Karoo Palaeontology Department
 
 

Research

Biology and Ecology of South African Permo-Triassic vertebrates

The study of the interaction between organisms and their natural environment provides important information on the nature and mechanisms of evolutionary change. Studying the palaeobiology and palaeoecology of extinct organisms sheds light on how life adapted to environmental changes and allows us to better understand the workings of modern ecosystems.

Current research in the Palaeontology Department comprises morphological and biostratigraphical examinations in order to shed light on the biology and ecology of Permo-Triassic vertebrates in South Africa. This data will improve our phylogenetic understanding of Early Triassic vertebrates by studying their Permian ancestors.


The End-Permian extinction

This research programme focuses on the survivorship and recovery rates of South African vertebrates after the end-Permian extinction event and their various adaptive strategies to an arid Early Triassic environment. This event is considered to be the most catastrophic mass extinction in Earth’s history. It was an event that changed the structure and composition of both marine and terrestrial ecosystems forever.

Permo Triassic Boundary

Studying this extinction, the events leading up to the extinction and the subsequent recovery provides important information on the manner in which organisms adapt to adverse conditions and offers reasons for the preferential survival of organisms. This in turn has implications for the study and conservation of modern ecosystems.

The South African Karoo Basin is ideal for such studies as it is one of the very few localities to contain an almost continuous terrestrial Permo-Triassic sequence and gives us the unique opportunity for studying the Permo-Triassic transition.

Studying the Permo-Triassic boundary (PTB) in the southern Karoo Basin of South Africa, can provide vital information on the varying abundance of terrestrial vertebrate taxa across the boundary. The PTB in South Africa is positioned approximately 40 stratigraphic metres below the base of the Katberg Formation and is recognized by the last appearance datum of the therapsid dicynodont Dicynodon lacerticeps, a negative excursion in d13C and a distinctive laminated sandstone-shale unit.

Recent work on the terrestrial PTB sequences in the southern and central Karoo Basin has provided palaeoenvironmental and biostratigraphical information allowing the patterns of disappearance of various vertebrate taxa across the PTB to be documented in greater detail. However, there is still little understanding of the recovery of diversity among the terrestrial vertebrates immediately after the extinction event and the strategies that these Early Triassic vertebrate communities used to survive and radiate in what appears to have been a much harsher environment than before the event.


Lystrosaurus

Reconstruction of Lystrosaurus
Lystrosaurus reconstruction

Very few Permian vertebrates are present in Early Triassic strata. So far, the dicynodont Lystrosaurus and the therocephalian Moschorhinus are the only therapsids to have been found in both Late Permian and Early Triassic strata, but Lystrosaurus is the only therapsid genus to be found in large numbers. Research on Lystrosaurus has revealed that of the four Lystrosaurus species only L. curvatus and L. maccaigi are found in the Late Permian Dicynodon Assemblage Zone. Lystrosaurus maccaigi did not survive the end-Permian extinction and L. curvatus is limited to an interval that straddles the PTB. Two new species of Lystrosaurus, L. murrayi and L. declivis, appeared in the Early Triassic and became the most abundant species to be found in the Early Triassic Lystrosaurus Assemblage Zone. These results have important implications for South African biostratigraphy as L. maccaigi can be used as a biostratigraphical marker to indicate latest Permian strata and L. curvatus may be used as a biostratigraphic indicator of the PTB interval.


Other Vertebrates

Lystrosaurus burrow
Lystrosaurus burrow

From detailed logging of multiple sections through the PTB sequence, results have shown that after the initial extinction, there was a second, lesser extinction event about 160 000 years later, including the taxa that managed to cross the PTB. The Early Triassic recovery fauna comprises proterosuchian archosauromorphs, small amphibians, small procolophonoids, medium-sized dicynodonts and small insectivorous cynodonts.

Burrowing specimens are found in abundance, which may reflect taphonomic bias towards the preferential preservation of burrowing animals or a successful adaptive strategy for survival in a harsh, arid Early Triassic environment.


Bone histology

Studying the bone histology (or bone microstructure) of an animal provides meaningful information about its biology, including such aspects as individual age, ontogeny (development), growth patterns, lifestyle adaptations and indirectly, physiology. By comparing the bone histology of living animals with that of extinct animals we are able to decipher the extinct animal's life history. The structural organization of a bone is usually retained even after fossilization. Although the osteocytes (bone cells), blood vessels (vascular canals) and collagen fibres are destroyed, their position usually remains intact, which facilitates the examination of fossil bone microstructure. The histological patterns observed in fossil bones are assessed according to the tissue organization seen in living animals. Bone is classified according to such factors as vascular arrangement, the type of bone tissue and the stratification (layering) of the primary bone tissue.

Current research interests lie in using bone histology to resolve questions relating to the growth patterns of South African therapsids.

Bone Histology Diademodon.jpg
Diademodon

This information can be used to decipher aspects of the physiology of these animals, including the origin and evolution of mammalian growth rates and even traits relating to endothermy.

Bone Histology Cynognathus
Cynognathus

In some cases, we can even distinguish between morphologically similar taxa. For example, the cynodonts Diademodon and Cynognathus were similar-sized, contemporaneous animals that both lived in the South African Karoo Basin during the Middle Triassic. Although their skulls are quite distinct, their skeletons are virtually indistinguishable from one another, making it difficult to positively identify these animals when skull material is absent (or too fragmentary for positive identification). However, the analysis of the bone histology of these animals has revealed very different bone tissue patterns. Diademodon exhibits a cyclical growth pattern reflected by alternating zones and annuli. The zones consist of fibro-lamellar bone tissue and the annuli consist of lamellar bone tissue. In contrast, Cynognathus exhibits a sustained, rapid growth pattern reflected by the presence of highly vascularised uninterrupted fibro-lamellar bone tissue. As these animals were contemporaneous and experienced the same environmental conditions, the differences in their bone histology are likely due to different inherent physiological factors. Thus, the differences in their bone tissue patterns have allowed us to distinguish between these animals, when other avenues are not possible.