Dinosaurs like T. Rex were more tyrannical than we thought

Large, carnivorous dinosaurs assumed the roles of several species as they grew older, resulting in a shocking lack of ecological diversity during the Mesozoic, new research shows.

Megatheropods – gigantic two-legged carnivores like Tyrannosaurus, Allosaurus, and Daspletosaurus– did not instantly dominate the ecological space belonging to monstrously huge dinosaurs. Like other dinosaurs, they hatched from eggs and had to survive during their transition to adulthood. As new research paper published in Science shows, these stages of development were not just stepping stones for megatheropods; These were times when dinosaurs, when they were juveniles, were still ecological forces to be reckoned with.

“This study puts numbers on something we suspected but didn’t really prove: that larger carnivorous dinosaurs occupied different niches in the food chain as they moved from miniature hatchlings to larger adults. than buses, ”Steve Brusatte, a paleontologist at the University of Edinburgh who is not involved in the new research, said in an email.

The authors of the new study, led by Katlin Schroeder, a doctoral student at the University of New Mexico, came up with a new term to describe this phenomenon: “morphospecies”. Essentially, this means that megatheropods, while maturing, growing and changing their hunting habits, have taken on the role of several species.

“Morphospecies is a really cool term,” said Holly Woodward, an Oklahoma State University paleontologist who is not affiliated with the new research, said in an email. “A young T. rex for example is always a T. rex, but it plays the role of smaller carnivore species, without being a different species.

By assuming the role of multiple species, however, megatheropods have managed to oust competitors and dominate multiple ecological niches, resulting in a striking lack of species diversity – and a notorious fossil divide, according to the research. This discrepancy exists throughout the Mesozoic, with possible explanations being the presence of non-dinosaurs in these niches (such as medium-sized mammals or crocodile-like creatures), or a selection bias in terms of fossils found.

“Our study confirms the persistence of a gap in medium-sized carnivorous dinosaurs from many different communities across space and time,” Schroeder wrote in an email. “We knew that megatheropods, especially the Cretaceous megatheropods, changed a lot as they grew, but we didn’t know what effect this had on the structuring of their ecosystem. The discovery that juveniles fit into this loophole, and may have overtaken medium-sized carnivorous dinosaurs, explains why they are largely absent from the fossil record.

Indeed, the new study clearly explains the lack of diversity of species experienced during the three periods of the Mesozoic: the Triassic, the Jurassic and the Cretaceous. As the fossil record shows, megatherapods – weighing over 2,200 pounds (1,000 kilograms) – were prolific, but medium-sized carnivores, known as mesocarnivores, were surprisingly rare. This is a strange result, because environmentalists are used to seeing the opposite, at least in mammals. By modern analogy, it would be as if only bears and lions existed, as well as small carnivores like cats, weasels, and civets, but no medium-sized predators like wolves, coyotes, and hyenas. This essentially describes the Mesozoic, a period in which mid-sized dinosaurs between 220 and 2,200 pounds (100 to 1,000 kilograms) were rare, and dinosaurs weighing less than 130 pounds (60 kilograms) were common.

“This seems to be a constant trend among dinosaurs, especially in Cretaceous communities, towards the end of their reign,” Brusatte said. “There were few species of mid-adult carnivorous dinosaurs, and this is because juveniles and adolescents and sub-adults of large despotic dinosaurs like T. rex controlled these niches.

Dromaeosaurs like Velociraptor were very successful, but unlike how they’re portrayed in the movie, they were actually quite small.

“Fans of jurassic park ‘Velociraptor’ might be a bit disappointed to find out that the real Velociraptor was actually only about the size of a turkey, “Schroeder said.” Even relatively large dromaeosaurs like Deinonychus weighed only about 80 kilograms [176 pounds]. “

That said, there were medium sized dinosaurs called megaraptors, such as Utahraptor, but they were rare, living only in places where megatheropods were rare, Schroeder explained. But there was one exception. Dakotaraptor, found in the Hell Creek Formation in South Dakota, weighed about 660 pounds (300 kilograms), “but when the next largest carnivorous dinosaur in the community is the 7 ton Tyrannosaurus rex, there is still a substantial gap, ”she added.

This gap is well known to paleontologists, and the new article is not the first to propose this theory – that large, carnivorous dinosaurs have filled multiple niches throughout evolutionary history. However, “despite their morphological disparity, adults and juveniles continue to be grouped together in diversity. [indexes], which is taxonomically correct but not ecological, ”the authors wrote in the new study. As the paper points out, the new analysis is unique in that it “demonstrates the influence that juvenile megatheropods would have had as morph-species on their community.”

To do this analysis, Schroeder and his colleagues examined 43 different dinosaur communities from seven continents across more than 136 million years of ecological history. The team analyzed over 550 species of dinosaurs, categorizing them by weight and diet, allowing them to compile meaningful community groupings made up of small, medium, and large dinosaurs.

The results showed that mesocarnivores were largely absent in communities ruled by megateropods, and this was true regardless of time period or geographic location. That said, this ecological divide appears to be most pronounced in the Cretaceous Period, which is hardly surprising given that megatheropods were prolific at the time.

The team also ran the numbers to see if those results made sense. Taking into account factors such as growth and survival rates, the team was able to estimate the proportion of juvenile megatheropods in the various dinosaur communities.

“The fact that we observed the gap between carnivorous dinosaurs in many different communities that have different climates, at very different points in time, clearly indicated that it was caused by [juvenile megatheropods]Schroeder said. “Adding the megatheropod juveniles into these communities and seeing them fit right into the gap was a very clear indication that they were at least part of the reason we were seeing a decrease in dinosaur diversity.”

This approach, in which the researchers examined individual communities and dinosaurs of all shapes and sizes were compared, is “the first attempt to quantitatively identify the ecological factors behind massive dinosaur distributions,” Schroeder said.

Brusatte really liked the new study, but he feared paleontologists were not sampling small dinosaurs from the fossil record.

“We sample so many fossils from small mammals, but that’s because their enduring teeth store well as fossils, and are so complex that we can even use tooth fragments to identify mammalian species. This is not the case with dinosaurs, ”he says. “This might affect some of the results of this study, but not the main finding that there is a gap in the body size distribution of carnivorous dinosaurs, with juveniles of the larger species filling ecological niches that might otherwise be filled by animals. species of moderate adult body size. “

When asked about a possible selection bias in the fossil samples, Schroeder dismissed it as a problem.

“I don’t think selection bias comes into play at all, since we’ve looked at many of the best-known and best-sampled formations, spanning 136 million years and representing all continents,” she said. declared. “Our data set includes almost half of all known dinosaur species, so our data is unlikely to be representative of dinosaurs as a whole.”

“It is difficult to say whether one agrees with the conclusions of this article, because as the authors say, no one has ever tried to quantify so rigorously the ontogenetic niche change in dinosaurs, so we really don’t have anything else to compare it to, ”Woodward said.

By ‘ontogenetic niche shift’ she refers to the changing ecological role of dinosaurs as they age and grow.

“But I think their study has reached the right level of detail and brevity,” added Woodward. “This will promote discussion on the topic and likely encourage more targeted investigations by other researchers.”