Why No Dinosaur Was Ever Smaller Than a Can of Soup

For more than 180 million years, the smallest known non-avian dinosaurs weighed about 400 grams as adults. That’s roughly the weight of a can of soup. A new study set out to crack one of paleontology’s most overlooked mysteries: why dinosaurs were seemingly locked out of small body sizes, and how birds, the only living dinosaurs, may have eventually broken free.

Published in the journal Evolution, the study tested whether a well-known scientific framework, one that links an animal’s body size to how efficiently it gathers energy and converts it into offspring, could explain why different groups of animals come in the sizes they do. Researchers applied this framework across birds, mammals, turtles, lizards, snakes, and crocodilians, and then turned it on extinct non-avian dinosaurs. What they found reshapes how scientists think about body-size evolution throughout the entire history of amniotes, the group that includes reptiles, birds, and mammals.

Across most living groups, body size is shaped by a balance between two competing forces: how well a creature acquires energy from its environment and how efficiently it turns that energy into young. Smaller animals are faster at converting resources into offspring, but less efficient at gathering those resources. Larger animals gather energy well but reproduce more slowly. This balancing act, the study found, explains the body size patterns of birds overall, mammals, and turtles well, with one clear exception among flightless birds. No version of the model, though, could explain why the smallest adult dinosaurs weighed several hundred grams instead of just a few, like the tiniest birds.

Testing the Energy Model Across the Animal Kingdom

Researchers gathered body mass data for thousands of species. Their dataset included 4,062 mammal species, 9,385 bird species, 6,275 lizard species, 515 non-avian dinosaur species, 21 crocodilian species, and 260 turtle species.

For each group, the team calculated two numbers: how quickly animals of different sizes acquire energy from their environment, and how quickly they convert that energy into reproductive effort. From these numbers, the model predicts the most common body size for each group, meaning the weight at which an animal reproduces most successfully Comparing those predictions to the real-world distribution of body sizes across each group reveals whether energy alone explains the patterns.

For mammals, the model predicted the most common body size to be about 100 grams. Actual mammals cluster right around that range. For birds, the predicted sweet spot was about 33 grams, which closely aligns with the most common bird size. Turtles also fit the model reasonably well, with predicted and observed sizes falling in a similar range.

Lizards, snakes, and crocodilians told a different story. For lizards, the model predicted the most common body size to be about 2.7 kilograms, but the actual most common lizard weighs only about 20 grams. For crocodilians, the model predicted a most common size of roughly 1.6 kilograms, while the actual most common crocodilian weighs about 295 kilograms. In these groups, ecological constraints may be overriding the internal energetic logic that governs body size.

Why Dinosaurs Could Never Go Small

When the researchers turned their model on non-avian dinosaurs, the pattern stood out. Even when they pushed the model to its absolute limits, using parameter combinations drawn from all living groups, the largest predicted optimum it produced was about 3 kilograms. But actual dinosaur body sizes ranged far above that, and more puzzlingly, no non-avian dinosaur species in the fossil record had adults smaller than roughly 400 grams, a lower limit the authors argue is a genuine feature of the group rather than a gap left by the fossil record.

Researchers also accounted for a known problem in dinosaur science: smaller fossils are harder to preserve and discover than larger ones. To address this, the team examined specific fossil sites known for preserving small animals well, including the Jehol Group in northeastern China and the Djadokhta Formation in Mongolia. Both sites preserve small-bodied tetrapods including mammals, lizards, and birds in the sub-400-gram range. Adult non-avian dinosaurs in that size range are simply absent, even there.

“The absence of non-avian dinosaur species with adult sizes smaller than 300 g, across all fossil assemblages globally, is likely genuine because tetrapods of these sizes (e.g., mammals, squamates, amphibians) are preserved in the fossil record, whereas adult dinosaurs in similar size ranges are not,” the authors write.

So if the energetic model does not forbid small dinosaurs and preservation bias cannot fully explain their absence, what was stopping them? The researchers point to ecological competition, particularly from early mammals. During the age of dinosaurs, small-bodied mammals were abundant and diverse. Small body sizes may simply have been ecologically occupied territory, with mammals already filling every niche available to a creature in that size range.

How Flight May Have Unlocked Tiny Birds

To test whether external ecological pressures actually push body sizes away from their energetically predicted values, the researchers compared mainland and island populations of lizards and flightless birds. Islands tend to have fewer predators and fewer competing species, meaning animals are freer to evolve toward whatever size biology alone might favor.

Island lizards showed a higher proportion of larger-bodied species than their mainland counterparts, moving closer to the energetically predicted optimum. Island flightless birds, conversely, showed a higher proportion of smaller-bodied species than continental flightless birds, bringing their distribution closer to what the model predicts for birds generally. In both cases, the lower ecological pressure on islands nudged body sizes toward the range the energy model predicts, backing the idea that competition and predators on the mainland push animals away from the sizes their biology alone would favor

Flightless birds on the mainland, the study notes, show a body-size pattern much like that of non-avian dinosaurs: mostly larger species, with almost no small ones. Without the advantages of flight, predation and competition appear to keep their body sizes high.

Living birds include species weighing as little as 1.74 grams, the bee hummingbird, about 200 times lighter than the smallest adult non-avian dinosaur. That shift happened abruptly in the fossil record, during the Early Cretaceous, around the same time that the sophisticated flight traits defining modern birds first appeared.

Flight may have done more than lift dinosaurs off the ground, the researchers propose. It may have triggered a release from the ecological constraints that had kept all other dinosaurs locked into larger body sizes. Once early birds were flying, they may have entered a new ecological space with different competitors and predators, or perhaps fewer of both. That ecological release, the researchers suggest, could have allowed the energetic logic of body size to take over, rapidly pulling bird sizes down toward the physiologically predicted range described by the model.

Whether early mammals were truly the force that kept dinosaurs locked out of small body sizes, or whether other ecological dynamics were at play, remains an open question. What the study makes clear is that body size is not shaped by biology alone. Ecology, competition, and the accidents of evolutionary history all leave their mark on the range of sizes a group of animals can actually achieve, sometimes overriding millions of years of energetic logic in the process.

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