How did the snarling robbers of the dinosaur era grow into giants? Apparently very different depending on the group they belong to, a study shows: Tyrannosaurus rex and its close relatives grew rapidly in strong bursts, while the representatives of the allosaurs developed slowly and steadily into giants. Possibly these different strategies were in turn linked to the growth patterns of the typical prey of the two groups of predatory dinosaurs, say the scientists.
They all start small, but how animals then develop differs significantly: mammals and birds usually go through a phase of strong growth relatively early. They then reach adulthood after a species-typical adolescence, with which the increase in size of the skeleton ends. However, this is not always the case with other animal groups. Many reptiles such as alligators, various lizards and snakes tend to grow steadily and continuously. An unusually large individual is therefore usually very old in their case. Both strategies have advantages and disadvantages: Rapid growth can make it possible to hunt certain prey at an early stage or even better avoid predators. On the other hand, growth spurts cost a lot of energy and resources. Lifelong, continuous growth is, in comparison, more economical and can therefore also be worthwhile. Apparently, depending on the animal group and lifestyle, one of the two advantages and disadvantages predominates.
On the trail of growth patterns
But how was it with the predatory dinosaurs, which were related to both birds and reptiles? As far as the “king” of the rabid dinosaurs is concerned, previous studies of bone structures have already shown that T. rex reached its length of up to 13 meters through a growth spurt in teenage years. So far, however, it was unclear whether this only applied to this species or its closest relatives, or whether it was the typical growth pattern in the various lines of development of the two-legged predatory dinosaur. “So we looked at a wider range of different theropods to better understand the growth and evolutionary patterns in this group,” says Tom Cullen of the Field Museum of Natural History in Chicago.
As part of their study, he and his colleagues examined certain structures in cross-sections of fossil bones. “During the life of an animal, tree-ring-like lines appear inside the bones that tell how old it is and how much it has grown each year,” explains Cullen. In addition to the fossils of a number of predatory dinosaurs from different families, the researchers also again analyzed a sample of a Tyrannosaurus rex. It came from the famous “SUE” fossil from the Field Museum. Another highlight was a sample from a recently discovered representative of the carcharodontosaurs from Argentina. These predatory dinosaurs belonged to the group of allosaurs and reached similar dimensions as T. rex. Among the theropods, however, they were most distantly related to the tyrannosaurs.
Some quickly, others slowly
The paleontologists came to the conclusion that the growth patterns of the dinosaurs differed significantly depending on their relatives. It has been confirmed that Tyrannosaurus rex and its relatives, the coelurosaurs, experienced a period of rapid growth during adolescence. When they reached a certain size, there was no more growth. Specifically, the investigations by SUE showed that this T. rex was 33 years old. However, the animal had already reached its full size by the age of 20. As a teenager, SUE put on about 18 to 22 kilograms per week, the researchers estimated.
With the more distant cousins of the tyrannosaurs – the representatives of the allosaurs – a different pattern emerged: They could become almost as big as T. rex, but they grew only slowly throughout their lives, with the oldest individuals reaching the largest dimensions. In the case of the examined Carcharodontosaurus from Argentina, the analyzes showed that it was around 50 years old when it died. Despite this advanced age, however, it had only stopped growing two or three years earlier, according to the analyzes.
The question now arises as to why there were these developmental differences between the predatory dinosaurs. According to the researchers, this may have been linked to how they interacted with other animals in their environment. It is known that T. rex lived alongside and presumably hunted herbivorous dinosaurs such as triceratops and duckbill dinosaurs (hadrosaurs). As the paleontologists explain, there is evidence that these animals also grew up quickly and in bursts. The slowly growing representatives of the allosaurs, on the other hand, were more adapted to the long-necked sauropods as prey, which probably took longer to reach their full size.
“There may have been selective pressures for the tyrannosaurs and their close relatives to grow quickly to keep up with their prey,” says Cullen. “And in the case of the allosaurs, it might have been more beneficial to keep growing, as their prey also increased in size. It is possible that the sauropods had so many offspring that there was always something small to eat for the predators, ”says the paleontologist. “So far, however, these explanations have remained rather speculative,” Cullen concludes.
Source: Field Museum, specialist article: Proceedings of the Royal Society B, doi: 10.1098 / rspb.2020.2258