But the question of how quickly a T rex achieved its maximum size has been a matter of debate.
A new study examining bone tissue microstructure in the leg bones of 17 fossil specimens has concluded that T rex took about 40 years to reach its maximum size of roughly eight tonnes, 15 years more than previously estimated.
The researchers identified previously unknown growth marks in these bones that could only be seen using polarised light.
“This growth trajectory is more gradual than expected,” said paleohistologist Holly Woodward Ballard of the Oklahoma State University Centre for Health Sciences, lead author of the study published this week in the journal PeerJ. “Rather than T rex ratcheting up to adult size quickly, it spent a lot of its life at juvenile to sub-adult sizes.”
The researchers scrutinized annual growth rings – akin to those present in tree trunks – in T rex leg bones from various specimens, which ranged from smaller juveniles to large adults.
“We also found that growth-ring spacing in individual T rex was variable. T rex had a flexible growth pattern. Some years it didn’t grow much, while other years it grew a lot,” Prof Woodward said.
“This likely depended on resource – food – availability or environmental conditions. In other words, if conditions weren’t great, it didn’t spend energy on growing, but when conditions were good, it could grow larger. This flexibility allowed it to survive harsh times while growing larger than other carnivores, so it could outcompete others for resources. Ultimately, T rex was only competing against other T rex for food,” Prof Woodward added.
The Alamosaurus was among the prey of the T rex
The T rex prowled western North America during the Cretaceous period at the twilight of the age of dinosaurs, before an asteroid struck Mexico’s Yucatan Peninsula 66 million years ago. This calamity doomed the dinosaurs and wiped out three-quarters of Earth’s plant and animal species.
A T rex could reach more than 40 feet (12.3 meters) long. It had a massive head and tremendous bite strength, walked on two strong legs, and had small arms with just two fingers.
Previous research had found that the T rex had a lifespan of around 30 years. The new study, according to paleontologist and study co-author Jack Horner of Chapman University in California, found that the lifespan could have been 45 to 50 years.
This study involved more T rex specimens – many of them held at the Museum of the Rockies in Montana – than previous research. The authors also said they used a new statistical approach that considered growth records from different specimens to better estimate the growth trajectory of the species across all stages of life, producing a different conclusion than previous work.
“We don’t know for certain which of these estimates are more accurate since we don’t have living T rexes to measure, but these new estimates make more sense logically and statistically, considering the size these dinosaurs attain,” Mr Horner said.
The T rex preyed on various plant-eating dinosaurs, including the duck-billed Edmontosaurus and the horned Triceratops, as well as, in the southern part of its range, the long-necked Alamosaurus.
“Unfortunately we can’t know the evolutionary advantage of any particular characteristic, but a lengthened growth with an intervening growth hiatus allows the younger individuals a different food strategy than the older, larger individuals,” Mr Horner said.
“And, independent of this current paper, I think the older adults were much more opportunistic – utilizing more scavenging – than the younger, smaller individuals. The extended growth period would provide a longer period of time for the younger individuals to possibly acquire more live prey,” Mr Horner added.