The Plesiosaurians were possibly the most successful clade of Mesozoic Marine Reptiles, first appearing in the Late Triassic, and surviving till the End Cretaceous Extinction. Their basic bauplan was simple, with a rigid body and four paddle-like limbs used for propulsion, although within this framework they produced considerable diversity, with distinctive groups such as the Thalassophonean Pliosaurs and Cryptoclidian Plesiosaurs appearing in the Middle Jurassic and the Leptocleidids and Elasmosaurids appearing around the End of the Jurassic.
The middle Cretaceous was a period of about 14 million years during which there was significant turnover in many groups of organisms. It was during this period that the Ichthyosaurs disappeared and the Mosasaurs first appeared and diversified. Within the Plesiosaurians, the Pliosaurs vanished during this interval, while the Euelasmosaurida underwent a significant evolutionary radiation.
The Cambridge Greensand forms the lowest member of the West Melbury Marly Chalk Formation. This Albian–early Cenomanian bed comprises micaceous, glauconitic, silt marl with a basal lag of reworked phosphatic nodules usually associated with Vertebrate fossils and exotic clasts, often encrusted in small Oysters and other epibionts. The Cambridge Greensand contains one of England's richest Vertebrate fossil assemblages, and dates to the crucial middle Cretaceous period of high biotic turnover. However, many of the skeletons recovered from this deposit are fragmentary in nature, and some may have been reworked from the underlying Gault Formation. Furthermore, most of the sites from which they were recovered are not exposed on the surface; these sites were uncovered by phosphate miners in the nineteenth century, and have largely been covered over again.
In a paper published in the journal Acta Palaeontologica Polonica on 30 October 2025, Jose O'Gorman of the División Paleontología Vertebrados at the Museo de La Plata, and Roger Benson of the American Museum of Natural History and the Department of Earth Sciences at the University of Oxford, redescribe CAMSM X50356, a partial Elasmosaurid Plesiosaur skeleton in the collection of the Sedgwick Museum in Cambridge.
CAMSM X50356 was excavated near the village of Fen Ditton in Cambridgeshire. It is a disarticulated, partial skeleton, comprising 37 cervical centra, 3 pectoral centra, 20 dorsal centra, 5 sacral centra, 3 caudal centra, isolated neural arches, fragmentary ribs, part of a scapula, one almost complete propodial, and several propodial fragments. A basioccipital bone included with the original collection is rejected by O'Gorman and Benson as clearly belonging to an Platypterygiine Ichthyosaur.
The skeleton if thought to have come from an immature Animal, as the neural arches of the vertebrae have detached from the centra, a sign of incomplete ossification. Many of the elements show signs of abrasion, and some of encrustation by Oysters, suggesting that the remains were exposed on the seabed for some time before being buried.
CAMSM X50356 has 37 preserved cervical vertebrae, which can be identified as such by the presence of a ventrolateral parapophysys (bony ridge on the underside). This sequence is incomplete, with the atlas and axis (first two vertebrae, which form a joint with the skull and are slightly modified for this purpose), and potentially other vertebrae missing, giving a minimum count of 39. Furthermore, there are twenty preserved dorsal vertebrae, defined by the absence of parapophysis, which again is the minimum number, suggesting that CAMSM X50356 was long, even for an Elasmosaur.
A phylogenetic analysis recovered CAMSM X50356 as the basalmost known Elasmosaurid, not corresponding to any other described member of the group. It is also the only Elasmosaurid with cervical central longer than high but without lateral ridges, suggesting that this was the basal condition in the group, even if absent from all other members.
The Sedgwick Museum collection also contains several Elasmosaur specimens from the Cambridge Greensand listed under the names Plesiosaurus euryspondilus and Plesiosaurus euryspondilus, species for which no known formal description exists. These specimens were collected by the palaeontologist Hary Seely in the 1860s, who noted that 'These names are only intended for the convenience of students using the Museum, and not necessarily to take rank as names of described species'. O'Gorman and Benson examined several of these specimens, and could not find any features which could be used to distinguish them from CAMSM X50356, however, they do not consider that either that specimen, nor any of Seely's material, show sufficient diagnostic features to be formally described as a species.
See also...
