The Ulster Museum - Object Of The Month

Ammonite slab

February

Snapshot of an ancient sea floor

This amazing natural accumulation of fossils represents a small area, 'frozen in time', of an almost 200 million year old sea floor. It was found in Dorset in southern England. The spirally coiled objects are the fossil shells of four different species of ammonite, extinct relatives of the modern squid and octopus. Most are simple ribbed shells, called Promicroceras, each about a cm or two across. A few larger shells, called Asteroceras, are easy to recognise because they have a distinct ridge running around the edge of the shell. The very large shell is of a type called Xipheroceras, and there are also a few tiny, fat, smooth ammonites called Cymbites.

Scattered among these ammonite shells are several pieces of fossil wood, ancient driftwood that became waterlogged and sank to the sea floor. The empty shells of dead ammonites were drifted along the sea floor by gentle currents, coming to rest against this ancient logjam like a snowdrift against a hedge.

Normally the weight of mud that slowly buried these remains would have soon crushed the hollow shells, but these are all beautifully preserved in 3-dimensions. As the shells were buried, they themselves acted as a focus for mineralising fluids to form a hard limestone concretion around the fossils while, at the same time, filling the shells with a mineral called calcite. This happened before the mud had accumulated deeply enough to crush the shells.

When this piece of ancient sea floor finally emerged from the Dorset cliffs, all that could be seen were a few pieces of fossil wood and the edges of some ammonites protruding from a hard limestone concretion. It took Andy Cowap, an expert fossil preparator, more than 300 hours of painstaking work, often with just a single needle, to pick away the limestone and reveal this remarkable fossil graveyard.

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