But why were there never tiny dinosaurs, like the size of a mouse? They hypothesise that because early mammals – most of them puny and shrew-sized – had filled ecological niches for small animals, their presence prevented dinosaurs from evolving tiny body sizes adapted to such niches. “A small animal typically requires fewer resources and can achieve reproductive maturity and reproduce more quickly than ⁠a large-bodied animal,” Lechki adds. The smallest dinosaurs included the likes of crow-sized Microraptor, weighing as little as 430 grams (15oz); Fruitadens, at 730 grams; and Anchiornis, at 530 grams. These were still much ‌larger than today’s smallest bird, the bee hummingbird (about 1.95 grams); smallest mammal, the Etruscan shrew (about 1.8 grams); and smallest lizard, the dwarf gecko (about 0.15 grams).