Five Questions about Plesiosaurs

This is the subject of a poster I presented at the SVPCA in Edinburgh in early September 1999. Please Email me with any answers or ideas, no matter how wild


Why do plesiosaurs have hollow vertebral centra?

Scans and sections show an apparent cavity in the middle of the centra.
Is this an actual cavity, or did it contain soft tissue?
Or is it an artefact of fossilisation?

Why do they have nutritive foramina in the vertebral centra?

The presence of nutritive foramina is used diagnostically to identify vertebral centra as being plesiosaurian. These formina are found on the underside of cervical and caudal, and on the sides of dorsal centra, and on the base of the neural canal. They are either tubes joining the neural canal to the underside of the spinal column, or openings into a space filled with some sort of specialised tissue within the body of the centrum. They occur on virtually all centra. What sort of metabolic system serves the entire body, and runs dorsally/ventrally rather than anterior/posterior?

How did they evolve single-headed ribs?

Most reptiles have double headed cervical ribs. The space between the heads of the ribs is an important route for parts of the circulatory system. During plesiosaur evolution, this route is gradually closed off. What happened to the vessels that ran along that route?

Can propodials grow from the middle outwards?

The text books describe the growth of propodials as the development of a shaft between epiphyses at each end. Plesiosaur propodials have an unusual internal structure. The endochondral bone forms a conical insert into the perichondral outer shaft, coming to a point roughly midway along the shaft where a large foramen marks the entry of a blood vessel to the middle of the bone. Is it possible for a bone to grow from the middle outwards?

Why do they have long necks?

Long necks are characteristic of plesiosaurs, yet it is hard to understand what advantage they offered. In the early part of the last century Conybeare remarked :
"its long neck must have impeded its progress through the water; presenting a striking contrast to the organisation which so admirably fits the Ichthyosaurus for cutting through the waves"
Yet there can be no doubt that this long neck was an extremely good adaptive device. Long-necked plesiosaurs were successful for 130 million years, and the length of the neck increased in that time. What were they doing?

A relevant quotation?