Paleontologists at Virginia Tech have found that muscle-scarred fossil leg bones of one of the closest cousins of dinosaurs that lived approximately 240 million years ago can shine new light on a large unknown: How early dinosaurs grew from hatchlings to adults.
Published this month in the Journal of Vertebrate Paleontology, the findings are surprising: dinosaurs and their close relatives had much more variation in growth patterns then ever expected, and this variation does not appear to be related to differences between males and females.
Lead author Christopher Griffin, a geosciences master's student in the College of Science, focused his study on muscle scars etched into the fossil bones of the Asilisaurus kongwe, a dinosaur cousin that lived roughly 10 million years earlier than the oldest known dinosaurs.
"Variation in muscle scars were thought to indicate sexual difference in early dinosaurs, but we know that in many modern animals these features are related to growth, not sex," said Griffin of Redding, California. "Because of this, we thought that similar variations that we saw in Asilisaurus would not turn out to split into two groups, which would be evidence for a sex difference, and instead be more on a spectrum. As we looked at more Asilisaurus fossils of different sizes, because we had such a great sample size, we found this to be supported: with a large sample size, they don't split into two clean groups."
Added Sterling Nesbitt, study co-author and an assistant professor with the Virginia Tech Department of Geosciences: "The earliest dinosaurs grew just like their closest relatives, and there are very few features that make dinosaurs unique from their closest relatives."
Asilisaurus lived during the Triassic Period, roughly 240 million years ago in present-day Africa. With four legs and a long tail, the animal was about the size of a Labrador retriever, and likely maxed at 65 pounds, according to previous studies of the animal. Its exterior skin appearance remains unknown.
Fossils of Asilisaurus kongwe - a combination of Swahili and Greek works meaning "ancient ancestor reptile" - are vital because a large number of specimens were found, largely intact and varying in size and age. Such findings are so rare that paleontologists have struggled with understanding how the first dinosaurs grew, as most species of early dinosaur are known from only a handful of fossils.
The Asilisaurus fossils initially were discovered during a 2007 expedition in southern Tanzania, with additional field excursions taking place for the next eight years.
The length of the field excursions and the number of specimens of fossils resulted in several smaller individual specimens appearing to be more mature than larger finds, and individuals of the same size appeared to be at different stages of growth.
In studying the anatomy and bone tissue of Asilisaurus and how each changed during growth, Griffin and Nesbitt found the although these individual animals lived in roughly the same location at the same time, they grew differently. Griffin compared this finding to any modern family with siblings and cousins differing in height or body mass, for instance, one brother smallish, and another taller; one naturally muscular, another prone to thinness.
Griffin and Nesbitt studied bone scars on the Asilisaurus leg bones, focusing on spots where muscles and tendons attach to bone.