Dinosaurs are best known for their enormous size. Tyrannosaurus rex was a powerful predator, while long-necked giants such as Apatosaurus reached extraordinary weights. However, new research suggests that one of the most important mysteries in dinosaur evolution may involve the opposite question: Why did non-avian dinosaurs apparently never become extremely small?
In a study published in the journal Evolution, researchers from the American Museum of Natural History and Princeton University used mathematical models to investigate how body size evolved across vertebrates. The models successfully explained size patterns in mammals, birds, and turtles by examining energy use and physiology. However, those same factors could not explain why the smallest non-avian dinosaurs remained relatively large.
The researchers propose that ecology may have created a lower size limit for dinosaurs. Competition from early small mammals, in particular, may have prevented dinosaurs from occupying mouse-sized and sparrow-sized ecological niches that are common among vertebrates today.
“Everyone loves a giant dinosaur,” said study co-author Roger Benson, the Macaulay Curator of Dinosaur Paleobiology at the American Museum of Natural History. “But we decided to investigate the other end of the size spectrum. The absence of tiny dinosaurs may be just as scientifically interesting as the existence of giant ones. We already knew that dinosaurs may have limited the evolution of large mammals before the end-Cretaceous mass extinction. Our study suggests that mammals may, in turn, have limited the evolution of very small dinosaurs.”
Why Were Non-Avian Dinosaurs So Large?
The difference between dinosaurs and modern animals is striking. The largest dinosaurs weighed more than 80 tons, while the smallest known non-avian dinosaurs weighed approximately one pound, or about 450 grams—roughly the size of a large rabbit.
That makes the smallest known non-avian dinosaurs more than 200 times heavier than the bee hummingbird, the world’s smallest bird, which weighs about 1.75 grams. The Etruscan shrew, the smallest living mammal, weighs approximately 1.8 grams. Some reptiles are even smaller: the dwarf gecko can weigh only about 0.15 grams.
Small-bodied animals are also extremely common in modern ecosystems. Approximately 75 percent of living mammal species and 90 percent of living bird species are smaller than the smallest known non-avian dinosaurs.
“Small animals dominate modern ecosystems,” said Stephanie Lechki, lead author of the study and a postdoctoral fellow at Princeton University. “To understand how modern biodiversity developed, we also need to understand why tiny dinosaurs appear to have been absent.”
Could Tiny Dinosaur Fossils Be Missing?
One possible explanation is that very small dinosaurs existed but have not yet been discovered. Small animals are often difficult to identify in the fossil record because their delicate bones are less likely to survive fossilization and erosion.
Benson and Lechki argue that the available evidence makes this explanation unlikely. Fossil sites that preserve dinosaurs also frequently contain much smaller animals, including mammals, lizards, amphibians, and other vertebrates.
If mouse-sized dinosaurs had been widespread, scientists would expect at least some of their fossils to have been found alongside these other tiny animals.
Instead, the fossil record indicates that truly tiny non-avian dinosaurs were either exceptionally rare or may never have existed at all.
Mathematical Models Highlight a Dinosaur Evolution Mystery
To investigate what may have prevented dinosaurs from becoming smaller, the researchers used mathematical models that predict how natural selection can influence body size. The models include physiological factors related to energy intake, growth, and reproduction.
The basic premise is that evolution should favor body sizes that allow animals to use available energy efficiently and produce offspring successfully.
When applied to different vertebrate groups, the models reproduced many of the body-size patterns observed in mammals, birds, and turtles. They were less effective at explaining size variation in snakes, lizards, and crocodilians.
Dinosaurs posed an even greater challenge. Even after testing a broad range of physiological conditions, the models could not explain why most non-avian dinosaurs were substantially larger than expected.
This finding suggests that dinosaur body size was not determined by physiology alone. Ecological factors—including competition, predation, food availability, and access to specific habitats—likely influenced how small non-avian dinosaurs could become.
How Birds Escaped the Dinosaur Size Limit
The study may also help explain one of the most important developments in vertebrate evolution: the emergence and diversification of birds.
Birds evolved from dinosaurs, yet modern birds can be far smaller than any known non-avian dinosaur. Soon after the first birds appeared during the Early Cretaceous, some evolved body sizes smaller than those seen in other dinosaur groups.
The researchers suggest that physiology alone cannot explain this rapid evolution toward miniature bird species.
Powered flight may instead have transformed the ecological opportunities available to early birds. Flight allowed them to reach habitats, food sources, and lifestyles that were inaccessible to many of their ground-dwelling dinosaur relatives. This change may have helped birds escape the ecological pressures that had restricted dinosaur body size.
“The ability to fly may have opened entirely new ecological opportunities,” Lechki said. “After birds entered these new niches, they could evolve body sizes that had not been possible for other dinosaurs.”
A Small Dinosaur Mystery With Major Evolutionary Implications
Future research could reveal which ecological pressures favored certain dinosaur body sizes while preventing others. Scientists may also learn more about how competition between dinosaurs and early mammals shaped the evolution of both groups.
Understanding these forces could help explain how dinosaurs evolved into some of the largest animals ever to walk on land while apparently failing to occupy the smallest size ranges that are so common among modern animals.
“This creates an unusual evolutionary pattern: dinosaur ancestors could be small, and the living descendants of dinosaurs—birds—can also be tiny. Yet non-avian dinosaurs themselves seem to have been restricted from reaching very small body sizes,” Benson said. “We do not yet fully understand why, but it is an important question to continue exploring if we want to understand dinosaur evolution and biology.”
Source: www.sciencedaily.com