The Speeton Plesiosaur

The substantially complete skeleton of a plesiosaur was found by an amateur collector, Nigel Armstrong, in 2001. He recognised the importance of the find and alerted the local museums who organised a scientific excavation led by Will Watts, Dinosaur Coast Officer and Phil Manning of the Yorkshire Museum.

The site is on a bleak and exposed cliff overlooking the North Sea, and exposed to the winds howling in from Siberia. I spent a couple of days there helping with the dig and have never been so cold in my life! The specimen in well-preserved in 3-D, and looks as if the body has been bundled up in a sack and dropped on the bottom of the sea. The area which includes the main body is a complex of overlapping bones about half a metre deep. Phil decided to take the risky but from a scientific point of view necessary decision to extract all of this main portion of the skeleton in a single large plaster jacket. The alternatives of removing bones individually or breaking up the mass and jacketing smaller blocks would have destroyed a lot of valuable information. With a find of this type the scientific interest lies not only in the bones themselves, but also in the other fossils - ammonites and other invertebrates, fish bones, coprolites and so on that are in the clay that surrounds them as well as the clay itself. The presence of a large carcase leaves a geochemical residue of organic material, and the study of who this varies around the specimen is an avenue of investigation that may lead to valuable insights into how the process of fossilisation occurs.

The skeleton was removed October 2002 in a huge block, weighing in the order of a ton and a half, and a number of smaller blocks containing the neck and other elements which lay outside the main block. It has been prepared by Mike Marshall in his workshop in Whitby, and has been put on display in the newly refurbished Rotunda Museum in Scarborough.

Our initial impression of the animal was that it was an elasmosaur, a family of long-necked plesiosaurs known mainly from North America from deposits of the Upper Cretaceous. Now that the bones are free of matrix, it looks rather more complicated than that. This find sits neatly in the middle of a large gap in our knowledge of the evolution of the long-necked plesiosaurs.

The evolution of long-necked plesiosaurs is turning out to be more complex than we thought. We now think that the trend to the extreme elongation of the neck occured in at least two, and probably three pleisoaur lineages. This is very puzzling when you think of the shapes of animals which live in water, especially active hunters such as plesiosaurs.

Animals which move quickly through water tend to have what we call a tuniform body shape. If you look at most fishes, whales and seals, they have a streamlined shape, thickest about a third of the way from the head to the tail. Plesiosaurs, on the other hand are sticking an enormously long neck ahead of their body, something which has to be a great hindrance to moving easily through water. We don't know how plesiosaurs hunted, but whatever they were doing they were doing very well. Not only were they successful predators for 150 million years, but the way in which they used their long necks was so effective that it evolved independently in several lineages.

It is too early to draw any firm conclusions about the evolutionary relationships of this specimen, but first impressions are that it is related to a genus from New Zealand called Mauisaurus, and Muraenosaurus from the Oxford Clay of eastern England. This is a rather contentious idea, as recent work on the relationships of plesiosaurs suggests that they are on different branches of the plesiosaur family tree. An implication is that the family we call the elasmosaurs is in fact two separate lineages, and that the shared characters which define the family are the result of evolutionary convergence.

Having said that, there are some rather unusual features of the skeleton which may complicate matters further. The upper limb bones - the propodials - are remarkably small relative to the size of the body. The lower limb bones, the epipodials, are rather elongated which is not a character which we would expect to find in a plesiosaur of this age. The head and the front of the neck are missing, but judging from the vertebrae which are preserved it looks as if the animal had a very long neck with 50 or more vertebrae.

This is still early days. Although the specimen is now on display, there is a lot of work still to be done. The bones need to be recorded, photographed, drawn and measured so that we can compare them with those of other plesiosaurs. We need to track down any other plesiosaur material which has been found in the Speeton Clay to see if this is an isolated specimen or if there more of the same kind have been found. When all this information is gathered, there is the matter of writing it up as a paper which can be submitted to a scientific journal.

So watch this space for news.