Fossils always have a story to tell. Here a number from the world-class collection of Oxford University Museum. Click on the photographs to see a higher-resolution image.

Phylum: Echinodermata Class: Asteroidea Species: Helianthaster rhenanus Most starfish have only 5 arms – this species has 16. Apparently it was short-lived, for it occurs only in the world-famous Hunsrück Slate at Bundenbach, Germany (not far from the river Rhine, whence the species name ‘rhenanus’). Mud was deposited in an offshore basin and subsequently baked into fissile slate when the ancient continents Euroamerica and Gondwana collided in the Variscan Orogeny. The formation is Early Devonian in age and has a total thickness of 3,750 metres.

In the earliest stages of the collision, sediment-laden turbidity currents buried communities on the muddy sea-floor under beds a metre or more thick. Arthropod trails and crinoids with rooting structures demonstrate that the communities were living where they were fossilised – not buried by some global flood that began tens of kilometres lower in the stratigraphic column! Conditions for preservations were exceptional – not only periodically rapid sedimentation but also the right sediment chemistry and bacteria to convert the organic matter into iron sulphide (pyrite). Occasionally even soft tissue, such as eyes, intestines and tentacles, were preserved. The 15-cm body of this unusual starfish is a fine example.

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Phylum: Echinodermata Class: Crinoidea Species: Pentacrinites fossilis Although stemmed crinoids are often called ‘sea lilies’, they are actually animals. Some can even walk! Similarities in fundamental structure show that they are related (amazingly) to starfish, brittlestars, sea urchins and sea cucumbers. In addition to these 5 extant classes within the phylum, 15 others went extinct, all in the Palaeozoic – so there was far more diversity in the past than today. Nine of the classes had evolved by the Cambrian, when echinoderms first appear.

The tangled group above comes from a 2-metre interval within the Lower Jurassic Black Ven Marls around Lyme Regis. The unoxidised organic matter and unburrowed laminae of this interval show that the sea bottom was anoxic. Unlike most crinoids, however, Pentacrinites colonised floating logs, not the sea floor, and the creatures are therefore termed ‘pseudoplanktonic’. The driftwood was also sometimes fossilised, with the crinoids still attached, but only on the part that was under water. Tiny white discs on the upper surface mark where larvae tried to get a foothold as the rest became submerged. In a matter of days the log sank completely, into the anoxic zone which brought the lives of the colony to an end. The same deadly conditions preserved them as fossils.

Pseudoplanktonic crinoids are among the largest crinoids known. The first to evolve such a life style were the genus Traumatocrinus, in the Triassic. Their stems grew to a length of over 11 metres. Similar lengths were reached by the closely related Seirocrinus, whose swirling arms grace what is arguably the most spectacular of all fossils, the 15-metre coalified log in the Urmuseum Hauff, at Holzmaden.

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Phylum: Echinodermata Class: Echinoidea Species: Temnocidaris sceptifera Sea urchins, or echinoids, first appeared in the Ordovician. While their origins are obscure, they seem to have diverged from a long-since extinct echinoderm class called ophiocistioids. The sea urchin illustrated here is from much later, when most of Britain was submerged in warm shallow seas and chalk sediments were accumulating.

Sea urchins live either on the sea floor or within the sediment itself. Those that live on top of the sediment feed by scraping and grazing – usually on algae, though some also eat a range of invertebrate animals. Their skeletons, known as tests, are covered by protective spines which articulate with tubercles (visible here in the centre of the hexagonal plates), and muscles attached to the spines enable the animals to move. Sea floors may be very muddy, and long spines are adaptations to this kind of environment. Within hours of death the spines drop off. Thus, when they are fossilised alongside the skeleton, it is clear that the organism must have undergone only minimal transport. By the same token, sedimentation rates were fast enough for the hard parts of the animal to be buried almost intact.

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