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Lesothosaurus muscle function studied with computational modeling

From: Ben Creisler

A new paper not yet mentioned on the DML:

Bates, K. T., Maidment, S. C. R., Allen, V. and Barrett, P. M. (2012)
Computational modelling of locomotor muscle moment arms in the basal
dinosaur Lesothosaurus diagnosticus: assessing convergence between
birds and basal ornithischians. Journal of Anatomy (advance online
doi: 10.1111/j.1469-7580.2011.01469.x

Ornithischia (the ‘bird-hipped’ dinosaurs) encompasses bipedal,
facultative quadrupedal and quadrupedal taxa. Primitive ornithischians
were small bipeds, but large body size and obligate quadrupedality
evolved independently in all major ornithischian lineages. Numerous
pelvic and hind limb features distinguish ornithischians from the
majority of other non-avian dinosaurs. However, some of these
features, notably a retroverted pubis and elongate iliac preacetabular
process, appeared convergently in maniraptoran theropods, and were
inherited by their avian descendants. During maniraptoran/avian
evolution these pelvic modifications led to significant changes in the
functions of associated muscles, involving alterations to the moment
arms and the activation patterns of pelvic musculature. However, the
functions of these features in ornithischians and their influence on
locomotion have not been tested and remain poorly understood. Here, we
provide quantitative tests of bipedal ornithischian muscle function
using computational modelling to estimate 3D hind limb moment arms for
the most complete basal ornithischian, Lesothosaurus diagnosticus.
This approach enables sensitivity analyses to be carried out to
explore the effects of uncertainties in muscle reconstructions of
extinct taxa, and allows direct comparisons to be made with similarly
constructed models of other bipedal dinosaurs. This analysis supports
some previously proposed qualitative inferences of muscle function in
basal ornithischians. However, more importantly, this work highlights
ambiguities in the roles of certain muscles, notably those inserting
close to the hip joint. Comparative analysis reveals that moment arm
polarities and magnitudes in Lesothosaurus, basal tetanuran theropods
and the extant ostrich are generally similar. However, several key
differences are identified, most significantly in comparisons between
the moment arms of muscles associated with convergent osteological
features in ornithischians and birds. Craniad migration of the
iliofemoralis group muscles in birds correlates with increased
leverage and use of medial femoral rotation to counter stance phase
adduction moments at the hip. In Lesothosaurus the iliofemoralis group
maintains significantly higher moment arms for abduction, consistent
with the hip abduction mode of lateral limb support hypothesized for
basal dinosaurs. Sensitivity analysis highlights ambiguity in the role
of musculature associated with the retroverted pubis
(puboischiofemoralis externus group) in ornithischians. However, it
seems likely that this musculature may have predominantly functioned
similarly to homologous muscles in extant birds, activating during the
swing phase to adduct the lower limb through lateral rotation of the
femur. Overall the results suggest that locomotor muscle leverage in
Lesothosaurus (and by inference basal ornithischians in general) was
more similar to that of other non-avian dinosaurs than the ostrich,
representing what was probably the basal dinosaur condition. This work
thereby contradicts previous hypotheses of ornithischian–bird
functional convergence.