Sea-serpent sightings were popular subjects of nineteenth century fictional tales. One of the most famous sightings, the 1817 appearance in the harbor of Gloucester (Massachusetts), generated a report published by the Linnean Society of New England. In 1869, ED Cope introduced a new reptilian order, Pythonomorpha, comprising large Upper Cretaceous marine lizards (mosasaurs) that he thought rather well captured in historical depictions of sea-serpents. The name Pythonomorpha emphasized the many striking features that Cope found mosasaurs to share with snakes. Cope’s Pythonomorpha was resurrected in the late 1990s, as a clade including mosasaurs plus snakes. This was supported by the placement of mid-Cretaceous marine snakes with well-developed hindlimbs as evolutionarily intermediate between mosasauroids and snakes. Critics pointed to features indicating that those fossil snakes are instead evolutionarily advanced, which would imply that hindlimbs of these fossil snakes re-developed from rudiments such as occur in pythons. Recent molecular developmental studies confirmed that the embryonic limb bud of the python hindlimb conserves the genetic program to generate a complete limb.