Data for: Macroevolution in axial morphospace: Innovations accompanying the transition to marine environments in elapid snakes

Sea snakes in the Hydrophis-Microcephalophis clade (Elapidae) show
exceptional body shape variation along a continuum from similar forebody
and hindbody girths, to dramatically reduced girths of the forebody
relative to hindbody. The latter is associated with specialisations on
burrowing prey. This variation underpins high sympatric diversity and
species richness and is not shared by other marine (or terrestrial)
snakes. Here, we examined a hypothesis that macroevolutionary changes in
axial development contribute to the propensity, at clade level, for body
shape change. We quantified variation in the number and size of vertebrae
in two body regions (pre- and post-apex of the heart) for ~94 terrestrial
and marine elapids. We found Hydrophis-Microcephalophis exhibit increased
rates of vertebral evolution in the pre- versus post-apex regions compared
to all other Australasian elapids. Unlike other marine and terrestrial
elapids, axial elongation in Hydrophis-Microcephalophis occurs via the
preferential addition of vertebrae pre heart apex, which is the region
that undergoes concomitant shifts in vertebral number and size during
transitions along the relative fore- to hindbody girth axis. We suggest
that this macroevolutionary developmental change has potentially acted as
a key innovation in Hydrophis-Microcephalophis by facilitating novel
(especially burrowing) prey specialisations that are not shared with other
marine snakes.