Echinoids

BGS fossils and geological time

A modern sea-urchin.
Information icon

A modern sea-urchin. ©Public domain. NOAA OKEANOS Explorer Program , 2013 Northeast U. S. Canyons Expedition.

Expand icon

Echinoids have lived in the seas since the Late Ordovician, about 450 million years ago, which is about 220 million years before dinosaurs appeared. The remains and traces of these animals were buried in sediment that later hardened into rock, preserving them as fossils. The living representatives of echinoids are the familiar sea-urchins that inhabit many shallow coastal waters of the world. Fossil echinoids closely resemble some living sea-urchins which helps us to understand how they must have lived.

The animal

Simplified cross section through a living echinoid. The tube feet protrude through the pores which are concentrated into narrow bands radiating across the test. BGS ©UKRI. All rights reserved.
Information icon

Simplified cross section through a living echinoid. The tube feet protrude through the pores which are concentrated into narrow bands radiating across the test. BGS ©UKRI. All rights reserved.

Echinoids are marine animals belonging to the Phylum Echinodermata and the Class Echinoidea. They have a hard shell ( referred to as a test) covered with small knobs (tubercles) to which spines are attached in living echinoids. The test and spines are the parts normally found as fossils.

Tylocidaris clavigena, a fossil echinoid. With club-like spines attached.
Information icon

Tylocidaris clavigena, a fossil echinoid. With club-like spines attached. BGS ©UKRI. All rights reserved.

Expand icon

Echinoid tests have a variety of shapes; they can be globular or flattened, rounded or heart-shaped. The spines, held in place by soft tissue covering the test during life, usually became detached and fossilized separately. Occasionally, when fossilization was rapid, the spines and test are found preserved together. The most important function of the test was to support and protect the soft body inside.

The test and spines are made of the mineral calcite. The test contains hundreds of calcite plates, loosely held together in Palaeozoic species, but rigidly fused together in most species since the Mesozoic. The plates are arranged so that the test appears to consist of wedge-shaped segments, usually separated by narrow bands of tiny holes (pores) that radiate out across the test.

Hemicidaris from the Jurassic, with elongate spines.
Information icon

Hemicidaris from the Jurassic, with elongate spines. BGS ©UKRI. All rights reserved.

Calcite also forms the powerful downward pointing teeth found in some types of living and fossil echinoids. Concentrations of magnesium in the tips of the teeth add extra strength for scraping food items from rock surfaces.

Spines, some poison-tipped, help protect echinoids from their many predators, which include other echinoids, crustaceans, octopuses and fish. Some fossil echinoids made themselves less palatable as prey by having large solid spines. Echinoids also use their spines for moving around the sea-bed, and in some groups they are specially adapted for burrowing.

Echinoids and their enemies : A seascape showing the kinds of environments inhabited by modern and fossil echinoids. Burrowing sea-urchins (1 & 2) conceal themselves from predators, while non-burrowing types (3 & 4) cling to rocky surfaces and have defensive spines to protect against fish, lobster, starfish and octopus. ©Richard Bell.
Information icon

Echinoids and their enemies: A seascape showing the kinds of environments inhabited by modern and fossil echinoids. Burrowing sea-urchins (1 & 2) conceal themselves from predators, while non-burrowing types (3 & 4) cling to rocky surfaces and have defensive spines to protect against fish, lobster, starfish and octopus. ©Richard Bell.

In the Mesozoic, echinoids evolved into a variety of shapes adapted to burrowing beneath the sea-bed. Concealed from predators, they used numerous bristle-like spines for ploughing through the sediment rather than for protection, and some probably resembled the modern day Echinocardium.

Some echinoids wandered the sea-bed scavenging on a wide variety of food, including algae, plants and encrusting organisms. Suckered tentacle-like organs (tube feet) extended through the pores in the test, their powerful grip preventing the echinoid from being overturned in rough water, and enabling it to climb rock faces as well as assisting in feeding and respiration.

Food particles were extracted from the sediment swallowed by burrowing echinoids, and in some types holes and frills in the test and grooves around the mouth helped to maximise food-gathering ability.

The geologists’ tool

Fossil echinoids are a reliable indication that the rocks containing them were formed in a marine environment. However, they can also be a useful guide to the age of the rocks in which they occur. Their remains are sometimes more common and better preserved than other types of fossils normally used for biostratigraphy. This is the case in the strata of Late Cretaceous age, known as the Chalk Group, which form the famous White Cliffs of Dover.

Pioneering work by geologist A W Rowe (1858–1926) and R M Brydone (1873–1943) in southern England showed that different types of echinoid could be used to divide parts of the Chalk into Biozones and Sub-biozones of different ages. This is particularly important for the purposes of correlation in the Chalk, which superficially has a very uniform appearance.

Palaeozoic echinoids are less useful to geologists because they tend to be rarer; their primitive tests often quickly disintegrated before fossilisation could occur.

Temnocidaris (Stereocidaris) sceptrifera sceptrifera (Mantell, 1822), a long-ranging Upper Cretaceous echinoid.
Information icon

Temnocidaris (Stereocidaris) sceptrifera sceptrifera (Mantell, 1822), a long-ranging Upper Cretaceous echinoid. BGS ©UKRI. All rights reserved.

Expand icon

Echinoids through time

Echinoids are abundant today and fossil echinoids are common in rocks of Jurassic and Cretaceous age, especially the Late Cretaceous Chalk. Examples of echinoids through time.

Myths and legends

Early drawing of a fossil echinoid. ©Natural History Museum.
Information icon

Early drawing of a fossil echinoid. ©Natural History Museum.

The Echinodermata take their name from the Greek words for spiny skin, a very conspicuous feature of many living echinoids. Since ancient times they have been revered as objects of religious or superstitious power.

3D fossil models

Heterocidaris wickense. (SM J 6106 – Undifferentiated Type). Bathonian Age (Jurassic Period) – Aalenian Age (Jurassic Period) (166.1 – 174.1 Ma B.P.) See 3D fossils online.
Information icon

Heterocidaris wickense Wright. (SM J 6106 – Undifferentiated Type). Bathonian Age (Jurassic Period) – Aalenian Age (Jurassic Period) (166.1 – 174.1 Ma B.P.) See 3D fossils online. BGS ©UKRI. All rights Reserved.

Expand icon

Many of the fossils in the BGS palaeontology collections are available to view and download as 3D models. To view this fossil, or others like it, in 3D visit GB3D Type Fossils.

Reference

Woods, M A. 1999. Echinoids: fossil focus. Nottingham, British Geological Survey.

Need more information?

Please contact the BGS enquiries team

You may also be interested in:

Ammonite p521007

Fossils

What is a fossil and why do we study fossils? This section explains the different methods of fossil preservation and links to a set of detailed pages that describe 14 of the most common fossil types, including ammonites, belemnites, bivalves and trilobites.

Show more

Was this page helpful?

  • How can we make this section better?*

  • Please select a reason*

  • How can we make this section better?*