Study Finds Early Mammals Prevented Evolution of Tiny Dinosaurs

A new study published in the journal *Evolution* by researchers from the American Museum of Natural History and Princeton University used mathematical models to examine vertebrate body‑size evolution. The analysis shows that while energetics and physiology explain size ranges in mammals, birds and turtles, they do not account for the persistent absence of truly tiny non‑bird dinosaurs. The authors propose that early mammals, most of which were shrew‑sized, already occupied the ecological niches for very small animals, thereby blocking dinosaurs from evolving mouse‑ or sparrow‑sized forms.

The smallest known non‑bird dinosaurs, such as *Microraptor* (≈430 g), *Fruitadens* (≈730 g) and *Anchiornis* (≈530 g), were still orders of magnitude larger than the smallest modern birds, mammals and lizards. Co‑lead researcher Stephanie Lechki said, “A small animal typically requires fewer resources and can reach reproductive maturity more quickly than a large‑bodied animal.” Co‑author Roger Benson added that the lack of tiny dinosaurs suggests many small‑body niches remained available to mammals, shaping today’s biodiversity.