A trip to South Africa in search of therapsids
By Zoe Kulik
I study the paleobiology of our very distant mammalian ancestors, the therapsids. What’s a therapsid, you ask? Think of a cross between a lizard and a dog – sounds pretty amazing, right?
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| A reconstruction of Lystrosaurus murrayi, Dmitry Bogdanov [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)] |
This summer I spent about five weeks studying one type of therapsid that is arguably the most whacky looking therapsid out there. It is called Lystrosaurus, which translates from Greek to ‘shovel-nosed lizard’. Lystrosaurus lived around 250 million years ago in what is present day southern Africa, China, Mongolia, Russia, India, and Antarctica. At the time when Lystrosaurus was alive, all of the continents were connected as Pangea. One of the aspects of Lystrosaurus paleobiology that I am interested in is whether its global species distribution had a latitudinal gradient in body size. Because Lystrosaurus lived in polar regions as well as equatorial regions, we might expect local environmental pressures to preferentially increase the body size of higher latitude species, otherwise known as Bergmann’s Rule.
In order to figure out what variation exists in Lystrosaurus body size, I went to South Africa which has the longest and most complete fossil record of pretty much any therapsid, including Lystrosaurus. While in South Africa, I visited three museums, The Evolutionary Studies Institute in the capital city of Johannesburg, the Iziko Museum of South Africa in Cape Town, and the National Museum in Bloemfontein, a smaller city near Johannesburg. I timed my trip to overlap with the International Symposium on Paleohistology where I presented a talk on cranial elaboration in therapsids (read more about it here). This conference happens every two years and was an absolutely amazing opportunity to meet with other people who study the microstructure of fossil bones – a niche discipline in the already niche field of paleontology.
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| Me and about 60 other paleohistologists at The International Symposium on Paleohistology in Cape Town |
But getting back to Lystrosaurus. You might be wondering how you determine the body size of extinct organisms. Since we only have their fossilized bones, and often only isolated bones instead of an entire skeleton, paleontologists use proxies like skull length to extrapolate body size. I used a number of different cranial measurements to capture skull size in Lystrosaurus. I also took thousands of pictures to record the quality of the fossil I was measuring. In an ideal world, fossils would be perfectly preserved without being crushed, distorted, or exploded. That rarely happens, and if I limited my sample to only those that were perfectly preserved, I would have a sample size of about 4 instead of the nearly 400 skulls I was able to measure. On top of preservational differences, there are also differences in the amount of preparation that has been done (where someone carefully removes the rock matrix to reveal the bones underneath).
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| Lystrosaurus skulls in three main stages of preservation and preparation. All of the skulls are in right lateral view. |
Most of the time, Lystrosaurus skulls are found in nasty nodules. In other words, they are completely encased in rock. Only a trained eye can pick out a nodule’d skull since they blend into the surrounding landscape. When skulls are brought back to a museum, only some get prepared. Many of the skulls might be partially prepared – where only the top of the skull is cleared off – in order to make a species-level identification. In a case like Lystrosaurus, there are an estimated 2500 specimens held in the three museum I visited. A large number are prepared and easy to measure but an equally large number have not been touched since the day they were collected. This presented a problem for me since all fossils, regardless of whether they have been prepped or not, are packed in layers of bubble wrap inside cardboard boxes. There was no way of quickly sorting which boxes had nicely prepared fossils and which had the hunks of rock. So, I spent a lot of my time unwrapping and rewrapping fossils.
The best preserved and the biggest fossils are out on the display, which posed yet another challenge of disassembling the thick glass panels in order to take measurements.
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| Climbing inside the gallery display to measure this gigantic Lystrosaurus maccaigi. The skull, vertebral column and limbs are highlighted in red. |
When I wasn’t measuring Lystrosaurus skulls, I spent my time sightseeing around Cape Town. I tried sightseeing in Joburg but ran out of time and had the opposite issue in Bloem. There wasn’t anything to do. But Cape Town was fantastic. It’s got an ocean, a giant mountain, and a huge city all wrapped up into one. If you have a chance to visit, I highly recommend it!
Zoe is a 3rd year PhD student in Chris Sidor’s lab.
