Mongolia Monday Travel Log 5

This week’s post is all about my research in the lab. While it may not have the exciting drone footage that you get in the field, this is where our exciting discoveries in the field come to life.

I’ve been working the past few weeks to try and finish up a few projects. Each of the next travel logs will feature a different project. This week: Eleven and Twelve.

Research

These are two oviraptorid specimens that I’ve been working on for a while now: MPC-D 102/11 (“Eleven” for short) and MPC-D 102/12 (“Twelve”). While Eleven is poached, Twelve was collected in 1998 at Guriliin Tsav by a Japanese expedition. We went back to Twelve’s quarry this year and found a beautiful ungual. Check it out in 3D below:

These two skeletons share a lot of similarities with each other, as well as another specimen that I spoke about at SVP 2017 in Calgary. One major difference, however, is the size of these individuals. Both have femora, so we can estimate their weights. Eleven would have weighed about 33 kg, and Twelve was more than double the mass at 74kg! This is really exciting because it allows us to trace the changes in the skeleton as the animals grow bigger. But are these differences related simply to size, or do they reflect age as well? To tell, we had to go deeper: Ovinception.

This is where histology comes in: by thin-sectioning fragments of the hindlimbs, we can count the growth marks and determine a minimum age for the individuals. Twelve was easy: there are 4 lines of arrested growth (LAGs), which correspond to at least 4 years of age. Based on the spacing of these lines and the way they group together at the edge of the bone, we can be sure that Twelve was an adult.

102-12 Histo
A thin-section of MPC-D 102/12, showing lines of arrested growth (different-coloured bands) used for age estimation

Eleven, on the other hand, lacks any clear LAGs. It does, however, have an unusual zone of parallel-fibered bone close to the external surface. Some of our previous work (now published here) shows that, in oviraptorosaurs, these zones correspond to growth marks when the animal is growing so fast that it doesn’t completely stop growing. The results from Twelve support this interpretation: each LAG is preceded by a zone of parallel-fibered bone as growth slowed down. Following this, we can estimate that Eleven was at least one year old. We may be able to go even further and say that it was just over one year old, because there’s some evidence that this style of growth mark is characteristic of the first growth mark (see more in our paper).

Tibia Summary
A summary of our recent paper, showing the first growth mark in an oviraptorosaur tibia. It appears as a band of slower-growing bone, rather than a distinct line. A similar mark in Eleven tells us it was probably one year old too.

With the ages of these animals in hand, we can confidently say that the similarities we see between the skeletons are independent of age. This supports our hypothesis that they belonged to the same species. We can also say that there are some changes that happen during the lifetime of the animal (likely also related to increasing size). The shafts of the hindlimbs become more robust, and the leg is overall shorter compared to the body. This likely helped the animal compensate for its extra weight. Each vertebra fused its two parts, and the ankle began to fuse as well.

There’s lots more work ongoing, but Eleven and Twelve are helping to give us the best view of any oviraptorid currently known. Now we just have to finalize the manuscript and submit… easy, right?

 

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s