A new avimimid flock

In 2016 I led a study that described the first oviraptorosaur bonebed, from the Nemegt locality in Western Mongolia. We described an assemblage of as many as 22 individuals of a small animal called Avimimus, which we later determined to be a separate species.

Excavating the Nemegt Avimimus bonebed in 2016

One of the peculiarities about the site was that the group apparently lacked juveniles. We identified adults by fusion of compound bones, which has been used to infer adulthood in other dinosaurs and fossils. In the Nemegt bonebed, most of the specimens had fused compound bones, but even those that didn’t were more than 80% the size of the largest individual (meaning they, too, were near adulthood). This was perplexing, because usually we see the opposite pattern in dinosaurs: groups of juveniles with no adults, or groups of mixed ages. This is because juveniles benefit more from grouping than adults, in terms of predator avoidance, finding food, and other benefits.

Tibiotarsi from the Nemegt Avimimus bonebed, showing how similar in size all of the individuals are.

But something just didn’t sit right with us… Avimimus is a small animal even at adult size, and its proportions–long legs, big eyes–seem more like juveniles. We decided to look at the problem histologically, by making thin sections of the bones. Conveniently, another avimimid bonebed had been stumbled upon in China, and the material was available for us to describe and work on. This gave us an opportunity to learn more about groups of avimimids, and also to see if we could figure out their growth patterns.

The results of that study were published today in Scientific Reports, and, like the first paper, are open access for everyone to read.

The results challenge our conception that fused compound bones signify adulthood in avimimids. Our thin sections show that an individual that was 80% the size of an adult–and had already started fusing the tibiotarsus–shows all the hallmark signs of a rapidly-growing juvenile animal. In fact, we can’t see any annual growth marks in its bones, which suggests to us that it was possibly as young as one year old. In any case, the presence of rapidly-growing bone is unexpected, because previous studies have shown that other small theropod dinosaurs stayed small because they had slower growth rates. Our avimimids were growing just as rapidly as larger oviraptorosaurs, but they grew for much shorter periods. This is remarkably similar to modern birds, which grow quickly, but often reach adult size in a year or less.

A young avimimid tibia, showing rapidly growing bone despite being close to adult size and beginning to fuse

More importantly, however, we also sectioned an individual with a fused tibiotarsus, and it shows that, despite being only a little bigger, it was significantly older. Its bone had slowed in growth considerably, and the bone right at the external surface (which is most recently deposited) is very slow growing and shows at least two growth marks. This tells us that, after the fusion event, this animal had essentially stopped growing for the next two years.

An adult avimimid tibia, with a completely fused tibiotarsus and slow-growing bone around the edges

So we have contrasting results. The fused individual had minimal growth after the fusion event, so fusion is a good indicator of adult body size. However, fusion starts even in very young, rapidly growing animals, so fusion isn’t a good indication of maturity. Altogether, this is an interesting pattern that suggests that avimimids grew much like modern birds: very rapidly at first, reaching adult body size and fusing their bones quickly, but then very slowly afterwards.

It also tells us that we were probably wrong to assume that the Nemegt bonebed has no juveniles. It’s very likely that the unfused bones at that site are indeed juveniles, but that avimimids grew so rapidly to adult size that we need to section the bones to determine their age, rather than relying on relative size. This isn’t a new idea by any means, and more and more studies are showing that body size is a poor indicator of maturity, but it remains odd that avimimids reach adult body size so quickly. Of course, our study is constrained by a small sample, because we didn’t expect to find this pattern and because we had limited material available to us. A follow-up study using the better Nemegt bonebed sample would probably let us nail down exactly when fusion started, and reconstruct a growth curve to show the trajectory of avimimid growth.

A hypothesized trajectory for fusion of the ankle, based on the specimens we sampled

In any case, we show that avimimids were more like other dinosaurs in that they had groups of mixed ages, both adults and juveniles. They’re different, however, in that there was minimal difference between adults and juveniles in terms of body size and probably their diet and ecology. What this means for their behaviour is unclear at the moment, but it shows a hidden diversity of behaviour that we should probably look for in other dinosaurs!

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