DALE DRINNON: On "Discosaurus" and the possibility of Plesiosaurian Survival

Joseph Leidy had written several articles about the earliest finds of Plesiosaurs in North America, and one of them was the disputed 'Discosaurus' in Alabama, possibly originating from the same beds as 'Zueglodons' (Basilosaurus). He was writing in the 1850s and one of the comparable early finds was from the Greensands of New Jersey, thought to have been of Late Cretaceous age. The specimens in this case were named 'Cimoliasaurus'; however, some of them turned out to be cetacean vertebrae of probably Pliocene age, probably some sort of a dolphin.

However, this was some of the vertebrae and not all: Leidy did think the other vertebrae were legitimate and were of that genus, and probably related to that

'Discosaurus' http://www.oceansofkansas.com/Leidy1865.html

However, it seems that both genus names are invalid. 'Cimoliasaurus' has been described as a 'garbage taxon' and several nondescript fossils from Europe and Australia have also been ascribed to this genus, much in the same way as the early tendency to call all early canivorous dinosaur finds.

'Megalosaurus' http://www.palaeos.com/Vertebrates/Units/220Lepidosauromorpha/220.820.html
In this case the really interesting thing is that the New Jersey fragmentary Plesiosaur is found in association with Pliocene dolphin fossils, mixed up together and only separated out later, and the Alabama fossils Leidy considered probably the same genus are labelled as coming from the Eocene zueglodon beds. In the case of the New Jersey Greensands, there is independant evidence that they are not only Cretaceous but also Tertiary: another site gives a paper in which several genera of O. C. Marsh's 'Cretaceous' birds from the New Jersey Greensands are actually of Eocene date or later. http://elibrary.unm.edu/sora/Auk/v084n02/p0260-p0262.html

The characteristics of these fossils has placed them tenatively in the same family as Cryptocleidus and Muraenosaurus, and they were thought to have been like the Elasmosaurs but with shorter necks. This is also along the lines of what the surviving Plesiosaurs would have to have been to give rise to our Long-necked Sea-serpents: long-necked, but not excessively long-necked, not so specialised as the extreme Elasmosaurs, and generalised enough to be versatile, possibly enough so that they could pursue other avenues of evolution that became open to them.

That makes a good deal of sense and I am willing to arrange the theory of Plesiosaurian survival on those terms alone.