Mar 01, 2021 I Paul Seaburn

Here’s Why There Were No Medium-Sized Carnivorous Dinosaurs (Except in the Movies)

There seems to be two rules in dinosaur movies: 1) the dinosaurs are almost all carnivorous (who wants to see a movie about giants eating palm trees?) and 2) they come in three sizes: cute dog-sized babies, wild mid-sized teens and huge people-eating adult monsters. This doesn’t match modern carnivores, whose adults come in all sizes from tiny felines to midsized wolves to huge grizzlies. It doesn’t match what paleontologists have found either – they’ve found fossils of tiny adult carnivorous dinosaurs and T-Rex-sized giant carnivores. What’s missing is mid-sized, fully-grown dinos. That conundrum is the focus of a new study which may have found the missing size mediums. Any guesses?

“We found that megatheropods (more than 1000 kg) such as tyrannosaurs had specific effects on dinosaur community structure. Although herbivores spanned the body size range, communities with megatheropods lacked carnivores weighing 100 to 1000 kg.”

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Why are you looking at me?

There’s a hint from the study published in the journal Science and summarized in Science Magazine. Megatherapods -- Tyrannosaurus, Allosaurus, Daspletosaurus and other giant two-legged carnivores – lived in areas where there were mysteriously no middleweight carnivores (mesocarnivores) in the 220-2200 pound range. That was the first interesting thing Katlin Schroeder, a Ph.D. student at the University of New Mexico (UNM), Albuquerque, found when perusing the Paleobiology Database, a global collection of fossil data. She searched for 550+ dinosaur species in 43 ancient ecosystems across 136 million years and seven continents for clues to answer the question that puzzled her and the rest of her team.

“It’s as if you went to the savanna and saw nothing in size between a bat-eared fox and lion.”

And that was not just in one spot or one time period but around the globe over millions of years, although it was more pronounced in the Cretaceous – when the tyrannosaurs and the abelisaurs were dominant species, and they also looked "very different as juveniles than they do as adults" -- than in the Jurassic period which preceded it. However, it would have been impossible for adult megatheropods to have eaten all of those creatures, so Schroeder and her fellow paleontologists looked at juvenile megatherapods, which would have outnumbered the adults and, being in the middle of a growth spurt, would have been far hungrier than the adults. (Sound like any teens you know?)

“We knew that megatheropods, significantly Cretaceous megatheropods, modified quite a bit as they grew, however we didn’t know what impact that had on the structuring of their ecosystem. The discovering that juveniles match into that hole, and should have been out-competing medium sized carnivorous dinosaurs, explains why they’re largely absent from the fossil document.”

Schroeder told The Times Hub that the team modeled the growth curves of juvenile megatheropods based on numbers found mass death fossil beds and found that the teens dominated (that means ate) the other species and then filled the gap where the mesocarnivores should have been, keeping the ecosystem functioning normally. Is it time to drop the mic?

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We were hungry!

"Even in relatively well-explored places around the globe, new dinosaur species are discovered each year, so this is not implausible."

Michael D'Emic, an associate professor in the Department of Biology at Adelphi University in New York who reviewed the study for Live Science, proposes that mesocarnivores were there but their fossils haven’t been found yet. It’s also possible that some of the fossils thought to be juvenile megatheropods are misidentified – a common occurrence in paleontology. Whatever the case, the study raises new questions about the relationships between predator and prey and how they affected their ecosystem.

Is it time for a “Jurassic Kiddie Park” movie?

Paul Seaburn

Paul Seaburn is the editor at Mysterious Universe and its most prolific writer. He’s written for TV shows such as "The Tonight Show", "Politically Incorrect" and an award-winning children’s program. He's been published in “The New York Times" and "Huffington Post” and has co-authored numerous collections of trivia, puzzles and humor. His “What in the World!” podcast is a fun look at the latest weird and paranormal news, strange sports stories and odd trivia. Paul likes to add a bit of humor to each MU post he crafts. After all, the mysterious doesn't always have to be serious.

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