Explaining a lack of evolutionary change: why is the ‘plesiosaur-like’ morphotype restricted to a single lineage of sea snakes?

Abstract A common enigma is that some traits do not evolve when we expect them to. For example, a plesiosaur-like body shape (tiny heads, slender forebodies, and bulky hindbodies) has evolved seven times in the Hydrophis group of sea snakes but not in any other snakes. It enables access to prey in narrow burrows, but why has a similar morphology not evolved in other burrow-foraging snakes? Possible explanations include: (i) lack of selective advantage, (ii) unique developmental mechanisms, and (iii) features that preclude ‘normal’ burrow-foraging because of physical constraints on fecundity. We evaluated these hypotheses from published literature and measurements of museum specimens. By reducing friction in mucus-lined burrows, the lack of enlarged ventral scales in Hydrophis-group species may necessitate a powerful hindbody to provide leverage, especially given buoyancy under water. Males as well as females exhibit plesiosaur-like morphology, inconsistent with fecundity selection as an explanation for heavyset hindbodies. We suggest that novel challenges in the ocean impose a fitness benefit to large overall body size via lower vulnerability to predation, increased lung capacity for diving, and more effective swimming. The only ways for a large snake to enter a burrow are either enlarged ventral scales that provide traction, or a plesiosaur-like morphology.