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Permo-Triassic therocephalian bone growth (free pdf)

From: Ben Creisler

New in PeerJ:

Adam K. Huttenlocker & Jennifer Botha-Brink (2014)
Bone microstructure and the evolution of growth patterns in
Permo-Triassic therocephalians (Amniota, Therapsida) of South Africa.
PeerJ 2:e325
doi:  http://dx.doi.org/10.7717/peerj.325

Therocephalians were a speciose clade of nonmammalian therapsids whose
ecological diversity and survivorship of the end-Permian mass
extinction offer the potential to investigate the evolution of growth
patterns across the clade and their underlying influences on
post-extinction body size reductions, or 'Lilliput effects'. We
present a phylogenetic survey of limb bone histology and growth
patterns in therocephalians from the Middle Permian through Middle
Triassic of the Karoo Basin, South Africa. Histologic sections were
prepared from 80 limb bones representing 11 genera of therocephalians.
Histologic indicators of skeletal growth, including cortical
vascularity (%CV) and mean primary osteon diameters (POD), were
evaluated in a phylogenetic framework and assessed for correlations
with other biologically significant variables (e.g., size and
robusticity). Changes in %CV and POD correlated strongly with
evolutionary changes in body size (i.e., smaller-bodied descendants
tended to have lower %CV than their larger-bodied ancestors across the
tree). Bone wall thickness tended to be high in early therocephalians
and lower in the gracile-limbed baurioids, but showed no general
correlation with cross-sectional area or degree of vascularity (and,
thus, growth). Clade-level patterns, however, deviated from previously
studied within-lineage patterns. For example, Moschorhinus, one of few
therapsid genera to have survived the extinction boundary,
demonstrated higher %CV in the Triassic than in the Permian despite
its smaller size in the extinction aftermath. Results support a
synergistic model of size reductions for Triassic therocephalians,
influenced both by within-lineage heterochronic shifts in survivor
taxa (as reported in Moschorhinus and the dicynodont Lystrosaurus) and
phylogenetically inferred survival of small-bodied taxa that had
evolved short growth durations (e.g., baurioids). These findings
mirror the multi-causal Lilliput patterns described in marine faunas,
but contrast with skeletochronologic studies that suggest slow,
prolonged shell secretion over several years in marine benthos.
Applications of phylogenetic comparative methods to new histologic
data will continue to improve our understanding of the evolutionary
dynamics of growth and body size shifts during mass extinctions and