loss of 20 tooth positions!
T. rex was the lone top predator in Laramidia during that last million years of the age of dinosaurs. The type (name-bearing) skull of Nanotyrannus, a purported “pygmy tyrant” from the same time and place as T. rex, has a narrow snout, wide forehead and cheek region, and many other features shared with adult T. rex, which, in addition to its small size, reveal that it is in fact an juvenile T. rex. In fact, the specimens thought to represent “Nanotyrannus” form a transitional series between juvenile and subadult T. rex.
So far, there is no evidence for sexual dimorphism in the teeth or bones of T. rex, although in life the sexes might have been different from each other in terms of coloration. Like other tyrannosaurids, the face of T. rex was crocodile-like in having a covering of large flat scales and lipless mouth, the evidence for which is based on the coarse texture of the facial bones of tyrannosaurids, which is identical to the condition seen in crocodylians. The facial skin of tyrannosaurids might have also been hyper-sensitive, indicating that touch was for them an important source of information about the world. Based on this, it is easy to visualize a parent T. rex tending to its nest of fragile eggs (Note: as of yet, no T. rex nests, or eggs for that matter, have yet been discovered!).
Thomas D. Carr, is an Associate Professor of Biology at Carthage College in Kenosha, Wisconsin. Thomas studies the evolution and growth of Tyrannosaurus rex and its closest relatives. He is currently working on a follow-up to his June 2020 study of growth in T. rex, a synthesis of the growth and evolution of all of the immediate relatives of T. rex – namely, the tyrannosaurids.
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