Dinosaurs like T. Rex were more tyrannical than we think

Fossil of T. rex.

Fossil of T. rex.
Picture: UNM Department of Biology

Large carnivorous dinosaurs assumed the role of multiple species as they grew, resulting in a shocking lack of ecological diversity during the Mesozoic, according to new research.

Megatheropods, giant two-legged carnivores like tyrannosaurus, Allosaurus, Y Daspletosaurus“It did not instantly dominate the ecological space belonging to monstrously huge dinosaurs.” Like other dinosaurs, they hatched from eggs and had to survive during the transition to adulthood. As new research paper published in Science shows, these developmental stages weren’t just idle stepping stones for megaheropods; they were periods when dinosaurs, when they were young, were still ecological forces to be reckoned with.

“This study puts numbers on something we suspect but haven’t really proven: that the largest carnivorous dinosaurs filled different niches in the food chain as they grew from miniature hatchlings to adults larger than buses,” Steve Brusatte, paleontologist at the University of Edinburgh who is not involved in the new research, he said in an email.

The authors of the new study, led by Katlin Schroeder, a Ph.D. candidate at the University of New Mexico, have proposed a new term to describe this phenomenon: “morphospecies.” It basically means that megaheropods, as they matured, grew and changed their hunting habits, they assumed the role of multiple species.

“Morphospecies is a very nice term,” Holly Woodward, a paleontologist at Oklahoma State University who is not affiliated with the new research, said in an email. “A juvenile Tyrant Saurus Rex for example is still a Tyrant Saurus RexBut it is playing the role of smaller carnivorous species, without being a different species. “

However, by assuming the role of multiple species, megaheropods managed to squeeze out competitors and dominate multiple ecological niches, resulting in a striking lack of species diversity and a notorious fossil gap, according to the research. This gap exists throughout the Mesozoic, and possible explanations are the presence of non-dinosaurs in these niches (such as medium-sized mammals or crocodile-like creatures), or a selection bias in terms of the fossils found.

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

Infographic showing the size distribution between carnivorous mammals and dinosaurs, with an obvious gap between medium-sized dinosaurs.

Infographic showing the size distribution between carnivorous mammals and dinosaurs, with an obvious gap between medium-sized dinosaurs.
Picture: UNM Department of Biology

In fact, the new study explains very well the lack of species diversity experienced during the three Mesozoic periods: Triassic, Jurassic and Cretaceous. As the fossil record shows, megathepods, weighing more than 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 among mammals. As a modern analogy, it would be as if only bears and lions existed, and also small carnivores like cats, weasels, and civets, but not medium-sized predators like wolves, coyotes, and hyenas. This basically describes the Mesozoic, a time during which medium-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 consistent pattern in dinosaurs, especially in Cretaceous communities, towards the end of their reign,” Brusatte said. “There were few species of carnivorous dinosaurs with a moderate adult body size, and that is because the young, adolescent and sub-adult of the despotic large dinosaurs like Tyrant Saurus Rex they were controlling those niches. “

Dromaeosaurs like Velociraptor They were quite successful, but contrary to how they are portrayed in the movie, they were actually quite small.

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

That said, there were medium-sized dinosaurs called megaraptors, such as UtahraptorBut they were rare, living only in places where megaheropods were rare, Schroeder explained. But there was an 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, “he added.

Paleontologists know about this gap, and the new paper isn’t the first to propose this theory: Large carnivorous dinosaurs filled multiple niches throughout evolutionary history. However, “despite their morphological disparity, adults and young people continue to be grouped in diversity [indexes], which is taxonomically accurate but not ecologically accurate, ”the authors wrote in the new study. As the document points out, the new analysis is unique in that it “demonstrates the influence that juvenile megaheropods as morphospecies would have had on their community.”

To do this analysis, Schroeder and his colleagues looked at 43 different dinosaur communities on seven continents throughout more than 136 million years of ecological history. The team analyzed more than 550 species of dinosaurs, classifying them by weight and diet, which in turn allowed them to compile meaningful community groupings consisting of small, medium and large size dinosaurs.

The results showed that mesocarnivores were largely absent in megaheropod-ruled communities, and this held true regardless of time period or geographic location. That said, this ecological gap appeared to be most pronounced during the Cretaceous, which is not a surprise given that megaheropods were prolific at the time.

The team also looked at the numbers to see if these results made sense. By considering 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 in carnivorous dinosaurs in many different communities that have different climates, from very different points in time, clearly indicates that it was being caused by [juvenile megatheropods]Schroeder said. “Adding the megaheropod juveniles to those communities and seeing them fit perfectly into the gap indicates very clearly that they were at least part of the reason we were seeing a decline 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 drivers behind massive dinosaur distributions,” Schroeder said.

Brusatte really liked the new study, but was concerned that paleontologists weren’t taking samples from smaller dinosaurs in the fossil record.

“We sampled many fossils of small mammals, but that’s because their durable teeth are as well preserved as fossils and are so complex that we can even use tooth fragments to identify mammalian species. That is not the case with dinosaurs, ”he said. “This could 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 the juveniles of the largest species filling ecological niches that might otherwise be filled by different species of moderate adult body size. “

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

“I don’t think selection bias comes into play at all, as we examined many of the best-known and best-sampled formations, spanning 136 million years and representing all continents,” he said. “Our data set includes almost half of all known dinosaur species, so it is highly unlikely that our data is not representative of dinosaurs as a whole.”

“It is difficult to say if one agrees with the conclusions of this article, because as the authors state, no one has tried to quantify ontogenetic niche change in dinosaurs so rigorously, so we really have nothing else to do. compare it. Woodward said.

By “ontogenetic niche change,” he refers to the changing ecological role of dinosaurs as they age and grow.

“But I think their study reached the right level of detail and brevity,” Woodward added. “It will promote debate on the topic and probably encourage more specific research by other researchers.”


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