Large meat-eating dinosaurs took on the role of multiple species while growing up, resulting in a shocking lack of ecological diversity during the Mesozoic, according to new research.
Megatheropods—gigantic two-legged carnivores like Tyrannosaurus, Allosaurus, and Daspletosaurus—didn’t instantly dominate the ecological space belonging to monstrously huge dinosaurs. Like other dinosaurs, they hatched from eggs and had to survive while transitioning into adulthood. As a new research paper published in Science shows, these developmental stages weren’t just idle stepping stones for megatheropods; they were periods in which the dinosaurs, while juveniles, were still ecological forces to be reckoned with.
“This study puts numbers on something we’ve suspected but haven’t really proven: that the biggest meat-eating dinosaurs filled different niches in the food chain as they grew from miniature hatchlings into adults bigger than buses,” Steve Brusatte, a paleontologist at the University of Edinburgh who’s not involved with the new research, said in an email.
The authors of the new study, led by Katlin Schroeder, a PhD candidate at the University of New Mexico, have proposed a new term to describe this phenomenon: “morphospecies.” It basically means that megatheropods, while maturing, growing, and changing their hunting habits, took on the role of multiple species.
“Morphospecies is a really nice term,” Holly Woodward, a paleontologist from Oklahoma State University who’s not affiliated with the new research, said in an email. “A juvenile T. rex for example is still a T. rex, but it’s carrying out the role of smaller carnivore species, without being a different species.”
By taking on the role of multiple species, however, megatheropods 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 across the entire 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 the fossils found.
“Our study confirms the persistence of a gap in medium-sized carnivorous dinosaurs from many different communities across space and time,” wrote Schroeder in an email. “We knew that megatheropods, particularly 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 been out-competing 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 upwards of 2,200 pounds (1,000 kilograms)—were prolific, but medium-sized carnivores, known as mesocarnivores, were surprisingly rare. This is an odd result, because ecologists 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 no medium-sized predators such as wolves, coyotes, and hyenas. This basically describes the Mesozoic, a time during which medium-sized dinosaurs between 220 to 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 those communities in the Cretaceous, towards the end of their reign,” said Brusatte. “There were few meat-eating dinosaur species of moderate adult body size, and that’s because the juveniles and teenagers and subadults of the big despotic dinosaurs like T. rex were controlling those niches.”
Dromaeosaurs like Velociraptor were quite successful, but contrary to how they’re portrayed on film, they were actually quite small.
“Fans of the Jurassic Park ‘Velociraptor’ might be a bit disappointed to discover that the real Velociraptor was actually only about the size of a turkey,” said Schroeder “Even relatively big dromaeosaurs like Deinonychus were only reaching 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 were megatheropods were scarce, explained Schroeder. But there was an exception. Dakotaraptor, found in the the Hell Creek Formation in South Dakota, weighed around 660 pounds (300 kilograms), “but when the next largest carnivorous dinosaur in the community is the 7-ton Tyrannosaurus rex, there’s still a substantial gap,” she added.
This gap is known to paleontologists, and the new paper is not the first to propose this theory—that large carnivorous dinosaurs filled multiple niches throughout evolutionary history. Yet “despite their morphological disparity, adults and juveniles continue to be grouped together in diversity [indexes], which is accurate taxonomically but not ecologically,” wrote the authors 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 morphospecies on their community.”
To make this analysis, Schroeder and her colleagues looked at 43 different dinosaur communities from seven continents across more than 136 million years of ecological history. The team analyzed more than 550 species of dinosaurs, categorizing them by weight and diet, which in turn allowed them to compile meaningful community groupings consisting of small-, medium-, and large-sized dinosaurs.
Results showed that mesocarnivores were largely absent in communities ruled by megatheropods, and this held true regardless of the time period or geographic location. That said, this ecological gap appeared to be most pronounced during the Cretaceous, which is hardly a surprise given that megatheropods were prolific at the time.
The team also ran 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 over many different communities that have different climates, from very different points in time indicated strongly that it was being caused by [juvenile megatheropods],” said Schroeder. “Adding the juveniles of megatheropods into those communities and seeing them fit neatly into the gap indicated really strongly that they were at least part of the reason we were observing decreased dinosaur diversity.”
This approach, in which researchers examined individual communities and dinosaurs of all shapes and sizes were compared, is “the first attempt to quantitatively identify ecological drivers behind dinosaur mass distributions,” said Schroeder.
Brusatte really liked the new study, but he worried that paleontologists might not be sampling smaller dinosaurs in the fossil record.
“We sample so many small mammal fossils, but that’s because their durable teeth preserve well as fossils, and are so complex that we can use even fragments of teeth to identify mammal species. That’s not the case with dinosaurs,” he said. “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 meat-eating dinosaurs, with juveniles of the largest species filling the ecological niches that might otherwise be filled by distinct 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 come into play at all, since we examined many of the most well known and well sampled formations, covering 136 million years and representing every continent,” she said. “Our data set includes nearly half of all known dinosaur species, so it’s pretty unlikely that our data isn’t representative of dinosaurs as a whole.”
“It’s difficult to say whether one agrees with the conclusions of this paper, because as the authors state, no one has ever tried to quantify ontogenetic niche shifting in dinosaurs this rigorously, so we really don’t have anything else to compare it to,” said Woodward.
By “ontogenetic niche shifting,” she is referring to the changing ecological role of dinosaurs as they get older and bigger.
“But I think their study hit the right level of detail and brevity,” added Woodward. “It will promote discussion of the subject and likely encourage more targeted investigations by other researchers.”