'Cope's Law' might be wrong as the sauropods evolved their giant sizes many times

“It was previously thought that sauropods evolved their exceptional sizes independently a few times in their evolutionary history, but through a new analysis, we now know that this number is much higher, with around three dozen instances over 100 million years around the globe,” said paleontologist Michael D’Emic, assistant professor of biology at Adelphi University in New York and author of the study.

D'Emic and his team discovered this by compiling measurements of the circumference of sauropod weight-bearing bones and correlating them with the estimated weight of each. They then used ancestral state reconstruction to map the reconstructed body masses of nearly 200 sauropod species onto their evolutionary tree. The findings demonstrate that sauropods attained their exceptional sizes early in their evolution and that one or more lineages independently attained superlative rank with each new sauropod family.

“Before going extinct with the other dinosaurs (besides birds) at the end of the Cretaceous Period, sauropods evolved their unrivaled sizes a total of three dozen times,” he explains. “These largest-of-the largest sauropods were ecologically distinct, having differently shaped teeth and heads and differently proportioned bodies, indicating that they occupied the ‘large bodied’ niche somewhat differently from one another,” he added.

Microscopic study of their bones also revealed that sauropods had different growth rates, suggesting that the record-setters were metabolically distinct. This mimics the pattern observed in mammals, which rapidly grew in size after the extinction of the dinosaurs before plateauing at the huge mammoth range.

Previously, "Cope's Rule," the well-known 19th-century idea that claimed that animals' sizes change through time, conflicts with D'Emic's observations. Instead, the new study shows that animals develop diverse body sizes based on their ecological setting and whatever niches are available. This process can appear random when viewed on a global scale.

“While other researchers have explained sauropods’ immense size in general based on their unique combination of features, there is no feature or set of features that characterize the sauropods that did surpass terrestrial mammal size from the ones that didn’t,” he says.

The team's next step is to discover why this might happen. You can read the study for yourself in the journal Current Biology.