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Extant marsupials are less ecologically diverse than placentals, and this
is reflected by placentals exhibiting a greater diversity of locomotor
modes, including powered flight and fully aquatic swimming. One proposed
explanation for this discrepancy is that the development of more disparate
marsupial forelimbs is prevented by the neonate’s crawl to the pouch,
which requires precocious forelimb development for climbing adaptations.
To test predictions of this Developmental Constraint Hypothesis, we pursue
a comparative morphometric study on osteological traits of mammalian
limbs, with an emphasis on functional differentiation of marsupial limbs
among locomotor modes. We apply multivariate analyses to a large dataset
of limb metrics and a diverse sample of mammals, with the placental sample
limited to taxa whose locomotor modes are exhibited in marsupials.
Overall, we do not find consistent evidence in support of the
Developmental Constraint Hypothesis. Diprotodontia serves as an exception,
with comparisons of their forelimbs to hind limbs supporting the
Developmental Constraint Hypothesis. Our results suggest that
developmental constraints on marsupial forelimbs may have limited
marsupial diversity to some degree. Despite this, the marsupial locomotor
groups show unexpectedly high levels of morphological differentiation
relative to placentals of the same locomotor modes, indicating that
ecological functions may overcome developmental constraints on a
macroevolutionary scale.
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