Benoit_Lystrosaurus skulls Synchrotron

This is a project to Synchrotron scan at Grenoble (France) 8 specimens of Lystrosaurus to study the evolution of their brain size throughout the PT boundary (see attached project details). The application to the ESRF has been accepted. The specimens will be hand-carried by Julien Benoit, Erin Lund and Olebeng Mogane, from the Evolutionary Studies Institute. To the best of our knowledge, synchrotron scanning is harmless to the fossils.

Aim/rationale:

Some 252 million years ago, the end-Permian mass extinction (EPME) wiped out an estimated 90% of all species, making it the second most important event in the history of life on Earth since the origin of life itself 1. The intense volcanic activity of the Siberian Traps pumped copious amounts of greenhouse gases into the atmosphere, triggering a positive feedback loop of global warming1,2, not dissimilar to the climate change that we are experiencing today. This makes the study of the EPME of primary relevance to better understand the upcoming biodiversity crisis. Chief among the most discussed topics is how a lineage of medium sized mammalian-ancestors, Lystrosaurus, became one of the only survivors and thrived during this mass extinction event. Hypotheses include burrowing, a “reproduce fast, die young” life history, and the lilliput effect as possible survival strategies, among many others3,4. In a recently published paper, my colleagues and I proposed a new hypothesis that remains to be explored: changes in brain size5. It has been observed that modern mammalian species with a larger brain had a higher survival rate during the Holocene extinction than small-brained ones, a larger brain favouring behavioural flexibility to adapt climate change and food scarcity6. However, the very few data that we currently have suggests that brain size remained rather constant across the EPME5, although this dataset only includes a handful of species, and most noticeably, only one Lystrosaurus specimen from the Triassic. This is insufficient to properly address the question of the evolution of brain size across the EPME: did Lystrosaurus brain size increase to behaviourally cope with the changing landscape? Or, the brain being the most energy-consuming organ of the body, did it decrease in size to save resources? Or was it unaffected by the crisis as the current, yet much incomplete, data suggests?
To answer these questions, we here propose to SRCT-scan 8 specimens of Lystrosaurus from the South African Karoo Basin ranging in age from the latest Permian to the Early Triassic, and including samples coming directly from the EPME beds. The brain endocast (the cast of the brain within the braincase) will be segmented out and its volume measured digitally. The goal will be to directly document variations in the brain size and shape of this surviving taxon before, across, and after the EPME using the Encephalization Quotient (EQ, a metric of the relative brain size compared to body mass7). To complete this approach, specimens of different ontogenetic stages, ranging from neonate to large adults, will also be scanned in order to reconstruct patterns of ontogenetic variations of brain shape and size, and the dynamic of its development before and after the crisis.

References
1- Erwin DH (1990) Annu Rev Ecol Syst 21: 69-91.
2- Shen J et al. (2023) Nat Commun 14: 6.
3- Botha-Brink J et al. (2016) Sci Rep 6: 24053.
4- Botha-Brink J et al. (2017) J Vert Paleontol 37: e1365080.
5- Gigliotti A et al. (2023) Pal Afr 56: 142-170.
6- Dembizer J et al. (2022) Sci Rep 12: 3453.
7- Benoit J et al. (2023) Prog Brain Res 275: 25-72.

Methodology (short):
We will use propagation phase contrast synchrotron micro-Computed Tomography (PPC-SRµCT) on BM18 to image complete skull: large-size and medium-size skulls will be imaged at ca. 65 µm (33 µm optic in binning 2x2), small-size skull will be imaged at 10 µm, which is sufficient for neuroanatomy. Based on previous experiment on similar specimen, the scanning time for all specimens will total ca. 72 hours. Adding extra time for beamline preparation with 2 setups, as well as for centring and preparing acquisition, it will take 12 shifts (4 days) on BM18.
From the generated data, we will reconstruct the brain endocasts in 3D using manual segmentation on Avizo 2020 (FEI, Hillsboro, OR, USA). The volume of the brain and its individual parts (olfactory bulbs, forebrain, midbrain, hindbrain, pituitary, pineal) will be measured with the same software. Brain size (using the synapsid adjusted EQ) and shape will be compared between Lystrosaurus specimens and with other therapsids. Body mass to compute the EQ will be estimated from skulls length.

Statement why this study cannot be done in South Africa:
The Lystrosaurus specimens are larger than what can be scanned at a regular CT facility, and this is why we will use BM18 at the ESRF (Grenoble, France). The density of the material, which is rich in metallic nodules and has been through 250 million years of geological compression and diagenesis, requires a high energy beam. Additionally, previous studies on therapsids have demonstrated the undisputable superiority of synchrotron imagery to reconstruct their brain endocast, because the outline of the endocast in these species are indicated by very thin and delicate bony structures that require high resolution to be observed.