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Patterns of tetrapod diversification on land (and possible explanations)

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If you have a question about this talk, please contact Ben Johnson.

Tetrapods – mammals, birds and other reptiles – comprise around 30,000 species today and play important roles in terrestrial ecosystems. They evolved from a single ancestor species that lived c.360 million years ago. Patterns of both species richness and phenotypic disparity, and how they have changed through time, have been subject to debate, and the prevailing hypothesis has been one of near-continual expansion through time, driven by frequent innovation. This hypothesis is central to macroevolutionary hypotheses about biotic diversification on Earth. The Mesozoic ‘age of dinosaurs’ is though to have witnessed particularly large increases in species richness of tetrapods on land, but this has received surprisingly little scrutiny.

My work has focussed on quantitative approaches to characterising patterns of phenotypic diversification and of tetrapod species richness in a spatially-patchy fossil record. Dynamic evolutionary radiations, involving increases in phenotypic diversity, and expansion of species richness occur frequently on shorter timescales (~ 10 Ma) and local phylogenetic scales. However, longer-term, large-scale patterns are almost static, with species diversification rates of approximately zero over extended intervals. This background of little net change in species richness is punctuated by abrupt episodes of dramatic radiation, that substantially elevated species richness on land beyond previous levels. These are associated with post-extinction radiations, and with a small number of truly exceptional evolutionary innovations. Explanations for this pattern may be rooted in macroecology and energetics.

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