Why were dinosaurs never as small as a mouse?

A study published in the journal Evolution has raised new questions about why non-avian dinosaurs never evolved to the size of modern small mammals or birds, such as mice or sparrows. Researchers Stephanie Lechki from Princeton University and Roger Benson from the American Museum of Natural History argue that despite the immense diversity in dinosaur sizes, ranging from massive sauropods weighing over 80 tons to much smaller species, the smallest known non-avian dinosaurs were still significantly larger than today's tiniest vertebrates. The study suggests there was a notable gap at the lower end of the dinosaur size spectrum, with the smallest known species weighing around 450 grams, roughly equivalent to a large rabbit. The researchers point to Alnashetri cerropoliciensis, one of the smallest known non-avian dinosaurs, which weighed approximately 450 grams. This makes them more than 200 times heavier than the bee hummingbird (Mellisuga helenae), currently the world’s smallest living bird, which weighs just 1.75 grams. Even the smallest extant mammals, such as the Etruscan shrew (Suncus etruscus) at 1.8 grams, or the tiny gecko Sphaerodactylus aeneus at 0.15 grams, are far lighter than the smallest non-avian dinosaurs. Lechki and Benson suggest that this absence of true "mouse-sized" dinosaurs is not simply due to a lack of fossil evidence. They argue that while small animals are less likely to be preserved in the fossil record, the presence of other small vertebrates in well-preserved deposits indicates that such fossils should exist if they had ever been present. Their analysis of multiple sites shows that even in locations where small animals are commonly found, no dinosaur fossils of comparable size have been identified. To investigate further, the researchers used mathematical models based on physiological factors such as energy intake and reproductive costs. These models predict the body size that would allow an animal to convert energy into offspring most efficiently. For modern mammals, birds, and turtles, these models align closely with observed size distributions. However, they fail to explain the size distribution of non-avian dinosaurs. According to their calculations, the most common body size among dinosaurs should have been around three kilograms, yet the observed average is closer to 180 kilograms. This discrepancy suggests that the data might be skewed toward larger specimens because they are more frequently discovered. The researchers propose that ecological factors could explain the absence of very small non-avian dinosaurs. Competition with early mammals, which occupied the same niche, may have played a role. Mammals, which appeared during the Mesozoic era, were already filling the ecological roles typically associated with small body sizes. Predation pressure and the availability of suitable niches may have limited how small non-avian dinosaurs could evolve. The study highlights the complex interplay between physiology, ecology, and evolution in shaping the body sizes of ancient organisms. While some of the smallest non-avian dinosaurs, such as the feathered microraptors from the Early Cretaceous period, were relatively small compared to their larger relatives, none reached the size of modern small mammals or birds. This finding adds another layer to our understanding of dinosaur biology and the evolutionary pathways that led to the dominance of birds, which are considered the direct descendants of theropod dinosaurs. The research team continues to explore potential explanations for this phenomenon, including the possibility that certain environmental conditions or predator-prey dynamics prevented the emergence of truly minute non-avian dinosaurs. As more fossil discoveries are made and additional studies are conducted, scientists hope to gain deeper insights into the evolutionary constraints that shaped the size range of these prehistoric giants.