A team of paleontologists has found that fossilized leg bones of Asilisaurus kongwe — a dinosaur cousin that lived during the Middle Triassic epoch about 240 million years ago, roughly 10 million years earlier than the oldest known dinosaurs — can shed new light on how dinosaurs grew from hatchlings to adults.
Asilisaurus kongwe: in this artist’s rendering, the reptile is shown as it would walk and move about; the stripes are artistic license, although the animal’s proto-feathers are likely. Image credit: Andrey Atuchin.
The findings, published online March 4, 2016 in the Journal of Vertebrate Paleontology, 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.
According to the team, Asilisaurus kongwe belongs to Silesauridae, an extinct group of Triassic reptiles related to the dinosaurs.
This reptile lived in what is now present-day Tanzania. It was about the size of a Labrador retriever, had a long tail, and likely maxed at 30 kg.
Fossils of the reptile are vital because a large number of specimens were found, largely intact and varying in size and age.
In studying the anatomy and bone tissue of Asilisaurus kongwe and how individuals changed during growth, the paleontologists found that although these individual animals lived in roughly the same location at the same time, they grew differently.
They 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.
They studied bone scars on the Asilisaurus kongwe leg bones, focusing on spots where muscles and tendons attach to bone.
The more mature an individual was at death, the larger its bone scars appeared. As with any animal or person, an individual skeleton goes from possessing few scars to possessing many during life, with scars appearing in a particular order as the age of the individual increases.
Findings show that except for the smallest and largest individuals, which are the least and most mature, size is a poor predictor of skeletal maturity in Asilisaurus kongwe, and therefore likely in early dinosaurs as well.
Further, similar differences in early dinosaurs had been thought to represent a difference in sex, with more ‘mature’ individuals representing one sex and more ‘immature’ individuals representing another.
“Variation in muscle scars was 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 lead author Christopher Griffin, from the Virginia Polytechnic and State University’s Department of Geosciences.
“Because of this, we thought that similar variations that we saw in Asilisaurus kongwe 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 kongwe 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.”
“The earliest dinosaurs grew just like their closest relatives, and there are very few features that make dinosaurs unique from their closest relatives,” said study co-author Dr. Sterling Nesbitt, also from the Virginia Polytechnic and State University’s Department of Geosciences.
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C.T. Griffin et al. The femoral ontogeny and long bone histology of the Middle Triassic (late Anisian) dinosauriform Asilisaurus kongwe and implications for the growth of early dinosaurs. Journal of Vertebrate Paleontology, published online March 4, 2016; doi: 10.1080/02724634.2016.1111224
