Large meat-eating dinosaurs have taken on the role of several species as they grow, resulting in a shocking lack of ecological diversity during the Mesozoic, according to new research.
Megateropods – giant two-legged carnivores ca. Tyrannosaurus, Allosaurus, and Daspletosaurus– did not instantly dominate the ecological space belonging to the monstrously huge dinosaurs. Like other dinosaurs, they hatched and had to survive as they matured. As a new research paper published in scientific shows, these stages of development were not just inactive stages for megateropods; there were times when dinosaurs, while young, were still ecological forces to be reckoned with.
“This study puts numbers on something I suspected, but didn’t really prove: that the largest meat-eating dinosaurs filled different niches in the food chain as they grew from miniature seedlings to adults older than buses, ”said 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, proposed a new term to describe this phenomenon: “morphospecies.” It basically means that megateropods, as they mature, grow and change their hunting habits, have taken on the role of several species.
“Morphospecies is a very beautiful term,” Holly Woodward, a paleontologist at Oklahoma State University who is not affiliated with the new research, said in an email. “It simply came to our notice then T. rex for example it is another T. rex, but fulfills the role of smaller species of carnivores, without being a different species. ”
G / O Media may receive a commission
Taking on the role of several species, however, megateropods have managed to squeeze out competitors and dominate more ecological niches, resulting in a striking lack of species diversity – and a notorious fossil gap, according to research. This gap exists throughout the Mesozoic, the possible explanations being the presence of non-dinosaurs in these niches (such as medium-sized mammals or crocodile-like creatures) or a selection bias regarding the fossils found.
“Our study confirms the persistence of a gap in medium-sized carnivorous dinosaurs in many different communities in space and time,” Schroeder wrote in an email. “I knew that megateropods, especially Cretaceous megateropods, changed a lot as they grew, but I didn’t know what effect it had on structuring their ecosystem. The finding that young people fall into this gap and could have outperformed the competition of medium-sized carnivorous dinosaurs explains why they are largely absent from fossil records.
Indeed, the new study beautifully explains the lack of species diversity experienced in all three periods of the Mesozoic: Triassic, Jurassic and Cretaceous. According to fossil records, megaterapods – 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 in mammals. As a modern analogy, it would be as if there were only bears and lions and also small carnivores such as cats, weasels and civets, but they would not be medium-sized predators such as wolves, coyotes and hyenas. It practically describes the Mesozoic, a time when dinosaurs of average sizes between 220 and 2,200 pounds (100 to 1,000 kilograms) were rare, and dinosaurs weighing less than 130 pounds (60 kilograms) were common.
“It seems to be a consistent pattern for dinosaurs, especially in those Cretaceous communities, towards the end of their reign,” Brusatte said. “There are few species of dinosaurs that eat meat with moderate body size in adults, and that’s because young people and teenagers and subadults like large despotic dinosaurs. T. rex they controlled these niches. ”
Dromaeosaurs like it Velociraptor they were quite successful, but unlike the way they are presented on film, they were actually quite small.
“It simply came to our notice then Jurassic Park “Velociraptor” might be a little disappointed to discover that the real thing Velociraptor it was actually just the size of a turkey, “Schroeder said.” Even relatively large dromaeosaurs like Deinonic they weighed only about 80 kilograms [176 pounds]. ”
That being said, there have been medium-sized dinosaurs called megaraptors, such as Utahraptor, but they were rare, living only in places where megateropods were rare, Schroeder explained. But there was an exception. Dakotaraptor, found in Hell Creek in South Dakota, weighed about 300 kilograms (660 pounds), “but when the next carnivorous dinosaur in the community is 7 tons Tyrannosaurus rex“There is still a substantial gap,” she added.
This gap is known to paleontologists, and the new work is not the first to propose this theory – that large carnivorous dinosaurs have filled several niches throughout the history of evolution. However, “despite their morphological disparity, adults and young people continue to be grouped together in diversity [indexes], which is taxonomically correct 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 influence that juvenile megateropods as morphospecies have had on the community. their”.
To make this analysis, Schroeder and her colleagues analyzed 43 different dinosaur communities on seven continents over 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 significant community groups of small, medium and large dinosaurs.
The results showed that mesocarnivores were largely absent in megateropod-governed communities, and this was the case regardless of time period or geographical location. That being said, this ecological gap seemed to be the most pronounced during the Cretaceous, which is not a surprise, given that megateropods were prolific at the time.
The team also ran the numbers to see if these results made sense. Taking into account factors such as growth and survival rates, the team was able to estimate the proportion of juvenile megateropods in different dinosaur communities.
“The fact that we have noticed the gap of carnivorous dinosaurs over many different communities that have different climates, from very different times in time, has strongly indicated that it was caused by [juvenile megatheropods]”Schroeder said. “The addition of megateropod minors to those communities and their observation that they fit well into the gap really indicated that they were at least part of the reason why we were seeing a decline in 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 was worried that paleontologists might not sample smaller dinosaurs in the fossil record.
“We try so many small mammal fossils, but that’s because their durable teeth hold fossils well and are so complex that we can even use tooth fragments to identify mammal species. This 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 size distribution of meat-eating dinosaurs, with chicks of the largest species filling ecological niches that might otherwise be filled with species of moderate adult body size. “
When asked about a possible selection bias in fossil samples, Schroeder dismissed it as a problem.
“I don’t think the selection will be influenced at all, because we examined many of the best-known and best-formed samples, covering 136 million years and representing each continent,” she said. “Our data set includes almost half of all known dinosaur species, so it is quite unlikely that our data will be representative of dinosaurs as a whole.”
“It is difficult to say whether anyone agrees with the conclusions of this paper, because, according to the authors, no one has ever tried to quantify the ontogenetic niche that changes rigorously in dinosaurs, so we really have nothing else to do with it. “We compare,” Woodward said.
By “changing the ontogenetic niche”, it 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 conciseness,” Woodward added. “It will promote discussion on the subject and will probably encourage more targeted investigations by other researchers.”