Abstract
Studies of the effects of mass extinctions on ancient ecosystems have focused on changes in taxic diversity, morphological disparity, abundance, behaviour and resource availability as key determinants of group survival. Crucially, the contribution of life history traits to survival during terrestrial mass extinctions has not been investigated, despite the critical role of such traits for population viability. We use bone microstructure and body size data to investigate the palaeoecological implications of changes in life history strategies in the therapsid forerunners of mammals before and after the Permo-Triassic Mass Extinction (PTME), the most catastrophic crisis in Phanerozoic history. Our results are consistent with truncated development, shortened life expectancies, elevated mortality rates and higher extinction risks amongst post-extinction species. Various simulations of ecological dynamics indicate that an earlier onset of reproduction leading to shortened generation times could explain the persistence of therapsids in the unpredictable, resource-limited Early Triassic environments, and help explain observed body size distributions of some disaster taxa (e.g., Lystrosaurus). Our study accounts for differential survival in mammal ancestors after the PTME and provides a methodological framework for quantifying survival strategies in other vertebrates during major biotic crises.
Highlights
Mass extinctions reshape biological communities as a result of extensive biodiversity losses over short time periods and novel selective pressures that either trigger secondary extinctions or alter tempo and mode of evolution[1]
We examined growth patterns in representatives of all boundary-crossing therapsid clades from the South African Karoo Basin, using data from the largest histological database of Permo-Triassic non-mammalian therapsids compiled to date
To gain insights into the demographic structure of extinct populations before and after the PermoTriassic Mass Extinction (PTME), we investigated body size distributions in therapsids (Supplementary Appendix 2) as a framework for interpreting relative abundances of different age classes[15,24] and, to infer differences in survivorship rates
Summary
Mass Extinction received: 12 October 2015 accepted: 18 March 2016 Published: 05 April 2016. The most catastrophic crisis in Phanerozoic history, the Permo-Triassic Mass Extinction (PTME), was characterised by a rapid decrease in global biodiversity, leading to a radical restructuring of ecosystems 251.9 Ma2 Both marine and terrestrial communities showed reduced diversity immediately after the PTME, a likely consequence of primary productivity losses that caused secondary extinction cascades[1,3,4]. Special consideration is given to the dicynodont Lystrosaurus, the most iconic of all PTME survivors This very abundant genus (3000 + specimens in museum collections) dominated Early Triassic ecosystems worldwide for millions of years during the post-extinction recovery phase. It provides a sufficiently large sample for studying population structure and differential survival during mass extinctions. To gain insights into the demographic structure of extinct populations before and after the PTME, we investigated body size distributions in therapsids (Supplementary Appendix 2) as a framework for interpreting relative abundances of different age classes (using basal skull length [BSL in mm] as a proxy for body size)[15,24] and, to infer differences in survivorship rates
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