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Allosauroid locomotor anatomy and body mass evolution



From: Ben Creisler
bcreisler@gmail.com

New in Paleobiology:


Karl T. Bates, Roger B. J. Benson, and Peter L. Falkingham  (2012)
A computational analysis of locomotor anatomy and body mass evolution
in Allosauroidea (Dinosauria: Theropoda).
Paleobiology 38(3):486-507
doi: http://dx.doi.org/10.1666/10004.1
http://www.bioone.org/doi/abs/10.1666/10004.1


Supplementary materials deposited at Dryad: doi: 10.5061/dryad.09kf4g02


Abstract
We investigate whether musculoskeletal anatomy and three-dimensional
(3-D) body proportions were modified during the evolution of large
(>6000 kg) body size in Allosauroidea (Dinosauria: Theropoda). Three
adaptations for maintaining locomotor performance at large body size,
related to muscle leverage, mass, and body proportions, are tested and
all are unsupported in this analysis. Predictions from 3-D
musculoskeletal models of medium-sized (Allosaurus) and large-bodied
(Acrocanthosaurus) allosauroids suggest that muscle leverage scaled
close to isometry, well below the positive allometry required to
compensate for declining muscle cross-sectional area with increasing
body size. Regression analyses on a larger allosauroid data set finds
slight positive allometry in the moment arms of major hip extensors,
but isometry is included within confidence limits. Contrary to other
recent studies of large-bodied theropod clades, we found no compelling
evidence for significant positive allometry in muscle mass between
exemplar medium- and large-bodied allosauroids. Indeed, despite the
uncertainty in quantitative soft tissue reconstruction, we find strong
evidence for negative allometry in the caudofemoralis longus muscle,
the single largest hip extensor in non-avian theropods. Finally, we
found significant inter-study variability in center-of-mass
predictions for allosauroids, but overall observe that consistently
proportioned soft tissue reconstructions produced similar predictions
across the group, providing no support for a caudal shift in the
center of mass in larger taxa that might otherwise reduce demands on
hip extensor muscles during stance. Our data set provides further
quantitative support to studies that argue for a significant decline
in locomotor performance with increasing body size in non-avian
theropods. However, although key pelvic limb synapomorphies of derived
allosauroids (e.g., dorsomedially inclined femoral head) evolved at
intermediate body sizes, they may nonetheless have improved mass
support.