They grew in size over a long period of time. Fossils unearthed in Argentina, however, suggest that this occurred earlier than previously believed.
Gigantism Evolved About 30 Million Years Earlier
In a paper published in the journal Nature Ecology & Evolution, researchers reported the discovery of new dinosaur species that suggest gigantism evolved about 30 million years earlier than scientists previously thought. It also showed that there were different ways giant dinosaurs evolved.
"The evolution from small bipedal to giant quadrupedal sauropodomorphs involved numerous anatomical changes," the researchers wrote in their study.
The partial fossil of the creature, which lived between 237 million and 201 million years ago, was found buried at the Quebrada del Barro Formation in Argentina. The fossilized remains include a shoulder bone, several neck vertebrae, and some bones from the legs and tail.
The bones belonged to an herbivore called Ingentia prima. The species is classified as a sauropodomorph. The group comprises the Brontosaurus and their cousins and ancestors from the Triassic.
Triassic sauropodomorphs looked different from their later relatives. Some of them were tiny and many were bipedal. They had neither evolved the column-like limbs associated with massive bodies.
Cecilia Apaldetti, from the National University of San Juan, and colleagues analyzed the fossils along with three other specimens of closely related but previously documented dinosaur called Lessemsaurus sauropoides.
The researchers found that the weight of I. prima ranged between seven and 10 tons, and showed elongated neck and long tail albeit not as pronounced as those seen in their later relatives.
Researchers also reported the creature featured a bird-like cervical sac and a neck with characteristics that allowed the oversized animal to stay cool. It also has structures in the backbones and hind limbs needed for supporting large bodies.
According to the researchers, many of these adaptations were not actually specific to gigantism but provides greater mobility for I. prima. Unlike the younger sauropods, these creatures stood on bent legs and had bones that thickened through accelerated bursts instead of steady and rapid growth.
"We present a new non-eusauropod sauropodomorph and three new specimens of Lessemsaurus sauropoides, all from the Late Triassic of Argentina, which show the presence of a novel growth strategy that allowed them to attain large body sizes without having the anatomical traits previously regarded as adaptations to gigantism in eusauropods," the researchers said.