Contradicting a Popular Theory, New Study Explains How Sauropod Dinosaurs Achieved Different Body Sizes

Sauropods, which include the iconic long-necked dinosaurs like Brachiosaurus and Apatosaurus, have held the title of the largest animals to ever walk the earth, with no other dinosaur or land mammal even coming close.

However, a recent study conducted by Adelphi University sheds light on how these super-giants managed to achieve their record-breaking sizes over time, providing new insights into their evolutionary history.

“It was previously thought that sauropods evolved their exceptional sizes independently a few times in their evolutionary history,” explains paleontologist Michael D’Emic, author of the study published today in Current Biology, “but through a new analysis, we now know that this number is much higher, with around three dozen instances over the course of 100 million years around the globe.”

In a bid to delve into the evolutionary path of sauropod body size, D’Emic collated an array of data encompassing the circumferences of weight-bearing bones from hundreds of specimens, which were then correlated to the respective animal weights.

Utilizing the technique of ancestral state reconstruction, he successfully mapped the reconstructed body masses of nearly 200 sauropod species onto their evolutionary tree.

The outcomes of this extensive study illustrate that sauropods achieved their remarkable sizes at the early stages of their evolution and that each new sauropod family gave rise to one or more lineages, which independently achieved superlative size status.

“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 adds. “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.”

Microscopic analysis of sauropod bones revealed that the largest of these dinosaurs had unique growth rates and metabolism compared to their smaller counterparts.

The study also challenges the popular 19th-century theory known as “Cope’s Rule” which suggests that animal size evolves over time.

Instead, the research suggests that body size is determined by ecological factors and available niches, which can appear random when viewed at a large scale.

Interestingly, this pattern mirrors the evolution of large body sizes in mammals after the extinction of the dinosaurs, which quickly plateaued in the range of gigantic mammoths.

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

The upcoming phase of the research will involve deciphering the reasons behind the evolution of super-giant sizes in some lineages, while others did not undergo such transformation.

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