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

<p><span>Sea snakes in the <em>Hydrophis</em>-<em>Microcephalophis</em> 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 <em>Hydrophis</em>-<em>Microcephalophis</em> 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 <em>Hydrophis</em>-<em>Microcephalophis</em> 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 <em>Hydrophis</em>-<em>Microcephalophis</em> by facilitating novel (especially burrowing) prey specialisations that are not shared with other marine snakes.</span></p>