Introduction and biology

The origins of echinoderms lie early on in the Cambrian, or even potentially in the Late Cambrian. Today, most echinoderms are pentaradial/pentameral (it has five-fold symmetry), but this is not true of early fossil groups. There are five echinoderm groups alive today, but many more in the fossil record. Our first video looks at echinoderm body plan and fossil record.

A quick note before we get any further – whilst sometimes we mix it up (it's an easy mistake to make!), echinoderm = member of the echinodermata, echinoid = a sea urchin (which is a subgroup of the echinodermata). Isn't life confusing at times? Palaeontology too...


Key points to take away from this video are:

  • There are five living groups of echinoderm and many more in the fossil record.
  • These animals have a unique water vascular system, a skeleton comprised of calcite, and that calcite has a fine spongy (stereom) structure.
  • These are deuterostomes, and more closely related to hemichordates, then chordates, than they are to other animals.
  • Their fossil record shows the group evolves from bilateral symmetry, and then through no symmetry at all, on the way to their modern, pentaradial body plan.

Meet an echinoderm.

There are a few too many groups to go through each one at this stage (and indeed, because of this, our videos are a bit longer this week), so here you can find an echinoid, or sea urchin, with a few labels to give you a head start on the next video.

A modern sea urchin (Strongylocentrotus purpuratus, the purple sea urchin) from Cornwall, England. Maximum diameter ~12cm.


An elaborate terminology has built up surrounding this group, arguably more so than many other animals groups. As such there are a lot of new words. Sorry. But the key ones are spelled for you below. As previously, it will help you to label a diagram with these as you go along.

  • Calyx/aboral cup/theca
  • Brachials (=arms)
  • Holdfast
  • Brachioles (=holes)
  • Ambulacra
  • Interambulacra
  • Periproct
  • Peristome
  • Aristotle's lantern


Key points to take away from this video are:

  • Crinoids are sessile, they have a holdfast, stem and then calyx with brachials (arms) forming the crown.
  • We often find individual bits of their stems in rocks (=ossicles).
  • There are a series of interesting fossil forms with varying arrangements and types of symmetry.
  • Echinoids have an anus and specialised plates for reproduction on top, and a mouth with a complex jaw arrangement on their bottom surface.
  • They have evolved, in some groups, away from their pentaradial (regular) form to become bilateral again (irregular).
  • Starfish are relatively uncommon as fossils.

Let's now meet a member of each group properly.

In particular, you may want to try and get a feel for the bits that make these animals up, given that this is often what we find in the fossil record.


Let's start by meeting some crinoids. They are cool creatures - check this one out. If you're really luck, this is the kind of form you may find in sites with good preservation.

Ordovician crinoid Daedalocrinus. Specimen ~7.5 cm long.

More often than not, though, you'll find crinoid bits that look a bit like this:

Crinoidal limestone where ossicles comprise much of the rock. Found in the Lower Carboniferous Keokuk Limestone of Monroe County, Indiana. Specimen ~8.5cm max.

Extinct forms of Echinoderm

The odds of you finding one of these are pretty low, apart from at specific sites. But, they are really nice fossils, so I wanted to include one. Meet the blastoid Pentremites robustus. This is a group (sometimes called sea buds) had short stalks, and often lose their arms, so you are left with the calyx as you can see here.

Blastoid Pentremites robustus, Lower Carboniferous, found in Indiana. Specimen ~4.5 cm long.

If you would like to learn more about these creatures, there is a good introduction to be found on Palaeontology [online].


Here you can find a fairly typical regular echinoid fossil. Often we'll only find bits, but on occasion we get a nice whole animal like this. Make sure you catch the ambulacra and interambulacra and are comfortable with how these differ in appearance.

Fossil sea urchin Phymosoma texanum: Cretaceous in age, and discovered in Texas. Specimen ~4.5 cm in diameter.

Now compare that with an irregular echinoid. In particular, note the shift in the position of the mouth and anus (they're labelled for you), and also note the change of the shape of the test. Do you agree that this is better for an infaunal mode of life?

Fossil echinoid Eupatagus antillarum. Well, kind of a fossil, it's Eocene (Ocala Limestone of Levy County, Florida). Specimen length ~7 cm.


Even though these are rare as fossils, let's have a quick look at fossils from this group, which comprise the starfish (Asteroidea) and brittlestars (Ophiuroidea). One of each. Let's start with the former. Here is a fossil starfish:

A fossil starfish, from the Devonian of Tompkins County, New York. max dimension of rock ~6.5 cm.

Contrast this with the less chunky brittlestars - you can tell the difference because there is a clear line/separation between the arms and the central disk in brittlestars.

Fossil brittle stars (Ophiopinna elegans) from the Jurassic (found in Ardeche, France). Rock ~19.5 cm max.

Why should we care?

That was quite a lot of cool fossils! But we can finish by looking at how geologists can use echinoderms, and what their fossils look like.


Key points to take away from this video are:

  • Echinoderms are often highly fragmented, but those fragments can be really widespread and abundant.
  • Our most common fragments are echinoid plates and spines, and crinoid ossicles, which can be major components of numerous rocks.
  • Echinoids in particular are useful for late Mesozoic and Cenozoic biostratigraphy.
  • When it comes to biozones, these don't just have to be built from the occurrence of a single index fossil.

If you want to find out more about the kinds of biozones, you can find a good overview on the website of the Subcommission on Quaternary Stratigraphy.

Crinoidal limestone in section

This slide shows a section through a bioclastic limestone, with a wealth of skeletal grains, including crinoid ossicles, echinoderm fragments and spines, and microfossils (forams and ostracods). This is courtesy of the UK Virtual Microscope and the Open University.

To view either of the two spots marked under PPL and XPL, just click on the icons. The slider on the bottom zooms; scale at top.