Dinosaurs are famous for being big, from the towering Tyrannosaurus rex to the colossal Apatosaurus. But a new study published today in the journal Evolution by scientists at the American Museum of Natural History and Princeton University suggests that one of the greatest mysteries of dinosaur evolution isn't how some species became giants—it's why even the smallest dinosaurs never became truly tiny.
Using mathematical models to investigate the evolution of body size across vertebrates, researchers found that while energetics and physiology can explain the sizes of mammals, birds, and turtles, they cannot explain why the smallest non-bird dinosaurs remained unusually large throughout their evolutionary history. Instead, the findings suggest that competition with small-bodied early mammals may have prevented dinosaurs from becoming mouse- or sparrow-sized animals that are so common among vertebrates today.
"Everyone loves a giant dinosaur," said study co-author Roger Benson, the Macaulay Curator of Dinosaur Paleobiology at the Museum. "But we decided to look at the other end of the scale. The absence of tiny dinosaurs may be just as interesting as the existence of giant ones. We already knew that dinosaurs prevented mammals from evolving to large sizes before the end-Cretaceous mass extinction. Here we suggest that mammals in turn prevented dinosaurs from evolving to small sizes."
While the largest dinosaurs weighed more than 80 tons, the smallest known non-bird dinosaurs weighed nearly one pound (approximately 450 grams)—about the size of a large rabbit. This is more than 200 times larger than the world's smallest bird, the bee hummingbird, which weighs just 1.75 grams and the smallest mammal, the Etruscan shrew, which is about 1.8 grams. The smallest lizard, the dwarf gecko, is only 0.15 grams.
Roughly 75 percent of living mammal species and 90 percent of living bird species are smaller than the smallest non-bird dinosaurs.
"Small animals dominate modern ecosystems," said Stephanie Lechki, the lead author of the study and a postdoctoral fellow at Princeton University. "If we want to understand how today's biodiversity evolved, we need to understand why tiny dinosaurs appear to have been missing."
One possible explanation is simply that scientists haven't found fossils of the smallest dinosaurs. Tiny animals are harder to discover, and their delicate bones are less likely to fossilize. But Benson and Lechki argue that explanation is unlikely because fossil deposits that preserve dinosaurs routinely preserve much smaller animals as well, including mammals, lizards, amphibians, and other vertebrates. If mouse-sized dinosaurs had existed, scientists would expect at least some of them to appear alongside these other small animals.
Instead, the fossil record suggests that genuinely tiny non-avian dinosaurs were rare, or maybe they were absent altogether.
To investigate why, the researchers used mathematical models that predict how natural selection shapes body size, based on the physiology of energy uptake and reproduction. These models are based on the idea that animals should evolve into sizes that help them most efficiently turn energy into offspring.
This approach successfully predicted observed body size distributions of mammals, birds, and turtles, but it did not work for snakes, lizards, and crocodilians. And even after accounting for a wide range of physiological possibilities, the models could not explain why most non-avian dinosaurs were so much larger than expected.
The findings suggest that physiology alone cannot explain dinosaur body sizes. Instead, ecological factors—including competition, predation, and the availability of different ecological niches—likely played a large role in limiting how small non-avian dinosaurs could become.
The study also offers new insight into one of the most dramatic transitions in vertebrate evolution: the emergence of birds. Although birds evolved from dinosaurs, modern birds are dramatically smaller than even the smallest known non-avian dinosaurs. Birds evolved body sizes far smaller than any other group of dinosaurs in just a short time after they first appeared in the Early Cretaceous.
The researchers found that this shift toward miniature body sizes also cannot be explained by physiology alone. Instead, they propose that the evolution of powered flight fundamentally changed the ecological opportunities available to early birds. By taking to the air, birds may have escaped the ecological constraints that prevented their dinosaur ancestors from evolving into much smaller animals.
"The ability to fly may have opened entirely new ways of life," Lechki said. "Once birds entered those new ecological niches, they were free to evolve body sizes that had simply not been possible for other dinosaurs."
Future studies may reveal even more about the ecological pressures that determined 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," 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."
Study DOI: 10.1093/evolut/qpag117
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