Being big has its benefits. It means less competition for food and less trouble from predators. And these benefits undoubtedly served the sauropods, a clade of leaf-loving dinosaurs that just so happened to be the biggest creatures to ever traverse the earth.
But how did these humongous dinosaurs become so huge? A paper published in Current Biology provides new insight into this question. It reveals that at least three dozen lineages of sauropod evolved an enormous stature throughout the Jurassic and Cretaceous periods.
The Largest Life
Over the years, biologists have identified over 250 distinct sauropod species, including several species from the famous apatosaurus and brachiosaurus genera. Although each and every one of these species was substantially sized — with the smallest sauropods measuring 13 feet in length — some were especially enormous. In fact, specialists say that the biggest titanosaurs boasted a body length of around 120 feet and a body weight of around 75 tons.
Biologists say that the advantages of this enormous size were numerous. As adults, the sauropods could find food and avoid predators that other vegetarian dinosaurs simply couldn’t find and avoid. They were also better at traveling long distances as a result of their enormity, meaning that they could move to new areas whenever changing environmental conditions made their traditional homes less hospitable.
But, while biologists know that the sauropods were large, the specific body measurements of many sauropod species have remained elusive. Evolutionary studies limited the sauropod size to less than half of their identified species.
Now, the use of a new approach for predicting sauropod body measurements has expanded the scope of these evolutionary studies. The dinosaurs grew into giants more times in 100 million years of their evolution than ever imagined.
“It was previously thought that sauropods evolved their exceptional sizes independently a few times in their evolutionary history,” says Michael D’Emic, the biologist behind the new approach, according to a press release. “We now know that this number is much higher, with around three dozen instances over the course of 100 million years.”
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Amassing measurements of weight-bearing bones from an assortment of sauropod species, D’Emic approximated the total body weight of almost 200 separate sauropods. He then turned to ancestral reconstruction to trace how those body weights transformed over time and within the context of the sauropod family tree.
The results revealed that the sauropods became big in the early stages of their evolution. They also revealed that the sauropods became big several times over. “Sauropods evolved their unrivaled sizes a total of three dozen times,” D’Emic adds, according to the release.
Challenging Cope's Rule, the 19th-century theory that animal lineages slowly increase in size, the findings seem to suggest that the transition to gigantism can take place at any point in an animal's evolutionary history, contingent on the conditions of its surroundings.
Interestingly, there were no clear similarities between the sauropods that became big. “There is no one feature or set of features that characterize the sauropods that did,” D’Emic says. “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.”
According to D’Emic, future investigations will eventually examine why some sauropod lineages became big as opposed to others.