Why Medium-Sized Dinosaurs Are Often Missing From the Fossil Record

Study suggests huge carnivores like T. rex may have occupied the ecological roles of medium-sized predators as juveniles

Teenage T. rex
Juveniles from species of massive carnivorous dinosaurs, such as Tyrannosaurus rex, may have out-competed species that would have otherwise succeeded as medium-sized adults, according to new research. UNM Biology

In the age of the dinosaurs, it took a hatchling Tyrannosaurus rex about 20 years to reach its full adult size of between 12,000 and 18,000 pounds. Now, new research suggests that as they grew up, these massive meat-eating dinosaurs exerted tyrannical influence over their prehistoric ecosystems by out-competing medium-sized predators, reports Laura Geggel for Live Science.

The paper, published last week in the journal Science, seeks to explain the conspicuous absence of predators weighing between 220 pounds and 2,200 pounds as adults from the fossil record.

The study surveyed more than 550 species from 43 dinosaur communities across 136 million years. Broadly, the researchers found lots of species that weighed under 220 pounds and over 2,200 pounds as adults, but very few in between.

Speaking with Live Science, Katlin Schroeder, a paleontologist at the University of New Mexico and lead author of the study, proposes that juvenile T. rex and other immature big predators “may have outcompeted other medium-sized dinosaurs, resulting in deflated global dinosaur diversity."

Schroeder tells Riley Black for New Scientist, that “the implication is that we’re not missing medium-sized dinosaurs from the fossil record because they didn’t fossilize well or haven’t been collected, but that competition from juvenile megatheropods pushed them out of the ecosystem.”

This pattern stands in stark contrast to what we observe today in predator-rich landscapes such as the Serengeti in Africa, where African wild dogs and servals fill in the mid-sized range between mongooses and lions, according to New Scientist.

Why Medium-Sized Dinosaurs Are Often Missing From the Fossil Record
A graphic showing the relative sizes of carnivorous mammals from Kruger National Park, South Africa on top, and the sizes of carnivorous dinosaurs from Canada's Dinosaur Park Formation on the bottom. The top and bottom graphics are scaled to the largest carnivores in the ecosystems they represent. Schroeder et al. Science 2021 / AAAS

“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 was not involved with the new research, tells George Dvorsky of Gizmodo.

The basic idea is that a juvenile T. rex would have been slender and fast-running compared to a hulking, bone-crushing full-grown adult, which might have positioned it to go after different prey that might have otherwise been fodder for a separate, medium-sized species.

The essential problem for would-be, mid-sized dinos was that even though there don’t appear to have been many species of predatory dinosaurs that were medium sized as adults, functionally speaking, the presence of these lithe juvenile T. rex and their ilk meant were plenty of medium-sized predators running around. That meant there wasn’t actually a vacancy in the food web waiting to be exploited by species at that size.

Per New Scientist, this ecological pattern might have emerged in the age of the dinosaurs partly because herbivores evolved to reach large sizes early on, spurring predators to become even bigger themselves. But even the biggest dinosaurs still had to hatch out of an egg, which limited their maximum size as hatchlings and set the stage for ravenous teenage T. rex terrorizing the ecosystem’s lower ranks.

“Bottom line is that throughout that entire time, it’s difficult to be a medium-sized carnivore,” Shroeder tells New Scientist.