Mass extinction events have far-reaching effects on Earth’s ecosystems. Understanding their effects on biodiversity is critical for understanding how organisms adapt to changing environments and how these adaptations play a role in the evolution of new species and traits after the extinction. Studies on past ecosystems have shown that changes in diversity, body shape, abundance, and/or resource availability may determine the success of a certain group during these events. However, ecology (e.g. lifestyle habits, trophic level) and life history (e.g. growth patterns, physiology, reproductive timing, age at maturity, longevity) also play important roles in buffering taxa from extinction.
The end-Permian mass extinction, which occurred about 252 million years ago, was the most catastrophic mass extinction in Earth’s history. Temperatures increased and it became very dry. Many species only appear to have reached full recovery some 5-8 million years after the extinction during the Middle Triassic. Temperatures decreased slightly during the Middle Triassic, but increased again during the Late Triassic, culminating in a 3-4°C increase in global temperatures at the close of the Triassic. Research has shown that gas emissions from volcanic eruptions and the release of methane from gas clathrates triggered by the Central Atlantic Magmatic Provence some 201 million years ago are the likely causes for increased carbon dioxide levels, which caused a super greenhouse effect. Latest Triassic taxa had to adapt to these climatic changes or succumb to extinction during what is called the end-Triassic mass extinction.
Extinctions were not equal across clades. For instance among vertebrates in the marine realm, some studies have found that marine reptiles did not go completely extinct during the end-Triassic extinction, but a study on ichthyosaurs (a group of large extinct marine reptiles) showed that they experienced a bottleneck through the extinction, and although they rebounded in the Jurassic, biodiversity never reached previous levels. This apparent discrepancy is perhaps misleading because it is probably due to the scarcity of rocks recording the end-Triassic extinction, making it difficult to fully understand the effects of the extinction on marine reptiles. On land, dicynodont and traversodontid non-mammaliaform cynodont therapsids went extinct during the Late Triassic. Therapsids are the ancient ancestors of mammals. Only three cynodont clades survived the end-Triassic extinction, the tritheledontids, tritylodontids and the mammaliaforms, the latter giving rise to living mammals. Crurotarsan reptiles such as phytosaurs, rauisuchians, ornithosuchids and aetosaurs all went extinct at the end of the Triassic. Crocodylomorphs were the only crurotarsans to survive into the Jurassic, and although they suffered a decrease in body shape diversity during the Early Jurassic they radiated into numerous ecological niches to become a highly diverse group by the Cretaceous. The radiation of Late Triassic and Early Jurassic dinosaurs shows that, as a group, they were not negatively affected by the extinction, but the difficulty in finding body fossils in a complete or near-complete sequence containing the end-Triassic extinction has hampered our understanding of ecosystem reorganization during this time.
The Elliot Formation of the Stormberg Group, Karoo Supergroup in South Africa (Figure 1) contains an excellent continental record of uppermost Triassic and lowermost Jurassic rocks. The formation is divided into two zones, the lower Elliot Formation and upper Elliot Formation. Previous research relied on global assemblage correlations to date the Elliot Formation, but recent field work has produced a radiometric date (the use of ‘chemical signatures’ [radioactive isotopes] for dating rock samples) for the lower Elliot Formation, placing it in the latest Triassic. Samples are currently being analyzed for the upper Elliot Formation, which will soon provide a precise age for the upper part of the formation as well. Detailed studies on the lithostratigraphy (rock layers) have found a change in river style and climate between the lower and upper Elliot Formation. The lower Elliot Formation is characterized by meandering permanent rivers, whereas the upper Elliot Formation represents a time of increased dryness with temporary rivers and flash floods. The formation contains a wide array of terrestrial species, but is particularly noteworthy for its preservation of sauropodomorph dinosaurs. At terrestrial end-Triassic extinction sites in the Northern Hemisphere, researchers have used geochemistry to identify the event and, by using pollen and fern spores, have documented a change in plant species, but these researchers have had to rely on dinosaur trace fossils to document the changes in animal species, due to the scarcity of body fossils at these sites. However, the Elliot Formation contains an abundance of body fossils and reveals a turnover between the lower and upper Elliot Formation, with a considerable increase in species diversity and changes in body shape in sauropodomorph dinosaurs in the upper parts of the formation.
Sauropodomorph dinosaurs are by far the most common fossils found in the Elliot Formation. These dinosaurs are the early members of the sauropod lineage that led to the , incredibly long-necked, long-tailed plant-eating dinosaurs that include well known species such as Brontosaurus, Brachiosaurus and Diplodocus. The sauropodomorph dinosaurs in the Elliot Formation, however, are the precursors to these very large dinosaurs, weighing around 12 tonnes or less (as compared to around for the largest sauropod dinosaurs). These early dinosaurs have given us much information on how these animals went from a bipedal (two-legged) to quadrupedal (four-legged) stance and how gigantism evolved in this group. These dinosaurs did not decrease in either abundance or diversity during the end-Triassic extinction in South Africa. Instead some dinosaurs, such as Massospondylus carinatus, were clearly more abundant in the post-extinction environment (Figure 2). Larger and more diverse species evolved (Figure 3), showing that this group had a biological advantage that allowed them to not only survive the extinction, but become incredibly successful.
More research on this group will tell us why these sauropodomorph dinosaurs were so successful and what life history traits were advantageous in the extinction environment.
References:
McPhee, B. R. Benson, J. Botha-Brink, E. Bordy and J. Choiniere. 2018. A giant dinosaur from the earliest Jurassic of South Africa and the transition to quadrupedality in early sauropodomorphs. Current Biology 28: 3143-3153.
Bordy, E. M., M. Abrahams, G. R. Sharman, P. A. Viglietti, R. B. J. Benson, B. W. McPhee, P. M. Barrett, L. Sciscio, D. Condon, R. Mundil, Z. Rademan, Z. Jinnah, J. M. Clark, C. A. Suarez, K. E. J. Chapelle and J. N. Choiniere. 2020. A chronostratigraphic framework for the upper Stormberg Group: implications for the Triassic-Jurassic boundary in southern Africa. Earth-Science Reviews. 203: 103120.
Figure 1. The Elliot Formation of South Africa showing an increase in dinosaurs across the Triassic-Jurassic boundary (Bordy et al. 2020).
Figure 2. Massospondylus carinatus, an early sauropodomorph dinosaur from the early Jurassic of South Africa (reconstruction Dorling Kindersley).
Figure 3. Sauropodomorph dinosaurs showing an increase in size across the Triassic-Jurassic boundary, taken from McPhee et al. 2018).
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