Where Were All the Tiny Dinosaurs? Evolutionary Size Barriers

A recent study published on August 13, 2026, in the journal Evolution by researchers Stephanie Lechki and Roger Benson explores why non-avian dinosaurs rarely became truly tiny, despite immense evolutionary diversity. While some dinosaurs exceeded 80 tons, the smallest known non-bird dinosaurs weighed about one pound, or 450 grams, contrasting sharply with modern small vertebrates like the bee hummingbird weighing 1.75 grams or the Etruscan shrew weighing 1.8 grams. The researchers tested mathematical models of energetic fitness across thousands of living amniotes and 515 extinct non-avian dinosaurs to determine if physiology dictated this size floor. The models failed to explain the absence of tiny dinosaurs through metabolic constraints alone, indicating that physiological limitations were not the primary driver of their minimum body size. Instead, the authors argue that ecological competition served as a major barrier, with early mammals and other small vertebrates already occupying miniature ecological niches during the Mesozoic era. This creates a symmetrical picture where large dinosaurs restricted mammalian growth, while small mammals simultaneously prevented dinosaurs from shrinking into lower body mass categories. The study highlights that the emergence of powered flight allowed early birds to break through this evolutionary size barrier by accessing novel aerial and arboreal habitats. Ultimately, the research demonstrates that evolutionary pathways are shaped not only by physiological capabilities but also by existing ecological competition and accessible niches within ancient ecosystems.