Scientists have put forward an explanation as to why there were no tiny dinosaurs
Dinosaurs could weigh tens of tonnes, but among their non-avian relatives there were no known adult species the size of a mouse or a tiny sparrow. A new study has shown that metabolic and reproductive characteristics do not explain this unusual size limit. The authors suggest that environmental constraints, including competition with early mammals, may have prevented dinosaurs from becoming smaller.
The smallest known adult non-avian dinosaurs weighed around 400–450 grams — roughly the same as a large rabbit. These animals included, for example, the feathered predator Microraptor.
By the standards of modern fauna, this is quite a large size. Around 75 per cent of living mammal species and 90 per cent of bird species weigh less than the smallest known non-avian dinosaurs. Miniature modern vertebrates can weigh less than two grams.
Details
It is possible that palaeontologists have simply not found the remains of the smallest dinosaurs. Thin bones do not preserve as well, and tiny fossils are harder to spot during excavations.
However, significantly smaller mammals, lizards and amphibians are regularly found in the same deposits. If adult dinosaurs the size of a mouse had been common, at least some of their remains would probably have survived as well. The authors therefore believe that such species were indeed extremely rare or non-existent.
The researchers used energy models to predict what body size would be advantageous for an animal, taking into account energy acquisition, metabolism and reproduction.
For the analysis, they gathered data on thousands of modern mammals, birds and reptiles, as well as on 515 species of non-avian dinosaurs. The models reproduced the size distributions of mammals, birds and turtles fairly well, but were unable to explain the results for dinosaurs, lizards, snakes and crocodiles.
Even under different physiological scenarios, the calculations allowed for the existence of significantly smaller dinosaurs than those known from the fossil record. This suggests that body structure alone is not sufficient to explain the absence of miniature species.
The authors proposed an ecological explanation. In Mesozoic ecosystems, the niches of small terrestrial animals were already occupied by early mammals, as well as other small vertebrates.
It is known that large dinosaurs may have prevented mammals from occupying the niches of large terrestrial animals. The researchers suggest that the reverse effect also existed: mammals may have competed with potentially small dinosaurs for food, shelter and other resources.
However, this remains a hypothesis. The study does not prove direct competition between specific species. It shows that internal physiological factors do not provide a sufficient explanation and that competition, predation and the availability of ecological niches must be taken into account.
Why birds were able to become tiny
Birds are the only line of dinosaurs to have survived to the present day, so the claim that all dinosaurs were large is biologically incorrect.
During the early Cretaceous period, the avian lineage underwent a rapid reduction in size. Some ancient birds weighed just a few tens of grams — far less than any known adult non-avian dinosaur.
Scientists suggest that the development of active flight opened up new ways of moving around and foraging for food. By occupying aerial ecological niches, early birds were able to break free from the constraints that had prevented terrestrial dinosaurs from becoming any smaller. The study itself, however, does not prove that flight was the sole cause of miniaturisation.
Why this matters
Body size determines what an animal eats, where it lives, how it reproduces and which species it competes with. The absence of tiny non-avian dinosaurs may have had a significant impact on the structure of Mesozoic ecosystems.
The new study shows that the evolution of size cannot be explained by metabolism alone. Even if an organism is physiologically capable of becoming smaller, an ecological niche occupied by competitors may block this evolutionary path for it.
Source
The researchers compared the actual body mass distributions of modern amniotes and 515 species of non-avian dinosaurs with the predictions of energy models. The results showed that internal physiological factors do not explain the unusually large minimum size of dinosaurs.
Study: Stephanie C. Lechki, Roger B. J. Benson. The explanatory power of energetic fitness models across living amniotes and extinct non-avian dinosaurs.
Journal: Evolution, 2026.