Abstract
The extent to which mass extinctions influence body size evolution in major tetrapod clades is inadequately understood. For example, the ‘Lilliput effect,’ a common feature of mass extinctions, describes a temporary decrease in body sizes of survivor taxa in post-extinction faunas. However, its signature on existing patterns of body size evolution in tetrapods and the persistence of its impacts during post-extinction recoveries are virtually unknown, and rarely compared in both geologic and phylogenetic contexts. Here, I evaluate temporal and phylogenetic distributions of body size in Permo-Triassic therocephalian and cynodont therapsids (eutheriodonts) using a museum collections-based approach and time series model fitting on a regional stratigraphic sequence from the Karoo Basin, South Africa. I further employed rank order correlation tests on global age and clade rank data from an expanded phylogenetic dataset, and performed evolutionary model testing using Brownian (passive diffusion) models. Results support significant size reductions in the immediate aftermath of the end-Permian mass extinction (ca. 252.3 Ma) consistent with some definitions of Lilliput effects. However, this temporal succession reflects a pattern that was underscored largely by Brownian processes and constructive selectivity. Results also support two recent contentions about body size evolution and mass extinctions: 1) active, directional evolution in size traits is rare over macroevolutionary time scales and 2) geologically brief size reductions may be accomplished by the ecological removal of large-bodied species without rapid originations of new small-bodied clades or shifts from long-term evolutionary patterns.
Highlights
Body size is an important biological trait that affects organismal fitness and imparts a strong influence on ecological, physiological, and life history attributes of species [1,2,3,4]
On the other hand, body size evolution was driven by longerterm trends beginning in the Permian, a directional model with a negative mean step change should best fit the data, thereby refuting the rapid and short-term perturbation predicted by the Lilliput effect
Using ‘geiger,’ I evaluated the relative fit of four phylogenetic models in order to approximate the mode of body size evolution observed on the tree: Brownian motion (BM), Brownian motion with trend (BMT), Ornstein-Uhlenbeck (OU), and early burst Brownian motion (EB)
Summary
Body size is an important biological trait that affects organismal fitness and imparts a strong influence on ecological, physiological, and life history attributes of species [1,2,3,4]. Though originally reserved for within-lineage size decreases in survivor taxa [8], the term ‘Lilliput’ has been repurposed as a pattern that may encompass a number of other underlying processes, such as differential extinction of large-bodied taxa and rapid diversifications of new small-bodied clades following mass extinctions [9,10], or a combination of these [25] These patterns have been well documented in Triassic marine invertebrate communities following the end-Permian mass extinction [9,25,26,27,28] and anecdotally in terrestrial vertebrates of the earliest Triassic Lystrosaurus Assemblage Zone, Karoo Basin of. I argue here that extinctions were largely size selective, but were constructive, dependent upon existing patterns of trait evolution, and did not result in shifts from long-term phylogenetic patterns in the study group
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