Evolutionary biomechanics: hard tissues and soft evidence?

Biomechanical modelling is a powerful tool for quantifying the evolution
of functional performance in extinct animals to understand key anatomical
innovations and selective pressures driving major evolutionary radiations.
However, the fossil record is composed predominantly of hard parts,
forcing palaeontologists to reconstruct soft tissue properties in such
models. Rarely are these reconstruction approaches validated on extant
animals, despite soft tissue properties being highly determinant of
functional performance. The extent to which soft tissue reconstructions
and biomechanical models accurately predict quantitative or even
qualitative patterns in macroevolutionary studies is therefore unknown.
Here, we modelled the masticatory system in extant rodents to objectively
test the ability of current muscle reconstruction methods to correctly
identify quantitative and qualitative differences between
macroevolutionary morphotypes. Baseline models generated using measured
soft tissue properties yielded differences in muscle proportions, bite
force and bone stress expected between extant sciuromorph, myomorph and
hystricomorph rodents. However, predictions from models generated using
reconstruction methods typically used in fossil studies varied widely from
high levels of quantitative accuracy to a failure to correctly capture
even relative differences between macroevolutionary morphotypes. Our novel
experiment emphasises that correctly reconstructing even qualitative
differences between taxa in a macroevolutionary radiation is challenging
using current methods. Future studies of fossil taxa should incorporate
systematic assessments of reconstruction error into their hypothesis
testing and, moreover, seek to expand primary data sets on muscle
properties in extant taxa to better inform soft tissue reconstructions in
macroevolutionary studies.