Large carnivorous dinosaurs took on the roles of multiple species as they grew up, resulting in a shocking lack of ecological diversity during the Mesozoic Era, according to new research.
Megatheropods – giant two-legged carnivores like Tyrannosaurus Allosaurus, and Daspletosaurus– did not immediately dominate the ecological space of monstrously large dinosaurs. Like other dinosaurs, they hatched and had to survive through the transition to adulthood. As new research paper published in Science shows, these stages of development were not just vain stepping stones for megatheropods; they were periods when the dinosaurs, while still young, were still ecological forces to be reckoned with.
“This study provides figures for something we suspected but didn’t really prove: that the largest carnivorous dinosaurs filled several niches in the food chain as they grew from miniature broods to adults larger than buses,” Steve Brusatte, a paleontologist at the University of Edinburgh who is not involved in the new investigation, said in an email.
The authors of the new study, led by Katlin Schroeder, a PhD student at the University of New Mexico, have proposed a new term to describe this phenomenon: “morphospecies.” It basically means that as megatheropods grew up and changed their hunting habits, they took on the role of multiple species.
“Morpho species is really a fancy term,” said Holly Woodward, an Oklahoma State University paleontologist who is not affiliated with the new study, in an email. “A juvenile T. rex is still a T. rex, but it fulfills the role of smaller carnivore species, without being any other species. “
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By taking on the role of multiple species, however, megatheropods managed to express competitors and dominate multiple ecological niches, resulting in a marked lack of species diversity – and an infamous fossil divide, the study said. This gap exists all over Mesozoic, with possible explanations such as 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 persistent divide between medium-sized carnivorous dinosaurs from many different communities in space and time,” Schroeder wrote in an email. “We knew that megatheropods, especially Cretaceous megatheropods, 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 surpassed medium-sized carnivorous dinosaurs explains why they are largely absent from the fossil record. “
Indeed, the new study nicely explains the lack of species diversity experienced during all three periods of the Mesozoic: the Triassic, Jurassic, and 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 curious result, as ecologists are used to seeing the opposite, at least in mammals. As a modern analogy, it would be as if only bears and lions existed, as well as small carnivores such as cats, weasels, and civets, but not medium-sized predators such as wolves, coyotes, and hyenas. This basically describes the Mesozoic, a time when 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).
“This appears to be a consistent pattern with dinosaurs, especially those Cretaceous communities, towards the end of their reign,” Brusatte said. “There were few carnivorous dinosaur species of moderate adult body size, and that’s because the juveniles and teenagers and sub-adults of the large despotic dinosaurs like T. rex controlled those niches. “
Dromaeosaurs love Velociraptor were quite successful, but unlike how they are portrayed on film, they were actually quite small.
“Fans of the Jurassic Park ‘Velociraptor’ may be a little disappointed to find out that it is real Velociraptor was actually only about the size of a turkey, “said Schroeder.” Even relatively large dromaeosaurs like Deinonychus only reached about 80 kilograms [176 pounds]
That said, there were medium sized dinosaurs called megaraptors like Utahraptor, but they were rare and lived only in places where megatheropods were scarce, 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 second largest carnivorous dinosaur in the community weighed 7 tons. Tyrannosaurus rex, there is still a significant gap, ”she added.
This gap is well known to paleontologists, and the new paper isn’t the first to propose this theory – that large carnivorous dinosaurs filled multiple niches throughout evolutionary history. But “despite their morphological inequality, adults and youth are still grouped together in diversity [indexes], which is taxonomically but not ecologically correct, ”the authors wrote in the new study. As the paper points out, the new analysis is unique in that it “demonstrates the impact that juvenile megatheropods as morphospecies would have had on their communities.”
To make this analysis, Schroeder and her colleagues looked at 43 different dinosaur communities from seven continents in more than 136 million years of ecological history. The team analyzed more than 550 dinosaur species, categorized them by weight and diet, which in turn allowed them to assemble meaningful community groups consisting of small, medium, and large dinosaurs.
The results showed that mesocarnivores were largely absent in communities ruled by megatheropods, regardless of time period or geographic location. That said, this ecological divide turned out to be most pronounced during the Cretaceous Period, which is hardly a surprise given that megatheropods were fertile at the time.
The team also tested the numbers to see if these results were accurate. By taking into account factors such as growth and chances of survival, the team was able to estimate the proportion of young megatheropods in the different dinosaur communities.
“The fact that we observed the divide between carnivorous dinosaurs across many different communities with different climates from very different times strongly indicated that it was caused by [juvenile megatheropods], ”Said Schroeder. “Adding the megatheropod juveniles to those communities and seeing them fit neatly into the canyon really strongly indicated that they were at least part of why we observed reduced dinosaur diversity.”
This approach, in which researchers examined individual communities and compared dinosaurs of all shapes and sizes, is “the first attempt to quantitatively identify the ecological factors behind the mass distribution of dinosaurs,” Schroeder said.
Brusatte really liked the new study, but he was concerned that paleontologists might not sample smaller dinosaurs in the fossil record.
“We sample so many small mammal fossils, but that’s because their durable teeth are preserved just as well as fossils, and are so complex that we can even use fragments of teeth to identify mammal species. That’s 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 juveniles of the largest species filling ecological niches that otherwise would be filled by several 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 would play a role, as we examined many of the most well-known and well-sampled formations, spanning 136 million years and representing every continent,” she said. “Our data set includes nearly half of all known dinosaur species, so our data is unlikely to be representative of dinosaurs as a whole.”
“It’s hard to say if they agree with the conclusions of this paper because, as the authors state, no one has ever attempted to quantify the ontogenetic niche shift in dinosaurs so rigorously, so we really have nothing else to compare it to. , ‘Woodward said.
With ‘ontogenetic niche shift’ she refers to the changing ecological role of dinosaurs as they get older and larger.
“But I think their study achieves the right level of detail and brevity,” Woodward added. “It will advance discussion on the topic and likely encourage more focused research by other researchers.”