A model of Microraptor was recently added to a display in the Museum’s Hall of Saurischian Dinosaurs. Microraptor was one of the smallest known dinosaurs, weighing about one pound—roughly the size of a large rabbit.Alvaro Keding/© AMNH
From Tyrannosaurus rex to Patagotitan mayorum, dinosaurs are famous for their enormous size. But their most surprising evolutionary feature may be the opposite: even the smallest dinosaurs were bigger than most living species, throughout more than 150 million years of evolution.
Across vertebrates living today, tiny animals are everywhere. Mice, hummingbirds, geckos, and shrews all weigh just a few grams or less. 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.
To investigate this question, researchers from the Museum and Princeton University used mathematical models that predict the body sizes natural selection should favor based on how animals use energy and reproduce. The models accurately captured the body size patterns seen in mammals, birds, and turtles, but they failed for non-bird dinosaurs.
Even after testing a wide range of physiological scenarios, the dinosaurs remained unexpectedly large. In other words, no reasonable model of their biology can explain the absence of tiny dinosaurs.
During the age of the dinosaurs, early mammals were thriving in the smallest ecological niches. Rather than evolving into mouse-sized animals themselves, dinosaurs may simply have been excluded from those roles by competition and other ecological pressures.
“Everyone agrees that dinosaurs restricted mammals to small sizes, before the end-Cretaceous mass extinction,” said Roger Benson, the Macaulay Curator of Dinosaur Paleobiology at the Museum and co-author of a recent study, published today in the journal Evolution. “Here we suggest that mammals in turn restricted dinosaurs to large sizes, preventing the evolution of tiny dinosaur species.”
The study also offers a fresh perspective on the origins of birds. Although birds descended from dinosaurs, they quickly evolved body sizes below anything seen in their non-flying relatives. The researchers propose that powered flight allowed early birds to exploit entirely new ecological opportunities, freeing them from the constraints that had kept other dinosaurs comparatively large.
“The ability to fly may have opened entirely new ways of life,” said Stephanie Lechki, the lead author of the study and a postdoctoral fellow at Princeton University. “Once birds entered those new ecological niches, they were free to evolve body sizes that had simply not been possible for other dinosaurs.”
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,” 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.”