Fossils - Part 5: Echinoderms (Sea Lilies and Sea Urchins)

Crinoids (also called sea-lilies) and sea urchins (echinoids) are the most commonly found fossil echinoderms. Other examples of echinoderms today are starfish (asteroids) and brittle stars (ophiuroids).

Crinoids

Phlyum Echinodermata
Class Crinoidea

Crinoids evolved in the middle Cambrian Period around 525 million years ago and some species still live today.

Crinoids are also called sea-lilies because, due to their appearance, it was once believed that they were plants and not animals. They are marine animals living in shallow water.

The presence of crinoids (and echinoids) in a rock indicates that it was formed in a marine environment. It is rare to find a complete fossilised crinoid and the photo below shows a matrix (measuring 8.5 by 4.5 cms) containing stem and branched arm fragments.

Crinoid - Species: Scaphocrinites elegans from the Late Silurian Period (~400 million years old).



Its not possible from the above photo to get an idea of what the living animal looked like so below is a photo of a brief sketch I have done - don't laugh (drawing is not one of my talents!)




Crinoidal limestone, comprised of fossilized crinoid remains such as stems, is often polished and made into ornaments or jewellery.

In some areas of England the columnals from the stem were called "fairy money".

Sea Urchins

Phylum Echinodermata
Class Echinoidea

Sea Urchins evolved in the Late Ordovician Period (~450 million years ago) and species are still found in the present day.

They are entirely marine and, like crinoids, mainly inhabit shallow seas.

Sea urchins have a hard test (shell) composed of calcite plates often in a 5-rayed arrangement. Tube feet used for feeding, respiration and movement, project through small pores in the plates. Some species have spines which tend to fall off after the organism has died and the test and spines are composed of calcite. Spines were used as protection against predators and to aid movement across the sea floor.

A Cretaceous Echinoid - Cidaris sp.



Another Sea Urchin from the Cretaceous Period ((146 - 65 million years ago)
Toxaster sp.



Heliophoza



Some sea urchins dating from the Cretaceous Period were known as "Shepherds Crowns" in the past. The 5 rays meeting at the top look like the ribs of a crown.
Weathering of rocks on the downs of South England would have exposed these fossils which may well have been picked up by shepherds.

Reference

Fossil Detectives Field Guide, BBC and Open University
Discovering Geology, Fossil Focus, Crinoids and Echinoids, British Geological Survey