WWD: Coniophis precedens (Patreon)

Making a short appearance in the episode is an early snake (played by a modern day boa constrictor), its appearance has nothing to do with dinosaurs, but it's just to show that snakes were one of the newer groups of reptiles to appear on Earth (comparatively speaking). Indeed, early ophidians were beginning to experience their first true radiation and including some very large species by the Maastrichtian epoch, such as Madtsoia madagascariensis (seen in one episode of Prehistoric Planet), which could as large, if not larger, than the reticulated python, the longest living snake. 

Supplementary material identifies the snake as Dinilysia, but this makes even less sense than the Deinosuchus because it is about 20 million years too late and on the wrong continent. Two snake species are known from the Hell Creek Formation, Coniophis precedens and an unnamed boid species; although the boid species would be taxonomically close to the snake seen in the episode, there is hardly any information on beyond the fact it existed, so I chose the one which actually has a name, measurements, and papers describing it. Coniophis is a transitional stem-snake, with a lizard-like skull on a snake-like body, and reached maybe up to one metre in length.

It's been an ongoing debate whether the earliest snakes evolved their serpentine shape as an adaptation for swimming or burrowing, although current evidence is stronger for a burrowing ancestry. Coniophis was a likely burrower, but would have emerged periodically to hunt for small vertebrates, like mammals and lizards, on the surface. The cranial kinesis possessed by modern snakes that allows them to swallow large food items whole had just begun to appear, which likely contributed to their adaptive radiation during the Mid Cretaceous, as the ability to easily eat more types of prey opened up more ecological possibilities.