Why dinosaurs never became mouse-sized

Dinosaurs, often imagined as colossal creatures, rarely evolved to be truly tiny, unlike modern mammals and birds. A new study by researchers from the American Museum of Natural History and Princeton University delves into this evolutionary puzzle.

Using a mathematical model to study vertebrate body size evolution, the team found it could explain the sizes of modern mammals and birds. However, it failed to account for why even the smallest dinosaurs, weighing approximately 400 grams, were significantly larger than the smallest modern animals.

These smallest dinosaurs, such as the feathered Microraptors that lived during the early Cretaceous period, were still at least 200 times heavier than the world's smallest bird, the bee hummingbird (1.75 grams), and the smallest mammal, the Etruscan shrew (1.8 grams). The researchers propose that competition with small mammals for resources might have been the key factor preventing dinosaurs from occupying the ecological niche of very small animals.

"We already knew that dinosaurs prevented mammals from evolving into large animals before the mass extinction at the end of the Cretaceous period," said co-author Roger Benson. "Here we suggest that mammals, in turn, prevented dinosaurs from evolving into small animals." The model, based on energetic factors, describes the body size at which an animal can most efficiently reproduce. While it aligns with observed sizes in modern mammals, birds, and even turtles, it doesn't fit modern lizards or flightless island birds, suggesting ecological constraints may also play a role in altered size distributions.

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We already knew that dinosaurs prevented mammals from evolving into large animals before the mass extinction at the end of the Cretaceous period. Here we suggest that mammals, in turn, prevented dinosaurs from evolving into small animals.