Discovery of New Giant Dinosaur Rewrites Evolutionary History

Reconstruction of Ingentia prima, a dinosaur from the Late Triassic period. Jorge A. González

A new giant dinosaur that lived between 210 and 205 million years ago in what is now Argentina has been discovered, according to a study published in the journal Nature Ecology & Evolution.

The specimen, which has been named Ingentia prima, sheds new light on the evolution of dinosaurs, suggesting that gigantism in these animals emerged much earlier than previously thought.

I. prima likely measured around 33 feet long, 14 feet tall and weighed up to 10 tons. It belongs to a group of dinosaurs known as sauropods, which includes some of the largest land animals ever known to have existed.

Enormous, four-legged sauropods such as Brachiosaurus and Diplodocus, for example, are among the most iconic of all dinosaurs. However, the earliest examples of this group were small, bipedal creatures. Until now, it was thought that giant body sizes in dinosaurs—anything over 10 tons—first appeared around 180 million years ago during the Jurassic period.

But the discovery of I. prima shows that this was not the case, with gigantism seemingly evolving around 30 million years earlier than thought, according to Cecilia Apaldetti, from the Universidad Nacional de San Juan, Argentina, and her colleagues.

"Although the origin of gigantism in sauropodomorphs [sauropods] was a pivotal stage in the history of dinosaurs, an incomplete fossil record obscures details of this crucial evolutionary change," the authors wrote in the study.

I. prima was also found alongside three specimens of an already known species called Lessemsaurus sauropoides. Together, these dinosaurs belong to a sauropod subgroup known as the lessemsaurids, which lived between 237 and 201 million years ago (relatively soon after dinosaurs first appeared) in what is now Argentina but was then the southeast corner of the supercontinent Pangaea.

Previously, it was thought that for a dinosaur to grow to giant size, straight legs and continuous rapid, growth were essential. However, the newly discovered fossils show that, in fact, there is more than one way for gigantism to emerge in dinosaurs.

Like later sauropods, lessemsaurids had elongated necks and tails, as well as air sacs resembling those that are found in birds. These repiratory structures may have been necessary to keep such large animals cool. However, unlike the diplodocus and other later sauropod relatives, lessemsaurids stood on bent legs and had bones which grew in short, accelerated bursts.