While fossils may reveal what dinosaurs looked like - whether they were large-bodied carnivorous predators or long-necked herbivores - there is still much that remains unknown about these iconic prehistoric rulers. For instance, one long-standing mystery is how early dinosaurs grew from hatchlings to adults.
Now, paleontologists from Virgina Tech University have shed some light on the matter. When examining 240-million-year-old muscle-scarred fossil leg bones of Asilisaurus kongwe - a close dinosaur cousin that roamed the Earth 10 million years earlier than the oldest known dinosaurs - researchers found a surprising amount of variation in the species' growth patterns.
What's more is that the variation in growth patterns between dinosaurs and their close relatives doesn't appear to be related to differences among males and females.
"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," explained Christopher Griffin, lead author of the study and a geosciences masters student at Virginia Tech.
"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," Griffin added. "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."
Asilisaurus lived during the Triassic Period in what is now Africa. With four legs and a long tail, the ancient reptile was about the size of a Labrador retriever, weighing no more than 65 pounds.
"The earliest dinosaurs grew just like their closest relatives, and there are very few features that make dinosaurs unique from their closest relatives," co-author Sterling Nesbitt said.
However, most of what is known about early dinosaurs is from only a handful of specimens, making those of Asilisaurus a rare and important find.
Originally unearthed during a 2007 expedition in southern Tanzania, the Asilisaurus fossils were found largely intact and varying in size and age. With such a large number found, it took eight more years of additional field excavations.
Surprisingly, several smaller individual specimens recovered appeared to be more mature than larger finds, while individuals of the same size appeared to be at different stages of growth.
Examining the anatomy and bone tissue of Asilisaurus revealed that although these individual animals lived in roughly the same location at the same time, they grew differently. This, researchers explained, is similar to how modern families often have siblings or cousins of varying height or body mass, for example.
To learn more, Griffin and Nesbitt closely examined bone scars on the Asilisaurus leg bones, specifically locations where muscles and tendons attached to bone. Overall, the more mature an individual was at death, the larger its bone scars were. This suggests that size is a poor predictor of skeletal maturity in Asilisaurus, and likely in early dinosaurs, too.
"Our study includes more individuals and more bone scars, and with this increase in sample size we found that individuals fall on a trajectory that is more similar to maturity difference than sexual difference," Griffin said. "This suggests that similar variation in bone scars in early dinosaurs is variation in growth, not male and female difference. Because this variation appears to be widespread among early dinosaurs and their closest relatives, it is likely that high variation in growth between individuals characterized the most recent common ancestor of Asilisaurus and all dinosaurs."
Next, researchers virtually reconstructed growth sequences derived from bone scar evidence and studied bone tissue samples to determine not only each of the specimens' age but also their pace of growth.
"I'm fascinated by how much we can learn about the past through animals that are so unlike anything that we have today, and how that can help us understand how today's world came to be the way it is," Griffin concluded.
Their study was recently published in the Journal of Vertebrate Paleontology.
