Echinoderms

Echinoderms:

The name translates as ‘spiny skinned.’ In fact, only a few echinoderms have spiny skins, but they are the most familiar: the echinoids, or sea urchins, and to some extent the starfish. Today, there are also ophiuroids (brittle stars), holothurians (sea cucumbers), crinoids (sea-lilies) and concentricycloids (sea daisies). The last of these are only found on wood that is decaying in very deep water, and are therefore truly obscure. The others, despite some spectacular differences in appearance, share a few common traits – a calcite skeleton made of porous plates (reduced to tiny spicules in the holothurians), a remarkable ‘water vascular system’ that uses hydraulics to manipulate large numbers of tiny tube feet, and… wait for it… five-fold symmetry. Yes, they truly are weird and wonderful creatures.

But these are just the last remnants of what surely qualifies as the strangest of all animal groups. Go back to the Lower Palaeozoic, and they were weirder still. Even the names are strange and evocative: edrioasteroids, helicoplacoids, carpoids, cinctans, eocrinoids, parablastoids, echinocystids, ophiocystioids… in all, there are about thirty described classes (groups equivalent in their range of shape to the echinoids, or to the starfish) of echinoderm in the fossil record. Their diversity of form is enormous, but most share the same features seen in modern echinoderms. The Ordovician and Silurian are a period of great interest, because they show both the emergence of most of the modern groups, and the greatest diversity of the extinct ones.

For the palaeontologist, however, there is a problem with echinoderms. Although their skeletons survived the chemical trials of fossilization rather well, the same cannot be said for the physical aspects. With a few exceptions, the skeleton is made of a large number of small plates, or ‘ossicles,’ and the soft tissue is thin and insubstantial. When the animal dies, there is little holding it together, and very soon (within days or weeks), all that is left is a mass of plates. In order to preserve an echinoderm intact, you really need to bury it alive, or at least only very recently deceased. Without that, the isolated plates are often unidentifiable, unless you can compare them with plates in complete specimens. Frequently, we cannot even tell what class of echinoderm a plate has come from, although there are exceptions; some, such as the stem ossicles of crinoids, can be diagnostic even at species level, and are well worth looking at closely.

There’s only so much you can do with anything else, though, unless it’s articulated. If it is articulated, however, then you usually have a truly beautiful fossil in front of you. Echinoderms are amongst the most visually stunning of all fossil finds. Equally important, of course, is that a new specimen often reveals new information, because the rarity of complete fossilisation means that the record we have is very incomplete. It is still unclear how many species of crinoids there were in the Builth Inlier, and of the other groups we have very little trace.

To start with the crinoids, though, they are almost invariably found in the shallower, coarser sediments. There are currently seven species described from the area (but another three or four that we’re trying to make sense of), and all except one occur in relatively coarse sandstones. Most of the species can be assigned to the genus Iocrinus, a relatively simple, multi-armed creature that evolved into numerous slight variations, often with different ecological preferences. Cefnocrinus samgilmouri is also rather common, but the other species are scattered. Two are known from one articulated specimen each, and another from two specimens. The classification of crinoids is mostly dependent on the arrangement of plates in the cup-shaped body, or calyx, so that’s the thing to look for if you think you’ve got a nice specimen.

In contrast, starfish are extremely uncommon as complete fossils. They are even more fragile as coherent skeletons, and the isolated plates are not normally recognisable. However, at least six specimens have been seen in the Builth Inlier, including at least four species, and probably more. These are among the earliest starfish from Britain, and of great interest to palaeontologists. Such a relatively high diversity for the number of specimens indicates that there were originally many more specimens alive, but most have not been fossilized. Any starfish is a very interesting find, and a brittle-star (with a central disc, and long, flexible arms) would be even more so.

Other groups are rarer still. There is a single specimen so far of the cystoid Echinosphaerites, although there is also another new cystoid, and isolated plates suggest that more will one day be found. The cystoids are a diverse group of generally globular creatures, often seemingly without arms (technically they have brachioles, but they are so thin and fragile that they rarely preserve intact), and sometimes with hundreds of plates in the ‘theca’ (body). There is also an abundant mitrate from the teretiusculus Zone, and these creatures are extremely interesting indeed. They are also highly controversial. Depending on who you ask, they are either aberrant, highly derived echinoderms, an early stage of the echinoderm group (before pentaradial symmetry appeared), or an ancestral chordate (i.e. an early version of us). New specimens are always welcomed by those involved in the rather noisy debates! They will also be interested in the selection of new cornutes that is now turning up from the teretiusculus and gracilis biozones; some of them are rather nice, in fact.

Other than those, there is only a single example of an echinoid - but it is the world's oldest, so we can't complain. There was another, but I lost it before I realised it was interesting. Tut, tut. Undergrads, eh? A whole host of other things might also appear, at least theoretically. Normally they can be identified as echinoderms by the characteristic plated structure, but sometimes it really isn’t obvious. If you’re not sure, please, please, please don’t throw it away!

[4]Cornute indet. sp. A. A unique specimen so far, and a really odd beast. The fibrous bit at right is particularly strange - any ideas, please contact us. 10 mm across.

[4]Cornute indet. sp. B. Only two specimens so far, on one slab. Looks like a hanusiid (thanks to Bertrand!). 6 mm wide.

[2]Crinoid indet. sp. B. (Botting 2003). Crown diameter up to ~ 100mm.

[2]Crinoid indet. sp. C. Cladid crinoid known only from two poor, partial crowns, and several occurrences of columnals. Potentially an interesting species. Calyx diamter ~ 7 mm.

[1/2]Crinoids. [2]Cefnocrinus samgilmouri Botting 2003 (top left),[2]Caleidocrinus (Huxleyocrinus) turgidulus Ramsbottom 1961 (Botting 2003) (top right); [1]Iocrinus pauli Donovan & Gale 1989, [2]I. llandegleyi Botting 2003, [2]I. cf. whitteryi Ramsbottom 1961 (Botting 2003) (middle, left to right); [2]indet. crinoid sp. A (Botting 2003) (lower right). C. samgilmouri up to 250 mm tall; others to scale.

[4]Cystoid indet. Only one, or possibly two specimens from one bed. At least superficially similar to Regulaecystis, but more specimens are needed. Orientation uncertain, as there is no evidence of a stalk or of fistulipores; the associated hexagonal plate is also rather weird. Diameter ~ 5 mm.

[2]Echinoderm indet. sp. A. to Echinoderm sp. F. Individual ossicles typically ~ 2-3 mm diameter.

[2]Echinoderm indet. sp. G. Plates up to 4 mm across.

[2]Echinoderm indet. sp H. Individual plate up to 3 mm across.

[2]Echinoderm indet. sp. I. Individual plates up to 3 mm across.

[2]Echinosphaerites cf. granulatus M'Coy 1846 (Botting 2003). Up to 80 mm tall.

[4]Galliaecystis sp. nov.; a rare, primitive genus of cornute originally described fom the Lower Ordovician of France. The dashed parts are extrapolated from known species, to give some idea of the likely outline. Only specimen so far is 7 mm across.