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Keichousaurus (Triassic pachypleurosaur from China) sexual dimorphism and allometric growth

From: Ben Creisler

A new online paper:

Xue Yifan, Jiang Dayong, Ryosuke Motani, Olivier Rieppel, Sun Yuanlin,
Sun Zuoyu, Ji Cheng, and Yang Pengfei (2013)
New information on sexual dimorphism and allometric growth in
Keichousaurus hui, a pachypleurosaur from the Middle Triassic of
Guizhou, South China.
Acta Palaeontologica Polonica (in press)

Keichousaurus hui is a small pachypleurosaur (Reptilia: Sauropterygia)
from the Triassic of China. Many specimens of various growth stages
are known, making it ideal for ontogenetic research. We report 22 new
specimens from the Middle Triassic of Xingyi (Guizhou, south China),
and combined their skeletal measurements with those from 85 published
specimens to analyze the ontogenetic trajectory of sexual dimorphism.
An Exploratory Factor Analysis suggests that the largest factors
behind morphological disparity within the species are body size
followed by gender. Sexual dimorphism is most clearly reflected in
selected skeletal ratios that are more pronounced in males than in
females. We found that the relative length of femur to body size was
useful in gender identification, in addition to three ratios that are
traditionally used, namely a distal expansion of the humerus relative
to its shaft, humerus length relative to body size, and humerus length
relative to femur length. Two distinctive patterns exist in allometric
changes of these four ratios. The distal expansion of the humerus is
exceptional in that it is equally pronounced in juvenile and adult
males and therefore must have been fully established during embryonic
growth. The other three features are not pronounced at birth size and
subsequently become pronounced during postembryonic growth. However,
males and females already show different growth trajectories at birth
size even in these three. Therefore, the fate of sexually dimorphic
features seems to have already been set during embryonic growth in K.