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Eat medium-sized meat dinosaurs are absent from the fossil record, and paleontologists believe they have understood why.
Paleontologists have discovered gargantuan dinosaurs and small dinosaurs, but a new study finds there is a dire shortage of medium-sized carnivorous dinosaur species, especially the Cretaceous period (145.5 million to 65.5 million years ago).
Putting on their detective hats, researchers quickly found a suspect; megatheropods – the largest of the carnivorous dinosaurs, as Tyrannosaurus rex and Gorgosaurus, who weighed over 2,200 pounds (1,000 kilograms) as an adult. Juvenile megatheropods may have overtaken intermediaries, the researchers said.
“Juvenile megatheropods may have outperformed other medium-sized dinosaurs, resulting in deflated global diversity of dinosaurs,” said study principal investigator Katlin Schroeder, a doctoral student in the Department of Biology at the University of the New Mexico, in an email.
However, not everyone is convinced that there is a case of missing mid-sized dinosaurs; it is possible, for example, that there are fossils of medium-sized animals that have yet to be found, said Michael D’Emic, associate professor in the biology department at Adelphi University in New York City, who was not involved in studying it.
Related: Gory guts: Photos of a T. rex autopsy
Small medium large
As laying animals, all dinosaurs started out small, weighing no more than 33 pounds (15 kg) as hatchlings. As the dinosaurs grew, some likely occupied different niches and ate different foods than adults of the same species – for example, a young T. rex probably couldn’t take a Triceratops, and probably went after smaller prey.
To investigate the midsize mystery, Schroeder and his colleagues logged on to the Paleobiology Database, a nonprofit resource for paleontological data, and they have classified over 550 species of dinosaurs as small (22 to 220 pounds, or 10 to 100 kg), medium (220 to 2200 pounds, or 100 to 1000 kg), or large (over 2,200 pounds) or 1,000 kg). These dinosaurs lived in 43 communities (groups that lived at the same time and in the same place) on seven continents during Jurassic period (201 million to 145.5 million years ago) and the Cretaceous period.
The researchers found that while communities often had herbivorous dinosaurs in each size category, it was rare to spot a medium-sized carnivorous dinosaur in communities with megatheropods.
“It’s possible that the ‘divide’ was caused by juveniles of these large megatheropods, which may have eaten things different from their parents, and therefore competed with medium-sized carnivores,” Schroeder said.
The team found that the gap between medium-sized dinosaurs was more pronounced in the Cretaceous than in the Jurassic. During the Cretaceous Period, tyrannosaurs and abelisaurs were kings, and they also looked “very different as juveniles than as adults,” unlike Jurassic megatheropods, she said.
In other words, during the Jurassic period, megatheropods, such as Allosaurus, did not change much as they grew, “and may have shared food resources, such as giant sauropods [long-neck herbivorous dinosaurs] with their parents, “Schroeder said.” This may have allowed more carnivores to coexist in the same communities, resulting in a smaller [medium-size] hole in carnivores. “
But by the end of the Jurassic, many sauropods became extinct, as did dinosaurs like Allosaurus. “They may have been replaced by dinosaurs like Tyrannosaurus that used a greater variety of different resources as they grew, ”said Schroeder.
Next, Schroeder’s team wondered whether juvenile megatheropods had a greater effect on the composition of their communities than adults. To find out, the researchers calculated the number of juveniles and adults of each species in a community. Next, the team determined biomass – the number of individuals of a species multiplied by their mass at a certain age.
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Researchers have found that in some species of megatheropods, such as Allosaurus and Tyrannosaurus, juveniles made up a larger portion of the mass than adults, probably because it was a world of dinosaurs eaten by dinosaurs at the time, and megatheropods did not always reach adulthood. This indicates “that young people have had as much (if not more) of an effect on their community than adults,” Schroeder said in the email. In fact, there were so many mid-sized megatheropod juveniles that they could even be considered their own species, in a way.
“When we added minors to communities like theirs [species], the gap has largely disappeared, ”said Schroeder.
However, it’s possible that something else could explain this midsize mystery, D’Emic said. Twelve of the 43 paleocommunities examined in the study did not appear to follow the pattern inferred in the study – that medium-sized carnivorous dinosaurs were rare in communities with megateropods. In those 12 communities, “they have large and medium sized theropods,” D’Emic said. The study explains these exceptions in different ways, but maybe mid-sized carnivorous dinosaurs were there, it’s just paleontologists have yet to find their fossils in all of the communities, D’Emic said. “Even in relatively well-explored places around the world, new species of dinosaurs are discovered every year, so it’s not unlikely,” D’Emic told Live Science in an email.
It is also possible that some dinosaur species in the study were misidentified. It wasn’t until recently that paleontologists began to assess bone microstructure, which can reveal a dinosaur’s age at the time of death. “This may show that some small individuals of dinosaurs belonging to one species were simply juveniles of other species, or conversely, that some small individuals of dinosaurs considered to be juveniles of one species are rather adults of new dwarf species,” D’Emic said.
The study was published online on February 25 in the journal Science.
Originally posted on Live Science.
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