Rates and modes of body size evolution in early carnivores and herbivores: a case study from Captorhinidae

Neil Brocklehurst

doi:10.7717/peerj.1555

Abstract

Body size is an extremely important characteristic, impacting on a variety of ecological and life-history traits. It is therefore important to understand the factors which may affect its evolution, and diet has attracted much interest in this context. A recent study which examined the evolution of the earliest terrestrial herbivores in the Late Carboniferous and Early Permian concluded that in the four herbivorous clades examined there was a trend towards increased body size, and that this increase was more substantial than that observed in closely related carnivorous clades. However, this hypothesis was not based on quantitative examination, and phylogenetic comparative methods provide a more robust means of testing such hypotheses. Here, the evolution of body size within different dietary regimes is examined in Captorhinidae, the most diverse and longest lived of these earliest high fibre herbivores. Evolutionary models were fit to their phylogeny to test for variation in rate and mode of evolution between the carnivorous and herbivorous members of this clade, and an analysis of rate variation throughout the tree was carried out. Estimates of ancestral body sizes were calculated in order to compare the rates and direction of evolution of lineages with different dietary regimes. Support for the idea that the high fibre herbivores within captorhinids are being drawn to a higher adaptive peak in body size than the carnivorous members of this clade is weak. A shift in rates of body size evolution is identified, but this does not coincide with the evolution of high-fibre herbivory, instead occurring earlier in time and at a more basal node. Herbivorous lineages which show an increase in size are not found to evolve at a faster rate than those which show a decrease; in fact, it is those which experience a size decrease which evolve at higher rates. It is possible the shift in rates of evolution is related to the improved food processing ability of the more derived captorhinids rather than a shift in diet, but the evidence for this is circumstantial.

Highlights

Body size is among the most important traits of an organism (Bell, 2014)
A great deal of effort has been put into understanding the patterns and processes in body size evolution and how this varies between clades, through time and between different ecological groups
The majority of analyses (91) suggest this rate shift was an increase occurring at the same node: the clade containing Captorhinus, Captorhinikos valensis, Labidosaurus and the Moradisaurinae (Fig. 3), there are a minority where the rate increase is found to have occurred only in the genus Captorhinus

Summary

Introduction

Body size is among the most important traits of an organism (Bell, 2014). It influences, amongst other things, an organism’s potential diet range (Sinclair, Mduma & Brashares, 2003), the habitats it may occupy, its energy requirements (Oksanen et al, 1981), its ability to defend against predation (Roff, 1992), its development (Gillooly et al, 2002) and viableHow to cite this article Brocklehurst (2016), Rates and modes of body size evolution in early carnivores and herbivores: a case study from Captorhinidae. Body size is among the most important traits of an organism (Bell, 2014). In a recent paper, Reisz & Frobisch (2014) examined body-size evolution in the earliest terrestrial herbivorous vertebrate. The earliest members of each of these four clades are considered to be small carnivores or insectivores, and the herbivorous members of these clades appeared in each case to be noticeably larger than their carnivorous ancestors. They suggested that there was more pronounced increase in body size in the herbivorous members of these clades than in closely related carnivores

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Conclusion