New Clues to How the Biggest Dinosaurs Got So Big

https://www.nytimes.com/2018/07/11/science/giant-dinosaurs-evolution.html

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No creatures ever stomped across the planet quite like the creatures scientists call sauropodomorphs.

These long-necked plant eaters were the largest dinosaurs, and they included the mighty 70-ton titanosaurs, as well as the Brachiosaurus and Diplodocus. Paleontologists have long wondered how these lumbering 100-foot-long behemoths got so big.

Now a group of researchers has uncovered a new, early sauropodomorph. It differed from later species in its group, with seasonal growth spurts leading to its giant proportions, rather than continuous, gradual growth. The finding, published Monday in the journal Nature Ecology & Evolution provides insight into how certain dinosaurs became the biggest of the big.

“This new dinosaur changes our understanding of how dinosaurs became giants,” said Cecilia Apaldetti, a paleontologist from the Museo de Ciencias Naturales, Universidad Nacional de San Juan in Argentina and lead author of the study.

In 2015, Dr. Apaldetti and her colleagues discovered the new species while looking for Triassic Period fossils in northwest Argentina. They called it Ingentia prima, meaning “the first giant.”

“We didn’t expect a big dinosaur in the Triassic rocks, because we know that at that moment dinosaurs were in general small, no more than 3 tons,” Dr. Apaldetti said.

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But the prehistoric beast they found weighed an estimated seven to 10 tons — more than an African elephant — and measured about 33 feet long. It lived from 201 million to 237 million years ago, which was about 47 million years before its colossal cousins, the Brachiosaurus and Diplodocus, and 30 million years before the titanosaurs.

“Until now it was believed that the first giants to inhabit the Earth had originated during the Jurassic period,” Dr. Apaldetti said. That sauropod was known as Vulcanodon and it walked the Earth around 180 million years ago and measured about 20 to 35 feet long.

Back at the lab, they spent months cleaning the fossil and comparing it with other dinosaur species. Dr. Apaldetti and her colleagues studied the Ingentia prima fossils along with remains from a previously known and closely related species called Lessemsaurus sauropoides. Together the two species belong to a group known as “lessemsaurids.”

In studying the dinosaurs’ anatomies, she found that like the later Jurassic period giants, Ingentia prima had an avian-like respiratory system, meaning it had air sacs in its neck. But she also found differences.

“Just as growth seasons can be observed in a tree, the bony cuts in lessemsaurids show that it had cyclical, seasonal growth,” said Dr. Apaldetti. “But the style of the bone deposition during these periods of growth is different from the other sauropods or eusauropods we knew so far.”

The lessemsaurids grew through quick bursts that occurred seasonally, she said, while their later counterparts grew at a consistent rate until they became adults.

“The giant sauropods like the titanosaurs of the Jurassic or Cretaceous acquired big bodies through a slow and more complex way,” said Dr. Apaldetti. That included developing extremely elongated necks and forelimbs, smaller skulls and thick, trunk-like limbs.

In contrast, Ingentia prima did not have a greatly elongated neck and its legs were more flexible and bent rather than straight and pillar-like. These early giants showed that there was more than one way to climb to the top of the dinosaur world.