Lystrosaurus: The Most Unlikely Undertherapsid

Article by: Adam Manning
Edited by: Jack Wood and J. D. Dixon

Lystrosaurus curvatus, the only species of Lystrosaurus to survive the End-Permian extinction, was a dominant component of Early Triassic terrestrial faunas. Illustration by Jack Wood.

Name: Lystrosaurus
Age: Late Permian-Early Triassic
Diet: Herbivore
Size: Average 1m long approx. (varies by species)
Weight: Average 100kg approx. (varies by species)
Locations: Antarctica, China, India, Russia, and South Africa. 

Due to the sheer number of species, here the genus Lystrosaurus is examined, with notable species pointed out when necessary.

Lystrosaurus hardly has the awe-inspiring looks of other prehistoric creatures. Yet, this dog-sized animal has become a highlight of the Permian fauna because of one important reason; it survived the End-Permian mass extinction, otherwise known as “The Great Dying”, when approximately 95% of all species became extinct. Lystrosaurus is a “disaster taxa”, since the genus spread rapidly after catastrophic change, but was not dominant before the event. 

Lystrosaurus was a therapsid. These are a group of synapsids, most of which are now extinct, with the exception of contemporary mammals. Early therapsids, such as Lystrosaurus, have some features associated with both mammals and reptiles. Most notably, Lystrosaurus declivis is thought to most likely be endothermic, or warm-blooded. Lystrosaurus skulls show this as the maxilloturbinals in their nasal cavity are complexly constructed, and are situated directly in the respiratory airflow, filling out most of the lower nasal chamber, a feature found in mammals and birds to assist with thermoregulation and humidification.

A vegetarian, Lystrosaurus possessed many adaptations as a burrower, with a short, broad hand suited for digging and a wide knee joint for powerful foot muscles. This meant that it probably excavated burrows for itself, possibly to den in for protection, but it wasn’t committed to that particular lifestyle. It also had a barrel chest that allowed it to have relatively large lungs that could extract more oxygen from the air.

Lystrosaurus hedini skeleton at the Museum of Paleontology, Tübingen. Available at: https://en.wikipedia.org/wiki/Lystrosaurus

So how did Lystrosaurus survive the End-Permian mass extinction? This mass extinction began just before 252.28 ± 0.08 million years ago, when the Siberian Traps – a large igneous province located in what is now Siberia – erupted, expelling thousands of gigatons of carbon dioxide into the atmosphere, resulting in lethal climate change and problems such as ocean acidification and anoxia. Lystrosaurus, however, was uniquely qualified to survive this catastrophe.

As the atmosphere became ever more hostile, Lystrosaurus’ large lungs probably allowed it to breath more easily. It also lived in burrows, so had access to a more stable food source, as plants that grow underground are less affected by changes in the atmosphere. Furthermore, it had a huge distribution over the supercontinent of Pangaea, and may have been very good at roaming large areas of land. This probably made it harder for the genus to go extinct due to the sheer number of individuals, as well as the fact that it could exploit niches in many ecosystems. And like in any mass extinction, there was likely an element of luck as well.

Only one species of Lystrosaurus made it to the Triassic, Lystrosaurus curvatus, but as a genus, they were a huge success, and this single species dominated the Early Triassic, accounting for ~ 75% of the post-extinction terrestrial fauna. They even helped us understand our world better, as the global distribution of their fossil remains was used as evidence in the theory of continental drift, presented by the German meteorologist Alfred Wegener in 1912.

Image References
[1] Lystrosaurus curvatus. Illustration by Jack Wood
[2] Lystrosaurus hedini skeleton at the Museum of Paleontology, Tübingen. Available at: https://en.wikipedia.org/wiki/Lystrosaurus

Information References and Further Sources:
[1] Benton, M.J. (2014). Vertebrate Palaeontology. 4th ed. Oxford: Wiley-Blackwell. pp 143 and 165.
[2] Botha-Brink, J. (2017). Burrowing in Lystrosaurus: preadaptation to a postextinction environment?, Journal of Vertebrate Palaeontology, (37) 5. doi: https://doi.org/10.1080/02724634.2017.1365080. Available at:
https://www.tandfonline.com/doi/abs/10.1080/02724634.2017.1365080. Accessed 18th July 2019.
[3] Dorling Kindersley. (2009). “Triassic Vertebrates” in Prehistoric. Great Britain: Dorling Kindersley Limited. pp. 220.
[4] Brendan Murphy, J., and van Andel, T. H. (2019). ‘Plate tectonics’, Encyclopaedia Britannica, June 25th 2019. Available at: https://www.britannica.com/science/plate-tectonics/Development-of-tectonic-theory#ref14448
Accessed 17th July 2019.
[5] King, G. M. (1990). The aquatic Lystrosaurus: A palaeontological myth, Historical Biology, 4 (3-4), pp. 285-321. doi: https://doi.org/10.1080/08912969009386547. Available at: https://www.tandfonline.com/doi/abs/10.1080/08912969009386547. Accessed 18th July 2019.
[6] King, G. M., and Cluver, M. A. (1990). The aquatic Lystrosaurus: An alternative lifestyle, Historical Biology, 4 (3-4), pp. 323-341. doi: https://doi.org/10.1080/08912969009386548. Available at:
https://www.tandfonline.com/doi/abs/10.1080/08912969009386548. Accessed 18th July 2019.
[7] Laaß, M., Hampe, O., Schudack, M., Hoff, C., Kardjilov, N., and Hilger, A. (2010). New insights into the respiration and metabolic physiology of Lystrosaurus, Acta Zoologica, 92 (4), pp. 363-371.doi: https://doi.org/10.1111/j.1463-6395.2010.00467.x. Available at: https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1463-6395.2010.00467.x. Accessed 18th July 2019.
[8] Newitz, A. (2013) ‘Lystrosaurus: The Most Humble Badass of the Triassic’, National Geographic, May 28 2013, Available at: https://www.nationalgeographic.com/science/phenomena/2013/05/28/lystrosaurus-the-most-humble-badass-of-the-triassic/
Accessed 17th July 2019.
[9] Palaeobiology Database. Available at: https://paleobiodb.org/#/.. Accessed 18th July 2019.
[10] Shen, S-z., Crowley, J. L., Wang, Y., Bowring, S. A., Erwin, D. H., Sadler, P. M., Cao, C-q., Rothman, D. H., Henderson, C. M., Ramezani, J., Zhang, H., Shen, Y., Wang, X-d., Wang, W., Mu, L., Li, W-z., Tang, Y-g., Liu, X-l., Lu, L-j., Zeng, Y., Jiang, Y-f., and Jin, Y-g. (2011). Calibrating the End-Permian Mass Extinction, Science, 334 (6061), pp. 1367-1372. doi: 10.1126/science.1213454. Available at: https://science.sciencemag.org/content/334/6061/1367. Accessed 18th July 2019.