GEOLOGY | FOSSILS | ECHINOIDS
What are ECHINOIDS?
Echinoids are marine organisms that have existed since the close of the Ordovician, circa 450 million years ago. They are members of the phylum Echinodermata, which includes crinoids, starfish, sea cucumbers and sea urchins.
The remains and traces of these animals were buried in sediment which over thousands of years became 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.
Image: This is a fossil echinoid of the species Macraster denisonensis from the Cretaceous deposits of the Duck Creek Formation in Tarrant County, Texas, USA. This specimen is 4.6 cm wide.
Five-fold symmetry means that the test exhibits five planes of symmetry, each giving a mirror image. The pattern formed is radial.
Bilateral symmetry means there is only one plane of symmetry on the test as in irregular echinoids eg. Clypeus plotii.
The endoskeleton of the echinoid is known as a test.
The water vascular system is the hydraulic system that enables the organism to extend its tube feet by forcing surrounding water into its tissues. The volume of water in the tissues is moderated by the madreporite.
Tube feet are the soft tissue appendages, which extend out through pairs pores in the test.
Most members of the Echinodermata exhibit five-fold symmetry.
Irregular echinoids have bilateral symmetry.
The organism
Echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea. They have an endoskeleton, known 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. There are two main types of echinoids: regular echinoids show five-fold symmetry and irregular echinoids show bilateral symmetry. The latter evolved from the former.
Echinoid tests have a variety of shapes; they can be globular or flattened, rounded or heart-shaped. The most important function of the test was to support and protect the soft body inside. The spines, held in place by soft tissue covering the test during life, usually became detached and fossilised separately. Occasionally, when fossilisation was rapid, the spines and test are found preserved together.
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.
Image: Tylocidaris clavigena, a fossil echinoid, with club-like spines attached. BGS © UKRI.
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 seabed and, in some groups, they are specially adapted for burrowing.
Image: Hemicidaris from the Jurassic, with elongate spines. BGS © UKRI
REGULAR Echinoid anatomy
"In reality the mouth-apparatus of the urchin is continuous from one end to the other, but to outward appearance it is not so, but looks like a horn lantern with the panes of horn left out." Aristotle (367–347 BCE)
THe ENDOskeleton
The endoskeleton of the echinoid is known as a test and is hemispherical in shape. The test is constructed from individual plates of calcite. Each plate is formed from one, single crystal of calcite. There are two types of plates; ambulacra and interambulacra. Ambulacran plates are relatively narrow and bare pore pairs, whereas interambulacran plates are wider and may have tubercles.
Ambulacral zones consist of two adjacent rows of plates that radiate from the apex of the animal, these alternate with interambulacral zones.
The pore pairs found on the ambulacran plates would have been the location of the tube feet. The tubercles found on some interambulacran plates would have been the site where the spines would attach.
Image: Goniopygus agassiz Upper Jurassic (Tithonian) to Palaeocene
THE
MOUTH
In regular echinoids the mouth is in the centre of the lower (adoral) surface and is surrounded by a membrane called the peristome. The edges of the test, surrounding the peristome are turned inwards to create the perignathic girdle, which acts as an anchor for the muscles that support and control the lantern (chewing mechanism).
The lantern consists of five jaws, each with a sharp single tooth, capable of scraping algae and other food from the sea floor substrate, passing it inwards to the gut. The mouth is on the underside to enable feeding.
Image: The lantern of a living sea urchin
APICAL
Disc
This is located at the centre of the aboral (upper surface of the test), enabling waste material, eggs and sperm to be released into, and carried away by, the surrounding water. The apical disc comprises ten plates arranged in two rings surrounding the anus. Of the five genital plates, the largest is the madreporite. The madreporite has many tiny pores that enable the ingress of water, feeding the internal water vascular system. The five remaining smaller ocular plates sit between the genital plates. The anus is surrounded by a membrane known as the periproct.
Tube feet
The external expression of the water vascular system of an echinoid are the tube feet wihich protrude through the poor pairs in the ambulacral zones of the test. Water that has entered the echinoid through the madreporite supplies the water vascular system, eventually passing into the tube feet.
The tube feet enable locomotion and provide secure anchorage to sub-marine substrates.
The important exchange of oxygen and carbon dioxide occurs across the membranes of the tube feet.
Spine Attachment
Tubercles can be found on the interambulacral plates. Turbcles have two main parts: the wide-based boss and the central nipple-like structure known as the mamelon. Muscle attachments between the spine and the tubercle provide anchorage and co-ordinated movement of the spine, making them useful in locomotion and defence.
The largest spines were attached to the largest tubercles. The size of tubercles reduces towards the oral region of the body, so that spines do not inhibit access to the sea-floor substrate.
After death the soft tissues of the test and tubercle decay and the spines become disarticulated from the body. This is why fossilised echinoid spines are often found littering sedimentary rocks.
Diagram: Echinoid spine attachment
REGULAR ECHINOID
ADAPTATIONS
Regular echinoids live in high energy near-shore environments, usually on rocky substrate. Regular echinoids have therefore adapted to their environment by developing robust tests and spines.
They graze on algae that live on the rocky sea floor.
They cling to the rocks using tube feet and rotate their spines to enable locomotion.
Spines are also used for protection.
Irregular echinoid
ADAPTATIONS
Irregular echinoids live in low-energy environments, in shallow or deep burrows in soft sediments. The burrowing nature of irregular echinoids means that they have evolved adapted body forms: they are typically heart-shaped with bilateral symmetry; the petaloid amulacra became longer to enable efficient gas exchange and their bodies are often wedge-shaped so that they can move through sediments with ease. They are filter feeders and therefore they have a smaller mouth, with no jaw.
ECHINOID MODES OF LIFE
IRREGULar Echinoids: Mouth
Although the mouth parts are still located on the ventral surface of the test, it has migrated towards the anterior sulcus. The complex lantern jaw and perignathic girdle are absent. A labrum, which projects over the lower part of the mouth has evolved to protect the mouth from unwanted sediment. A modified set of interambulacral plates form the plastron. The plastron has small tubercle for the attachment of specialist spines, which are used to dig the burrow and maintain its stability.
Irregular echinoids: Petaloid ambulacra
Unlike regular echinoids where the ambulacra run from the apex of the test to the mouth, irregular echinoids have developed a flower like structure, called the petaloid ambulacra, populated by many small pore pairs for tube feet on the aboral surface of the test. The anterior petaloid ambulacra are larger and form the sulcus or groove. This anterior groove, known as the fasciole, is lined with cilia which pass food particles to the mouth.
Long tube feet extend from the anterior ambulacra, which are used by the animal to excavate and maintain its burrow.
irregular echinoids: sub-anal fasciole
The sub-anal fasciole is another modified area of the test, located close to the anus. It too has many beating cilia that remove waste material and particles, directing them away from the animal, down the sanitary tube. The tube feet that protrude from the two posterior ambulacra are adapted to keep the burrow clean and maintain the sanitary tube.
