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Saber-tooth marsupials and placentals compared, plus bird wing and leg morphology

From: Ben Creisler

Two new non-dino papers that may be of interest:

Stephen Wroel, Uphar Chamoli, William C. H. Parr, Philip Clausen, Ryan
Ridgely & Lawrence Witmer (2013)
Comparative Biomechanical Modeling of Metatherian and Placental
Saber-Tooths: A Different Kind of Bite for an Extreme Pouched
PLoS ONE 8(6): e66888

Questions surrounding the dramatic morphology of saber-tooths, and the
presumably deadly purpose to which it was put, have long excited
scholarly and popular attention. Among saber-toothed species, the
iconic North American placental, Smilodon fatalis, and the bizarre
South American sparassodont, Thylacosmilus atrox, represent extreme
forms commonly forwarded as examples of convergent evolution. For S.
fatalis, some consensus has been reached on the question of killing
behaviour, with most researchers accepting the canine-shear bite
hypothesis, wherein both head-depressing and jaw closing musculatures
played a role in delivery of the fatal bite. However, whether, or to
what degree, T. atrox may have applied a similar approach remains an
open question. Here we apply a three-dimensional computational
approach to examine convergence in mechanical performance between the
two species. We find that, in many respects, the placental S. fatalis
(a true felid) was more similar to the metatherian T. atrox than to a
conical-toothed cat. In modeling of both saber-tooths we found that
jaw-adductor-driven bite forces were low, but that simulations
invoking neck musculature revealed less cranio-mandibular stress than
in a conical-toothed cat. However, our study also revealed differences
between the two saber-tooths likely reflected in the modus operandi of
the kill. Jaw-adductor-driven bite forces were extremely weak in T.
atrox, and its skull was even better-adapted to resist stress induced
by head-depressors. Considered together with the fact that the center
of the arc described by the canines was closer to the jaw-joint in
Smilodon, our results are consistent with both jaw-closing and neck
musculature playing a role in prey dispatch for the placental, as has
been previously suggested. However, for T. atrox, we conclude that the
jaw-adductors probably played no major part in the killing bite. We
propose that the metatherian presents a more complete commitment to
the already extreme saber-tooth ‘lifestyle’.


Brandon M. Kilbourne (2013)
On birds: scale effects in the neognath hindlimb and differences in
the gross morphology of wings and hindlimbs.
Biological Journal of the Linnean Society (advance online publication)
DOI: 10.1111/bij.12110

Scale effects on whole limb morphology (i.e. bones together with in
situ overlying muscles) are well understood for the neognath forelimb.
However, scale effects on neognath gross hindlimb morphology remain
largely unexplored. To broaden our understanding of avian whole limb
morphology, I investigated the scaling of hindlimb inertial properties
in neognath birds, testing empirical scaling relationships against the
model of geometric similarity. Inertial property data – mass, moment
of inertia, centre of mass distance, and radius of gyration – were
collected from 22 neognath species representing a wide range of
locomotor specializations. When scaled against body mass, hindlimb
inertial properties scale with positive allometry. Thus, in terms of
morphology, larger bodied neognaths possess hindlimbs requiring
disproportionately more energy to accelerate and decelerate relative
to body mass than smaller bodied birds. When scaled against limb
length, hindlimb inertial properties scale according to isometry. In
the subclade Land Birds (sensu Hackett et al.), hindlimb inertial
properties largely scale according to positive allometry. The
contrasting results of positive allometry vs. isometry in neognaths
are due to how hindlimb length scales against body mass. Negative
allometry of hindlimb inertial properties, which would reduce
terrestrial locomotion costs, would probably make the hindlimb
susceptible to mechanical failure or too diminutive for its many
ecological functions. Comparing the scaling relationships of wings and
hindlimbs highlights how locomotor costs influence the scaling of limb
inertial properties.