Crinoids

WHEN WE were walking between Hope and Castleton in the Peak District on Wednesday, we came across crinoid fossils in some of the capstones of the drystone wall as we crossed a stile. Crinoids are also known as sea-lilies although they’re invertebrate animals, relatives of sea urchins and starfish. They’ve been called ‘starfish on stems’; creatures that spread their arms to catch food particles.

They’re usually found where a current wafts across a reef. At the time these fossils formed what is now the Peak District lay close to the equator.

This comic strip version of how fossils form is from my 1991 geology A-level notes:

Graptolites

500 MILLION YEARS AGO: in the world’s oceans, graptolites, colonial creatures, float with the currents. They are too small for their accumulated remains to build up into sediments on the sea floor but, given enough time, they can leave a thin layer in the fossil record. This can happen only where sedimentation is slow, for instance hundreds of miles away from land.

Their fossils look like calligraphy, hence their name, taken from graptos, the Greek for ‘marked with letters’.

Life on the Reef

330 MILLION YEARS AGO Castleton: a reef built up by algae separates a calm shallow sea to the south from a sea at least 150 metres deep to the north. In contrast to the graptolites, which are heading for extinction at this time, some organisms are so successful that they form sedimentary rocks. These include in particular; crinoids, corals, bivalves and brachiopods.

In my A-level notes I’ve written; ‘Allochemical : stays in place’ but this is misleading: allochemicals are chemicals that come from elsewhere. ‘Allo’ is from the Greek for ‘other’ or ‘different’. In the case of the reefs at Castleton ‘allochemical’ might refer to the chalky ‘cement’ that holds the shell fragments together. The cement-like matrix may have come from elsewhere but shelly sediment itself is autochthonous, meaning that it is ‘not transported’ that it formed ‘in place’.

Stone-lilies

‘. . . we find masses of limestone wholly composed of the stems of the stone-lilies, as they are rather poetically called. This stone is often seen polished in mantelpieces ; when exposed to the air, the softer parts, which are immediately between the stems, weather away, leaving the surface of the rock as if the fossils had been stuck on to it, as in the illustration of encrinitial limestone (Fig. 75). The heads of the animals which the stem supported were a corona of tentacules, with a mouth at the centre. These arm-like feelers waved about in the waters, bringing into their mouth the prey which they enclosed. As specimens, we give the Cyathocrinites planus (Fig. 76) and the Woodcrinus (Fig. 77).’