By the time dinosaurs no avian were extinct, the herbivorous sauropods such as 'Brontosaurus' had grown to gigantic proportions.
With a weight of up to 100 tons, these long-necked giants are the largest land animals that have ever walked the earth.
How they became so large from ancestors that were small enough to be found in a petting zoo today is still a mystery. A new and in-depth anatomical description of the best preserved specimens of a relative of the sauropods the size of a car, can help paleontologists unravel the mystery.
Adam Marsh, a paleontologist in the Petrified Forest National Park in the United States, directed the description of the dinosaur while obtaining his master's degree in the Austin Jackson School of Geosciences from the University of Texas. The findings are published in 'PLOS ONE' and Marsh co-authored the article with his advisor, the Jackson School professor, Timothy Rowe.
The dinosaur, called 'Sarahsaurus aurifontanalis', he lived about 185 million years during the Early Jurassic. It could contain important clues about the size of the sauropods because it belonged to the group of dinosaurs that preceded them. Its evolutionary location combined with the exquisite preservation of the specimens provides researchers with a detailed view of its anatomy and how it relates to its larger cousins.
"Sarahsaurus" retains in its anatomy whats anatomical changes that were happening in the Late Triassic and the Early Jurassic that were happening in the evolutionary lineage – says Marsh -. It can help us tell us how it gets big. "The description is based on two skeletons discovered in Arizona by Rowe in 1997.
The bones belong to the Navajo Nation, owner of the land where the fossils were discovered, and are preserved by the Jackson School Museum of Earth History Vertebrate Paleontology Collections. The bones are slightly crushed, and in some cases they are still attached to each other in parts of the body such as the hand and tail. The only important part that is missing is the skull.
"The specimens are well preserved in three dimensions and remarkably complete, which it's very rare in the fossil record – emphasizes the director of Collections Matthew Brown–. Such complete specimens help paleontologists better understand the remains of incomplete and fragmentary fossils we normally encounter. "
Marsh describes 'Sarahsaurus' as a dinosaur "Earth lazy". He stood up, walked on his hind legs and had powerful forelimbs with a large curved claw that covered the first finger of each hand. He had much in common with the first ancestors of sauropods, such as walking on two legs, but he was also beginning to show features that would predict how his mass relatives would evolve, such as an increase in body size and an enlargement of the neck vertebrae.
The size and length of the neck are characteristics that the sauropods would take to extremes as they evolved. By studying these traits and others in 'Sarahsaurus' and seeing how they compare with those of other dinosaurs, scientists can help reveal how these changes occurred throughout evolutionary history and how different dinosaurs relate to each other.
For example, the anatomical review helped to clarify the relationship between 'Sarahsaurus' and two other sauropod relatives who lived in North America during the early Jurassic. The researchers found that the three did not have a common ancestor in North America, but evolved from dinosaur lineages that came to North America independently.
Marsh is currently working on another study that could shed more light on how sauropods evolved. Led by Sterling Nesbitt, an assistant professor at 'Virginia Tech' and a research associate of the Jackson School's vertebrate collections, the project consists of trace the anatomical differences in the bones of the extremities of the dinosaurs to determine what characteristics relate to evolution and what to the age of an animal. Marsh points out that the two 'Sarahsaurus' skeletons examined for this document are a great addition to the project.