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Captorhinid (Sauropsida) body size evolution (free pdf)

Ben Creisler

A new paper in open access:

Neil Brocklehurst (2016)
Rates and modes of body size evolution in early carnivores and
herbivores: a case study from Captorhinidae.
PeerJ 4:e1555
doi: https://doi.org/10.7717/peerj.1555

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.