Bathyspondylus is an extinct genus of plesiosaur. It was first described in 1982 from a specimen originally found in 1774, now housed at the Devizes Museum. Because it is known only from its fossil vertebrae (and so few of those have been recovered), paleontologists are not entirely sure of the taxonomy of Bathyspondylus; the family it belongs to is not currently known.
The type, and only known, species of Bathyspondylus is Bathyspondylus swindoniensis, which was described from the same material as its genus.
The genus name Bathyspondylus is a compound of two Greek roots: βαθυς (bathys) 'deep' and σπονδυλος (spondylos) 'vertebra'. It can thus be translated as "deep-vertebrae".
The species name B. swindoniensis refers to the town of Swindon in Wiltshire, near which the holotype specimen was discovered.
Bathyspondylus had centra set fairly deep in the vertebrae relative to its length, as its name (the Greek words for deep-vertebrae) would suggest. The vertebrae themselves are short antero-posteriorly and can be flat or concave on their terminal faces. The holotype specimen, from 1774, appears to have features of both pliosauroids and plesiosauroids incorporated into its bones.
Bathyspondylus lived during the Kimmeridgian faunal stage of the Jurassic period, which occurred roughly 155 to 150 million years ago. The first of its fossils came from deposits near Swindon in Wiltshire, England.