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Re: Allosauroid locomotor anatomy and body mass evolution

Well... the conclusions are probably correct... Not in general terms
to ALL large theropods, but only to sauropod-hunting allosaurids.
Smaller species, as well as juvies, might have needed speed, but the
bigger boys became Bronto-hunter extraordinaires. Considering this,
logically, selection would result in relative speed needs dropping
within the group as body size increased.

Therefore, the intpretation I see of the evidence is not a restriction
due to body size, but an alteration to form and function due to
specialized hunting strategies.


On Thu, Jun 28, 2012 at 4:59 AM, Ben Creisler <bcreisler@gmail.com> wrote:
> 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.