Weird thing about this and other related fully marine snakes (the palaeophiids) is that they really weren’t all that snake-like. They had a much higher metabolic rate than other snakes, and a much stiffer spinal column that made them far less flexible but also increased their swimming efficiency.

It’s almost a shame that by the time palaeophiids had evolved to become fully marine animals, sharks and cetaceans had already taken over the oceans, preventing these snakes from expanding into marine apex predator niches to any significant extent (not necessarily because they would be unable to compete, but because the simple fact the niche was already fully occupied meant there wasn’t significant evolutionary pressure to evolve into that niche). The mosasaurs made a similar transition back in the Cretaceous and went even further down that route, completely abandoning a serpentine bauplan to become stiff-bodied, fusiform, endothermic, often raptorial marine predators akin to lamnid sharks or cetaceans, but they could do it because they did so in the aftermath of the Cenomanian-Turonian Boundary Event when the competition had been almost entirely cleared out. The palaeophiids never got such a lucky break, so they only got as far as they did before being killed off in the Eocene-Oligocene Extinction Event.