Behavioural leads in evolution: evidence from the fossil record

Abstract

There has been much discussion of the role of behaviour in evolution, especially its potential to lead morphological evolution by placing the organism in a novel selective environment. Many adaptations of living species can be imagined to have originated in this way, although documented examples are relatively few. A fruitful arena for testing hypotheses about behavioural innovation is the fossil record. Traditionally, the behaviour of fossil species has been deduced from their morphology, precluding the observation of a behavioural lead preceding morphological evolution. This circularity can be broken by examining behavioural proxies independent of the adaptive morphology itself. Examples applicable to fossil remains include dietary information (e.g. wear traces on teeth, stable isotopes) and trace fossils indicating locomotor mode (footprints). The signature of a behavioural lead would be an observed shift in behaviour from one horizon (or taxon) to another, followed later by a functionally-related morphological change. This pattern can be sought either in finely-stratified anagenetic sequences of fossils (stratophenetic approach) or among fossils with well-resolved species-level phylogenies (cladistic approach). An array of case studies from the literature is presented. These include feeding shifts in finely-resolved sequences of vertebrates ranging from freshwater fish to terrestrial ungulates, as well as locomotor changes crucial to major evolutionary transitions in the origin of tetrapods, birds, and humans. The latter examples highlight the role of behaviour in initiating exaptation (the requisitioning of structure for a new function). The case studies also illustrate the challenges of using fossil sequences to elucidate behavioural roles, including insufficient stratigraphic resolution and uncertainty over the adaptive function of observed traits. By the same token, they suggest criteria for choosing promising cases for research, as well as for formulating testable hypotheses about evolutionary modes. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 112, 315–331.