The Lower Jurassic rocks exposed in the sea cliffs around Lyme Regis, on the Dorset coast of southern England, are justly renowned for the wealth of fossils they have yielded to collectors over nearly two centuries. Although it is the vertebrates for which the site is best known, including some of the earliest discoveries of ichthyosaurs, plesiosaurs and pterosaurs, it is the invertebrates that account for the great majority of fossils. Impressive and beautiful though some of the ammonites and other fossils are, none of these invertebrates, or indeed any of the vertebrates, can match the beauty of what is perhaps the best-known fossil species found here, the crinoid Pentacrinites fossilis. The preservation of some specimens is extraordinary. Not only do they occur as large tangled groups with virtually every ossicle intact, but commonly they are coated with a thin film of pyrite, giving the impression that the whole fossil has been cast in bronze or gold. Some specimens have been justly claimed to be among the most beautiful fossils ever found (Fig. 189).

How have they come to be preserved in this way? The answer is an equally remarkable story that has been a source of discussion for more than 150 years.

ANCIENT DORSET

Nearly all specimens of Pentacrinites fossilis from the Dorset coast have been recovered from a 2-m interval within the obtusum Zone of the Lower Jurassic (Sinemurian) Black Ven Marls (approximately 190 million years old). Early accounts considered the crinoids to be confined to a single impersistent band, the ‘Pentacrinite Bed’, but it is now known that the thin crinoid lenses may occur at any level within this 2-m thickness and occasionally outside it.