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Moschorhinus (therocephalian) growth-rate changes across Permian-Triassic boundary



From: Ben Creisler
bcreisler@gmail.com


A new non-dino paper:



Adam K. Huttenlocker and Jennifer Botha-Brink (2013)
Body size and growth patterns in the therocephalian Moschorhinus
kitchingi (Therapsida: Eutheriodontia) before and after the
end-Permian extinction in South Africa.
Paleobiology 39(2):253-277
doi: 10.1666/12020
http://www.bioone.org/doi/abs/10.1666/12020


The continuous fossil record of therocephalian therapsids
(Eutheriodontia) across the Permo-Triassic boundary and their
differential survivorship of the end-Permian extinction offer an
exceptional deep-time perspective on vertebrate life-history evolution
during episodes of large-scale ecological perturbation. To examine
potential impacts of the extinction on body size evolution (e.g.,
“Lilliput” effects) and growth patterns, we investigated cranial sizes
and limb bone histology in the therocephalian Moschorhinus kitchingi
both before and after the end-Permian extinction, facilitated by
analysis of thin-sections of 23 limb bones from an ontogenetic sample
of ten individuals across the Permo-Triassic boundary. In general,
early subadult Moschorhinus displayed propodial cortices with
extensive woven- and parallel-fibered bone (PFB) with dense radial and
reticular vascularization and a moderately thickened bone wall with
few growth marks. The outer cortex of propodials and epipodials showed
a transition to PFB and lamellar bone with longitudinally oriented
canals in individuals interpreted as late subadults or adults (>80%
largest size). Most elements displayed several (3+) growth marks,
though growth marks were more faithfully recorded in the epipodials of
Permian individuals. Pearson product-moment correlation tests were
performed to examine the relationship between size and robusticity on
growth proxies (% cortical vascularity, mean primary osteon diameter),
but variation in histomorphology could not be explained by size alone.
Variation in body size may be affected by differences in juvenile
growth rate and duration, which are highly variable in environmentally
stressed extant reptile species. Geologic stage was a more consistent
predictor of cortical vascularity. We suggest that Permian and
Triassic Moschorhinus exhibited differential rates of early skeletal
growth, corroborating the hypothesis that increased environmental
variability in the earliest Triassic was associated with rapid growth
to a minimum body size requirement and, consequently, shortened
developmental times.