BP/1/5558_Burrow Synchrotron

The early diversification of mammalian forerunners (i.e. therapsids) was disrupted by several major biological crises, notably at the end of the Permian (252 Ma), when the most severe mass extinction event in Earth’s history occurred. In this context, the survival strategies employed by therapsids are the subject of major scientific interest. The Karoo Basin in southern Africa provides a particularly well-documented geological and palaeontological record that chronicles the evolution of therapsids throughout this extinction. Most interestingly, this record shows an increase in fossoriality by land vertebrates that occurred in line with an important aridification of the climate from the Late Permian to the Early Triassic. This combined information led to the hypothesis that therapsids were using burrows and torpor to respectively escape harsh climates and reduce their metabolic rate during prolonged lack of sustenance. However, while burrows do offer thermal refugia in modern ecosystems and modern vertebrates employ torpor to survive important droughts, the proposed hypothesis that therapsids employed torpor and burrowing as survival strategies only relies on circumstantial evidence and is yet to be fully tested.

In order to study fossoriality in therapsid a few fossil burrow casts from the Karoo have been imaged at the European Synchrotron Radiation Facility (ESRF, Grenoble, France) using synchrotron X-ray computed tomography in the 2010s. The first experiment of this kind on a specimen curated at the Evolutionary Studies Institute (specimen BP/1/5558, Wits University, Johannesburg) led to the unexpected discovery of two animals entombed together, the cynodont Thrinaxodon and the Temnospondyl amphibian Broomistega. Gathering multiple evidence from the data, the proposed hypothesis for the cohabitation of these two animals was that Thrinaxodon was in a deep torpor at the time of the flooding event. Yet, this evidence remains circumstantial.

Recent improvements have been made for propagation phase contrast synchrotron X-ray micro computed tomography (PPC-SRμCT) at the ESRF thanks to the ESRF-Extremely Brilliant Source upgrade and the new flagship beamline BM18. This new beamline and synchrotron upgrade now make it possible to image dental microstructure at sub-micron level of specimens preserved inside burrow casts. This will allow assessing the physiological life history of specific therapsids by analysing the microstructure of complete tooth rows over a full dental replacement cycle.

The project aims at looking for signs of decrease in physiological activity denoting the use of torpor in therapsids. This will be done by analysing the microstructure of the complete tooth row of fossorial (and non-fossorial) therapsids. The continuous tooth replacement pattern in many therapsids allows analysing up to three complete tooth replacement cycles in a single tooth row. Counting daily incremental layers of dentine will allow calculating the duration of a tooth replacement cycle. Analysing variations in the incremental pattern will inform on the physiological activity of the animal. Daily increment layers of the dentine were successfully imaged using PPC-SRμCT on ID19 during a test experiment on therapsids.
BP/1/5558 appears as a perfect candidate for this project as the well-preserved specimen in the burrow cast shows limited weathering and cracks, maximizing the chances to study teeth microstructure.

Methodology (short):
Perform Propagation Phase Contrast Synchrotron based micro Tomography (PPC-SRµCT)

Statement why this study cannot be done in South Africa: X-ray imaging of fossils suffers from the fact that fossilised bones often present a density similar to the surrounding rock which limits the visibility of structure in the generated images. Phase Contrast based techniques are hundreds of times more sensitive to density variation and allow accurate visualisation of all present fossilised structures. Using synchrotron radiation as a source, the protocol for phase contrast imaging only requires sufficient distance between the sample and the detector, which the BM18 beamline of the ESRF is specialised for. For the past 20 years it has proven to solve numerous palaeontological cases for which classic X-ray tomography was insufficient. The present principal researcher of this project is a scientist of the BM18 X-ray imaging beamline and a palaeontologist, dedicating his research to produce the most accurate images of fossils, having knowledge on the technique, the image processing and the scientific relevance of each case. Vincent Fernandez will use his personal inhouse beamtime to carry out the project.