How body size and development biased the direction of evolution in early amphibians

Abstract

Evolutionary change does not proceed in every direction with equal probability. Evolutionary biases or constraints are limitations on the mode, direction and tempo of evolution. Early tetrapods provide interesting examples, especially Paleozoic and Mesozoic amphibians. (1) Body size had a strong impact on morphology and development in early amphibians, resulting in manifold convergences imposed by design limitations. Miniaturisation had similar effects in a wide range of Paleozoic tetrapods, which are consistent with observations on extant salamanders. Gigantism was a common feature of Triassic temnospondyls, correlating with slow developmental rates similar to those of gigantic salamanders and the convergent evolution of bone density. (2) Ontogeny imposes constraints on evolution by canalised (buffered) developmental sequences. In Paleozoic temnospondyls, ontogenetic trajectories evolved by several different modes (truncation of the trajectory, shifting of events or condensation of events). Metamorphosis is an extreme example of a condensed developmental sequence, which first evolved in Paleozoic temnospondyls, increased in salamanders and culminated in anurans. It imposes strong biases that may be broken by three conceivable modes: (1) loss of the adult period (neoteny), (2) loss of the larval period (direct development) and (3) ‘unpacking’ of metamorphosis by re-evolving the plesiomorphic trajectory.

Keywords:

• constraint
• developmental events
• direct development
• metamorphosis
• neoteny
• ontogenetic trajectory

Acknowledgements

This paper would not have been written without the encouragement I experienced from Wolf Reif. Ever since we first met, Wolf has fascinated me more than any other researcher. He did so by a combination of two features: his openness to any subject and view and his deep thinking which has impressed everybody who met him. I thank Andrew Milner for numerous discussions, Dolf Seilacher for his stimulating questions and David Wake for a few but enlightening exchanges of thought. Feedback by Florian Witzmann and Nadia Fröbisch has helped me sharpen my views on the present and many related subjects. I thank the two reviewers and the editor for their constructive criticism, which made me think more deeply on various points.