Dinosaurs never became as small as a mouse. Two researchers from the American Museum of Natural History and Princeton University have now put forward an explanation. The limit was probably not physiology but competition from early mammals. The work appeared in the journal "Evolution".
Stephanie Lechki and Roger Benson tested vertebrate body size against a mathematical model. It calculates the weight at which an animal reproduces most efficiently in terms of energy. For today's mammals, birds and turtles the model holds. For non-avian dinosaurs it fails badly.
The model predicts a most common body mass of about three kilograms for dinosaurs. The most common mass derived from fossils is around 180 kilograms. Even the smallest known species weighed some 400 grams, about as much as a city pigeon. Among them are the feathered microraptors of the early Cretaceous, which began roughly 145 million years ago. They were at least 200 times heavier than the bee hummingbird, the smallest bird alive today at 1.75 grams, and the Etruscan shrew at about 1.8 grams.
The gap is not a gap in the record
The obvious objection is that tiny bones simply survive less well. Lechki and Benson consider that unlikely. The same deposits that preserve dinosaur bones regularly also hold mammals, amphibians and lizards. Had mouse-sized dinosaurs been widespread, some of them should have turned up.
That leaves ecology. The small niches, the pair argue, were already taken. "We already knew that dinosaurs prevented mammals from evolving into large animals before the mass extinction at the end of the Cretaceous. Here we suggest that mammals in turn prevented dinosaurs from evolving into small animals," Benson says.
For birds this yields a twist. They descend from dinosaurs and today fall well below their ancestors' size. The decisive factor, the researchers reckon, was active flight. It opened up habitats that were spoken for on the ground.
The model has limits, and the authors name them. It also fails for present-day lizards and for flightless island birds, "Spiegel" reports. Ecological constraints could be shifting the distribution there too. Large fossils are moreover easier to find than small ones, which probably biases the observed figure of 180 kilograms upwards. The question of why small animals dominate the planet today hangs on this gap. "If we want to understand how today's biodiversity came about, we have to understand why tiny dinosaurs were apparently missing," Lechki says.