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Giant herbivorous theropod body mass evolution

From: Ben Creisler

Notice and news release for a new paper not yet available online.
However, the abstract is available, so I'll post it anyway pending the
official links:



Lindsay E. Zanno & Peter J Makovicky (2012)
No evidence for directional evolution of body mass in herbivorous
theropod dinosaurs.
Proceedings of the Royal Society B

The correlation between large body size and digestive efficiency has
been hypothesized to have driven trends of increasing mass in
herbivorous clades by means of directional selection. Yet, to date,
few studies have investigated this relationship from a phylogenetic
perspective and none, to our knowledge, with regard to trophic shifts.
Here, we reconstruct body mass in the three major subclades of
non-avian theropod dinosaurs whose ecomorphology is correlated with
extrinsic evidence of at least facultative herbivory in the fossil
record – all of which also achieve relative gigantism (more than 3000
kg). Ordinary least-squares regressions on natural logtransformed mean
mass recover significant correlations between increasing mass and
geologic time. However, tests for directional evolution in body mass
find no support for a phylogenetic trend, instead favouring passive
models of trait evolution. Cross-correlation of sympatric taxa from
five localities in Asia reveals that environmental influences such as
differential habitat sampling and/or taphonomic filtering affect the
preserved record of dinosaurian body mass in the Cretaceous. Our
results are congruent with studies documenting that behavioural and/or
ecological factors may mitigate the benefit of increasing mass in
extant taxa, and suggest that the hypothesis can be extrapolated to
herbivorous lineages across geologic time scales.