Stretching 22 meters from snout to tail, Apatosaurus was one of the largest creatures to ever walk the planet. At 10 meters long with razor-sharp teeth, Allosaurus was one of the most fearsome. How did such animals come to dominate the planet? A new study suggests it was nothing more than dumb luck.
The predecessors of dinosaurs rose from the ashes of Earth's worst extinction. Prior to 251 million years ago, the dominant, large animals on land were the therapsids, early forerunners to mammals. These shrew- to hippo-size creatures came in a variety of forms, from tubby, tusked herbivores to agile, saber-toothed predators. At the end of the Permian period about 250 million years ago, however, rampant global warming and drastic drops in the atmosphere's oxygen content wiped out almost 90% of the planet's species. Many therapsids disappeared, and the few, diminutive lineages that survived faced competition from a different sort of creature—the archosauromorphs. These reptiles, the precursors of dinosaurs, crocodiles, and their closest relatives, would quickly rise to dominance.
To figure out how the archosauromorphs came to dominate other species, graduate student Roland Sookias of Ludwig Maximilian University of Munich in Germany and colleagues traced the evolution of body size in therapsids and archosauromorphs. They used femur length of over 400 species of fossil creatures spanning 100 million years to estimate body mass and tracked how body size changed in the two groups from the time of the Permian mass extinction to the heyday of the biggest Jurassic dinosaurs about 150 million years ago.
Sookias and collaborators confirmed that the archosauromorphs grew into a wider range of sizes—including the largest animals on land-while the therapsids remained small. One reason may be that archosauromorphs simply grew and bred faster. The quicker growth rate of the archosauromorphs, which had been discovered in previous studies of dinosaurs and their relatives, meant that these animals reached sexual maturity relatively earlier than the therapsids. Faster growth and breeding meant that the archosauromorphs quickly spread and adapted to overtake available habitats and the ecological roles of large herbivores and big predators before the smaller, slower-growing therapsids had a chance to put up a fight, Sookias says.
Not all of the archosauromorphs were large—over time, they diversified into a variety of body sizes throughout the Triassic and Jurassic, from the relatively tiny feathered dinosaur Anchiornis to Apatosaurus and Diplodocus, some of the biggest creatures to ever walk the planet. A key to evolving into such a wide range of sizes, Sookias says, may be in specialized features such as air pockets inside dinosaur bones that reduced the weight of their skeletons and opened up a wider range of possible sizes.
Thus, it appears that the forerunners of dinosaurs and mammals did not fiercely compete for space but rather used available space differently. After the Permian extinction, the archosauromorphs grew faster than the therapsids and effectively shut out the mammal precursors. The archosauromorphs filled up the ecological space so quickly that the therapsids were forced to stay small and use what was left over, the team reports online today in the Proceedings of the Royal Society B.
Paleontologist Jessica Theodor of the University of Calgary in Canada calls the research "intriguing" but notes a caveat about the methodology. While the new study used the femur lengths of therapsids and archosauromorphs to estimate body mass, Theodor says that the lack of modern, closely related species to the therapsids and archosauromorphs means that the estimates of body mass will be rough and dependent on how we reconstruct the posture of the ancient animals.
Still, Theodor points out that the new study indicates how archosauromorphs may have become dominant because they simply "prevented the remaining, smaller therapsids from evolving larger size." Instead of going head to head with dinosaurs, mammals and their ancestors may have simply been left in the shadows until they got their chance 65 million years ago, when another great extinction ended the reign of the dinosaurs and allowed mammals to flourish.