Dinosaurs' Missing Miniature Counterparts
New research suggests that dinosaurs may have been prevented from evolving into tiny animals due to competition with early mammals.
Dinosaurs are often remembered for their enormous size, but a new study suggests that one of the most intriguing questions about dinosaur evolution lies at the opposite end of the size spectrum: Why did dinosaurs apparently never become extremely small?
The study, published in the journal *Evolution*, used mathematical models to examine how body size evolved among vertebrates. The results show that energy use and physiology can account for the range of sizes seen in mammals, birds, and turtles. However, these factors cannot explain why even the smallest dinosaurs other than birds remained relatively large.
## Dinosaurs' Lower Size Limit
The contrast between dinosaurs and many modern animals is striking. The largest dinosaurs exceeded 80 tons, yet the smallest known dinosaurs outside the bird lineage weighed close to one pound (approximately 450 grams). This is more than 200 times heavier than the bee hummingbird, the smallest bird in the world, which weighs only 1.75 grams.
| | Smallest known dinosaurs outside the bird lineage | Bee hummingbird | Etruscan shrew | Dwarf gecko | | --- | --- | --- | --- | --- | | Weight | approximately 450 grams | 1.75 grams | 1.8 grams | 0.15 grams |
Tiny animals are also extremely common in modern ecosystems. About 75 percent of living mammal species and 90 percent of living bird species are smaller than the smallest known dinosaurs outside the bird lineage.
## Could Tiny Dinosaur Fossils Simply Be Missing?
One obvious possibility is that very small dinosaurs existed but have not yet been discovered. Small animals are generally more difficult to find in the fossil record because their fragile bones are less likely to survive the fossilization process. However, the researchers argue that this explanation does not fit the available evidence. Fossil sites containing dinosaurs also frequently preserve animals that were much smaller, including mammals, lizards, amphibians, and other vertebrates.
If dinosaurs as small as mice had been widespread, researchers would expect at least some of their fossils to have turned up alongside these other tiny animals. Instead, the fossil evidence indicates that genuinely tiny dinosaurs outside the bird lineage were either extremely uncommon or may not have existed at all.
## Mathematical Models Reveal a Dinosaur Puzzle
To understand what might have prevented dinosaurs from becoming smaller, the researchers turned to mathematical models designed to predict how natural selection influences body size. The models incorporate aspects of animal physiology related to energy intake and reproduction. When the researchers applied this framework to different vertebrate groups, it successfully reproduced the body size patterns seen in mammals, birds, and turtles. However, the models still could not account for why most dinosaurs outside the bird lineage were considerably larger than expected.
This result suggests that dinosaur size was not controlled by physiology alone. Ecological pressures, including competition, predation, and access to different ecological niches, probably helped determine how small these dinosaurs could become.
## How Birds Escaped the Dinosaur Size Constraint
The findings may also help explain a major turning point in vertebrate evolution: the rise of birds. Birds evolved from dinosaurs, but modern birds can reach sizes far below those known for dinosaurs outside the bird lineage. According to the researchers, physiology by itself cannot account for this rapid move toward miniature bodies either.
Instead, they suggest that powered flight transformed the ecological possibilities available to early birds. Once birds could fly, they gained access to habitats and ways of life that were unavailable to their terrestrial dinosaur relatives. That shift may have allowed them to escape the ecological forces that had previously prevented dinosaurs from evolving much smaller bodies.
"The ability to fly may have opened entirely new ways of life," said Stephanie Lechki, the lead author of the study. "Once birds entered those new ecological niches, they were free to evolve body sizes that had simply not been possible for other dinosaurs.
