New research published on August 7, 2026 in the scientific journal Evolution seeks to unravel a mystery that intrigues paleontology: the absence of truly small dinosaurs. Even the smallest known species, excluding birds, weighed around 450 grams, the equivalent of a large rabbit. This weight is considerably high when compared to most current mammals and birds, which, for the most part, are much lighter.
The size dilemma and discrepancy with modern animals
The classic image of dinosaurs refers to colossal creatures such as Tyrannosaurus rex and Apatosaurus. However, the study focuses on the other end of the size scale. The smallest known dinosaurs, such as Microraptor, weighed approximately 450 grams. This measure contrasts drastically with modern fauna:
- Mammals:About 75% of current species weigh less than 450 grams. The smallest known mammal is approximately 1.8 grams.
- Birds:Around 90% of bird species are lighter than 450 grams. The smallest recorded bird weighs about 1.75 grams.
This difference raises the question of why dinosaurs, ancestors of birds, never shrank to such small sizes in their development.
Learn more: Recent study explains why dinosaurs were not as small as birds
Research contradicts initial biology hypothesis
To investigate this question, researchers from the American Museum of Natural History, in the United States, and Princeton University developed complex mathematical models. These models simulated how biological factors, such as energy expenditure and reproduction, could have influenced the size of animals throughout evolution. The calculations managed to explain, in general, the size distribution observed among mammals, birds and turtles, but failed for dinosaurs.
Roger Benson, curator of dinosaur paleobiology at the American Museum of Natural History and one of the study’s authors, revealed his surprise at the results. “When we started the project, I was very excited by the idea that dinosaurs could have been prevented from becoming small by some characteristic of their biology,” he said. “We later discovered that this hypothesis was strongly rejected by our analyses. This was a big surprise to me.” The scientist emphasized that, in the field of science, it is essential to allow evidence to guide research, even if it overturns initial ideas, paving the way for alternative explanations.
The hypothesis of competition with primitive mammals
One of the possibilities suggested was that very small dinosaurs existed, but their fossils were not preserved or found, since smaller animals have more delicate bones. However, the authors consider this explanation incomplete. The same rock formations that hold dinosaur fossils also preserved the remains of much smaller mammals, lizards, amphibians and birds. Roger Benson explained that “fossils of small animals are rare. But they exist. In fact, some rocks contain many small animals, such as mammals, lizards and birds. So we find small animals, but they are never dinosaurs, with the exception of birds. This is very strong evidence that mouse-sized dinosaurs were absent or extremely rare.”
On the same topic: Study reveals crucial link between brain size and offspring in birds and dinosaurs, revealing giant eggs
The main theory raised by the study is ecological in nature: the small mammals that coexisted with dinosaurs probably already occupied the ecological niches available to small-sized animals. This competition for food, shelter, and other resources may have prevented the emergence of tiny dinosaurs. Thus, while large dinosaurs may have limited the evolution of larger mammals before the extinction that occurred 66 million years ago, mammals, in turn, would have exerted opposite pressure, hindering the development of small dinosaurs.
The role of flight in the evolution of birds and what is still being sought
An additional clue supporting the competition hypothesis comes from flightless birds. These species also exhibit a minimum size limit of approximately 450 grams, unless they live in environments where there are no mammals. Although this comparison is not definitive proof, it reinforces the idea that competition may have been a limiting factor in the size of dinosaurs.
Birds, which are direct descendants of dinosaurs, managed to break through this minimum size limit after they developed the ability to fly. By occupying air space, they had access to new food sources, shelters and different lifestyles, which effectively “opened” a new area of the environment for their evolution. Shortly after the emergence of flight, the first birds quickly decreased in size and went through a phase of great diversification. For researchers, these events suggest that flight may have freed the group from the size restrictions that their terrestrial ancestors faced. However, this interpretation still depends on the discovery of more fossils of the first birds, as the records found in China only help to understand the beginning of this transformation.