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Happy B/day T rex - new paper and others
Here's a birthday present for rex.
The following issue is now available for the Paleontological Journals Online
2005. Analysis of hindlimb muscle moment arms in Tyrannosaurus rex using a
musculoskeletal computer model: implications for stance, gait, and speed.
John R. Hutchinson, Frank C. Anderson, Silvia S. Blemker, Scott L. Delp,
Paleobiology 31(4): 676-701.
Abstract. ? Muscle moment arms are important determinants of muscle function;
however, it is
challenging to determine moment arms by inspecting bone specimens alone, as
curvilinear paths that change as joints rotate. The goals of this study were to
(1) develop a
three-dimensional graphics-based model of the musculoskeletal system of the
dinosaur Tyrannosaurus rex that predicts muscle-tendon unit paths, lengths, and
moment arms for a
range of limb positions; (2) use the model to determine how the T. rex hindlimb
muscle moment arms
varied between crouched and upright poses; (3) compare the predicted moment
arms with previous
assessments of muscle function in dinosaurs; (4) evaluate how the magnitudes of
these moment arms
compare with those in other animals; and (5) integrate these findings with
studies to produce a revised appraisal of stance, gait, and speed in T. rex.
model includes ten degrees of joint freedom (flexion/extension, ab/adduction,
or medial/ lateral
rotation) and 33 main muscle groups crossing the hip, knee, ankle, and toe
joints of each
hindlimb. The model was developed by acquiring and processing bone geometric
data, defining joint
rotation axes, justifying muscle attachment sites, and specifying muscle-tendon
paths. Flexor and extensor muscle moment arms about all of the main limb joints
and limb orientation was statically varied to characterize how the muscle
moment arms changed. We
used sensitivity analysis of uncertain parameters, such as muscle origin and
to deterimine how much our conclusions depend on the muscle reconstruction we
adopted. This shows
that a specific amount of error in the reconstruction (e.g., position of muscle
origins) can have
a greater, lesser, similar, or no effect on the moment arms, depending on
between components of the musculoskeletal geometry. We found that more upright
poses would have
improved mechanical advantage of the muscles considerably. Our analysis shows
assumed moment arm values were generally conservatively high. Our results for
muscle moment arms
are generally lower than the values predicted by scaling data from extant taxa,
suggesting that T.
rex did not have the allometrically large muscle moment arms that might be
expected in a
proficient runner. The information provided by the model is important for
determining how T. rex
stood and walked, and how the muscles of a 4000?7000 kg biped might have worked
in comparison with
extant bipeds such as birds and humans. Our model thus strengthens the
conclusion that T. rex was
not an exceptionally fast runner, and supports the inference that more upright
completely columnar) poses are more plausible for T. rex. These results confirm
about the relationship between size, limb orientation, and locomotor mechanics:
animals have a more limited range of locomotor abilities and tend to adopt more
upright poses that
improve extensor muscle effective mechanical advantage. This model builds on
phylogenetically based muscle reconstructions and so moves closer to a fully
three-dimensional model of stance, gait, and speed in T. rex.
Also in the same volume is:
Correlated trends in the evolution of the plesiosaur locomotor system.
F. Robin O'Keefe, and Matthew T. Carrano.
Paleobiology 31(4): 656-675.
Abstract. ? This paper investigates trends in the evolution of body size and
shape in the
Plesiosauria, a diverse clade of Mesozoic marine reptiles. Using measures from
plesiosaur specimens, we document and interpret evolutionary patterns in
relative head size, body
size, and locomotor variables. Size increase is a significant trend in the
clade as a whole, and
in constituent clades. The trend in relative head size is of variance increase;
sizes are both smaller and larger than ancestral values. In the locomotor
system, changes in
propodial and girdle proportions appear concomitant with body size increase and
are interpreted as
allometric responses to the physical constraints of large body size. Other
trends in the locomotor
system are significantly correlated with both body size and relative head size.
trends evolved convergently in several clades of plesiosaurs, and may have had
basis, although data are lacking to constrain speculation on this point. The
evolution of the
locomotor system in plesiosaurs sheds new light on the response of aquatic
tetrapods to the
physical constraints of foraging at large body size.
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