Pterosaurs from the Cambridge Greensand

Paul Pursglove (UK)

There is a wonderful story about a trilobite found within the footprint of a carnivorous dinosaur. This apocryphal tale is probably true. Fossil rocks can be eroded and fossils like trilobites, can be re-deposited in softer sediments at later times, perhaps even during the time of the dinosaurs.

The Cambridge Greensand is a deposit with rich fossil bone content. Many of the finds are fragmentary. Often species, which did not live together or even at the same time, are found next to each other in this geological deposit. As a result, this sediment was puzzling for a long time, until work was done on the nature of the deposits.

The Cambridge Greensands are re-deposited sediments which come from the erosion of the Gault Clays, just after they were deposited. This amounts to a period of nearly a million years. Local uplifting of the sea floor exposed the Gault to coastal erosion. This resulted in the re-deposition of sediments across a shallow sea, to a depth of between 25m and 30m. The sediment is sandy and consists also of chalky marl, with phosphatic nodules and variable pebble content. The redeposition process is illustrated in Figs. 1 and 2.

Following this period of erosion, the shallow sea that remained was subject to silt and calcium carbonate deposits, forming the Chalk Marl. As the marly sediments decreased, this gave way to the deposition of the Chalk.

Any fossils contained within the Upper Gault would have been eroded locally and redistributed in the Cambridge Greensand. This would dissociate skeletons and spread the damaged fossil remains over large areas. It is this problem that creates difficulties for palaeontologists, when working with the Cambridge Greensand.

I researched fossil pterosaurs for many years and I have seen thousands of such fossils from the Cambridge Greensand. They have survived in this sediment when they have not been preserved in the Gault Clay. It is possible that many of the pterosaur remains would have been preserved within the Cambridge Greensand sediments as they were deposited. However, it is difficult to be precise about this, as the fossils have been moved in the sediment by tidal turbation and longshore drift. This damages the remains and wears the surface, so the preserved fossils are usually small bone fragments with loss of surface detail.

The strongest bones survive, which are usually teeth and jaw fragments, the big joints of the wings, vertebra and pelvis fragments. Most of the other bones are lost due to the movement and wear within the shifting sediments before fossilisation. The great thing about the Cambridge Greensand is that, where pterosaur bones are usually crushed, compressed or misshapen in most deposits, they are naturally shaped and undistorted in this deposit.

Virtually all of the large pterosaur remains found around the world have crushed wing bones, which makes reconstruction very difficult. The nature of the fossil pterosaur bones in the Cambridge Greensand has enabled people like Hankin and Watson (1914) to reconstruct the wing joints of large pterosaurs and derive the position of the bones in flight and at rest. Bramwell and Whitfield (1974) were later able to apply biological calculations to the reconstructions and estimate tissue mass and flying speed as well as performance related data.

It is clear that, in isolation, the Cambridge Greensand specimens are of little value. When they are compared with the specimens from the rest of the world, they become a scientific goldmine of information that has enabled significant progress to be made in the reconstruction of some of the largest pterosaurs known. This research has lead to the construction of life size flying models of pterosaurs, the first being the Pteranodon model flown by Cherrie Bramwell and R G Whitfield over the Dorset coast in the summer of 1984. This was followed by a radio controlled high tech model of Quetzalcoatlus designed by Paul MacCready and his team at AeroVironment and flown in California in 1985.

This development has been made with the insight on pterosaur joint anatomy from the Cambridge Greensand specimens, and it is now possible to see convincing models of pterosaurs flying over European and American skies.

About the author

Paul Pursglove is a retired teacher and examiner of Biology and Geology having over 40 years experience with pterosaur fossils. He originally set up the website: The Pterosaur Database.

References

Charwin C. P., 1977, British Regional Geology, East Anglia and Adjoining Areas, HMSO (Fourth Edition).

Hankin, E. H. & Watson, D. M. S. 1914, On the flight of pterodactyls. Aeronautical Journal, 18, 324–335. Bramwell, C. D. & Whitfield, G. R. 1974, Biomechanics of Pteranodon. Philosophical Transactions of the Royal Society, London, B.267, pp.503-581.