Dinosaurs gave rise to gigantic animals, but almost never to terrestrial species as small as a sparrow. Why?
The observation has long been puzzling. Fossils do show small non-avian dinosaurs, that is, dinosaurs outside the lineage of modern birds. Yet truly small sizes remain rare.
Image Wikimedia
To understand this absence, researchers traced the evolution of body mass across several branches of their family tree. Their model compares size changes over time, then examines the effect of lifestyle, physiology and occupied environments.
No clear anatomical barrier appears. Some dinosaurs close to birds did gradually shrink in size. Their bodies therefore seemed capable of producing very small adults. But on land, another limit came into play before reaching the dimensions of the smallest birds.
This limit may come from young dinosaurs themselves. During their growth, dinosaurs successively occupy several niches, that is, several ways of feeding and living in the same environment.
Concretely, a young dinosaur growing large needs the food, shelter and space that a small adult species would have needed. A small adult would therefore have competed for these resources throughout its life with a large number of young individuals from other species.
Birds changed the game. Flight allowed them to reach branches, the air and resources that were largely inaccessible to strictly terrestrial dinosaurs. Their growth, reproduction and energy needs also evolved, leaving more room for small sizes.
An animal’s size therefore depends not only on its skeleton or metabolism. It also depends on what it can eat, available refuges, predators and competitors. To stay small, a species must find a lasting place that no one else already occupies.
One caveat remains: small bones do not preserve as easily and are harder to find. Future fossils could therefore change the known limit. Above all, they would help pinpoint when the first birds began to occupy ways of life inaccessible to their terrestrial cousins.