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Crocodile Mandibular Symphysis Proportions



From: Ben Creisler
bcreisler@gmail.com

New in PLoS ONE:

Christopher W. Walmsley, Peter D. Smits, Michelle R. Quayle, Matthew
R. McCurry, Heather S. Richards, Christopher C. Oldfield, Stephen
Wroe, Phillip D. Clausen & Colin R. McHenry (2013)
Why the Long Face? The Mechanics of Mandibular Symphysis Proportions
in Crocodiles.
PLoS ONE 8(1): e53873.
doi:10.1371/journal.pone.0053873
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0053873

Background

Crocodilians exhibit a spectrum of rostral shape from long snouted
(longirostrine), through to short snouted (brevirostrine)
morphologies. The proportional length of the mandibular symphysis
correlates consistently with rostral shape, forming as much as 50% of
the mandible’s length in longirostrine forms, but 10% in brevirostrine
crocodilians. Here we analyse the structural consequences of an
elongate mandibular symphysis in relation to feeding behaviours.

Methods/Principal Findings

Simple beam and high resolution Finite Element (FE) models of seven
species of crocodile were analysed under loads simulating biting,
shaking and twisting. Using beam theory, we statistically compared
multiple hypotheses of which morphological variables should control
the biomechanical response. Brevi- and mesorostrine morphologies were
found to consistently outperform longirostrine types when subject to
equivalent biting, shaking and twisting loads. The best predictors of
performance for biting and twisting loads in FE models were overall
length and symphyseal length respectively; for shaking loads
symphyseal length and a multivariate measurement of shape (PC1– which
is strongly but not exclusively correlated with symphyseal length)
were equally good predictors. Linear measurements were better
predictors than multivariate measurements of shape in biting and
twisting loads. For both biting and shaking loads but not for
twisting, simple beam models agree with best performance predictors in
FE models.

Conclusions/Significance

Combining beam and FE modelling allows a priori hypotheses about the
importance of morphological traits on biomechanics to be statistically
tested. Short mandibular symphyses perform well under loads used for
feeding upon large prey, but elongate symphyses incur high strains
under equivalent loads, underlining the structural constraints to prey
size in the longirostrine morphotype. The biomechanics of the
crocodilian mandible are largely consistent with beam theory and can
be predicted from simple morphological measurements, suggesting that
crocodilians are a useful model for investigating the
palaeobiomechanics of other aquatic tetrapods.


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