Dinosaurs evolved into some of the largest animals ever to walk the Earth, but a new mathematical model suggests that early mammals may have crowded them out of the ecological niches reserved for the smallest creatures.
Torvosaurus gurneyi. Image credit: © Sergey Krasovskiy.
“Across vertebrates living today, tiny animals are everywhere. Mice, hummingbirds, geckos, and shrews all weigh just a few grams or less,” said Princeton University’s Dr. Stephanie Lechki and Dr. Roger Benson from the American Museum of Natural History.
“Yet even the smallest known non-bird dinosaurs weighed around 450 grams — about the size of a large rabbit. That means the majority of living birds and mammals are smaller than the tiniest dinosaurs ever discovered.”
“So what prevented dinosaurs from becoming truly tiny? The answer, new research suggests, may lie not in dinosaur biology, but in their ecosystems.”
In the new research, they tested whether a mathematical model of ‘energetic fitness’ can explain why animal species come in the sizes they do, and specifically why the smallest non-avian dinosaurs were still hundreds of times heavier than the smallest living birds, mammals, or reptiles.
They compiled body-mass data for thousands of species across six groups: mammals, birds, lizards, snakes, turtles, and crocodylians, plus 515 non-avian dinosaur species.
Using statistical regressions on the data, they calculated the acquisition and conversion constants for each group and generated predicted body-size distributions.
They compared these model predictions against the actual, observed body-size distributions for each group.
The model worked well for mammals, birds, and turtles, but failed for lizards, snakes, and crocodylians.
No combination of realistic physiological parameters could explain the dinosaurs’ oversized minimum body mass.
This led the authors to conclude that ecological pressures (like competition or predation, possibly from early mammals), not pure physiology, likely kept small-bodied dinosaurs from evolving, until flight’s origin in birds abruptly lifted that constraint.
“Everyone agrees that dinosaurs restricted mammals to small sizes, before the end-Cretaceous mass extinction,” Dr. Benson said.
“Here we suggest that mammals in turn restricted dinosaurs to large sizes, preventing the evolution of tiny dinosaur species.”
“The ability to fly may have opened entirely new ways of life,” Dr. Lechki added.
“Once birds entered those new ecological niches, they were free to evolve body sizes that had simply not been possible for other dinosaurs.”
According to the team, future work is needed to fully understand which body sizes flourished and which never evolved at all.
“We have this unusual situation where the ancestors of dinosaurs could be tiny. The living descendants of dinosaurs — birds — they can be tiny. But dinosaurs themselves seemed to be forbidden from being tiny,” Dr. Benson said.
“And we don’t really understand that yet, but it’s a question we should continue to explore if we really want to understand dinosaurs and their fascinating biology.”
The study was published August 5 in the journal Evolution.
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Stephanie C. Lechki & Roger B.J. Benson. The explanatory power of energetic fitness models across living amniotes and extinct non-avian dinosaurs. Evolution, published online August 5, 2026; doi: 10.1093/evolut/qpag117
