Friday, 16 March 2012

Prehistoric cucumbers

Top o’ the mornin’ to ye. Lucy's still sunning herself in the desert, finding sponges, arthropods, and who-knows-what, so you're stuck with me again. Once she's back we're hoping to have - shock, horror - a brief holiday, and go somewhere spectacular. Before then, though… time for some more fossils.

This is based one of our papers, just out in the open-access online journal Palaeontologia Electronica, published by the Palaeontological Association. At some point we'll write about the whole debate over science publishing, but suffice to say that open access is a Good Thing. Did you know that authors have to sign away their copyright to the journal, and are often asked to pay a few hundred pounds for the privilege, before the journal sells the paper at £30-40 a go, and charges ridiculous subscription charges for academic libraries, with no royalties whatsoever to the authors? Palaeo Electronica isn’t the highest-impact journal, but it’s one of the ethical ones. Anyway, that’s for another day. The point here is that you get to read the paper for yourselves, should you be that way inclined:

In outline, what we’ve got is another fauna from the Ordovician of Wales with some exceptional preservation of squidgy things in it. Unlike the Llanfawr fauna we talked about before, this isn’t in some nice useful exposure like a quarry – it’s a tiny, crumbling bank of shale with trees on top, on the side of a stream valley that’s so dim you can barely see anything on a hot summer day due to the thick canopy. The shales are deeply weathered, and although there was pyrite there once (good for X-radiography, remember) it’s not there anymore. What we’re left with are rusty shapes on crumbly grey rock, which we can’t see until we’ve got out into the open.

The reason we bother with it is that there are – you guessed it - sponges! Lots of sponges. There are probably a dozen new species. There are also palaeoscolecid worms (we’ll go into those when the paper comes out in the next couple of months), trilobites (yawn… no, they’re cute, really), and various other gubbins. The gubbin that is most exciting, though, is a small number of sea cucumbers.

Sea cucumbers do not appear as fossils very often. Unlike their cousins the sea urchins and starfish (all included in the echinoderms), they don’t have a rigid skeleton. The plates of an urchin’s test have been reduced to tiny sclerites (rather like sponge spicules) embedded in a leathery skin. The only really robust bit is a ring of plates around the pharynx, which you can see in the photo. They’re truly delightful animals – the only things I know of whose defence mechanism is to spit their intestinal tract out at anything that bothers them. It’s ok – they grow a new one afterwards. Just down the road from us is a restaurant that advertises, “Specialist in Sea Cucumber!” so obviously someone else appreciates them too.

Anyhow, although they turn up regularly on reefs, sea cucumbers really come into their own on the abyssal plain. Great herds of them march across the muddy plains, filtering yummy marine snow out of the water, or digging through the mud for delicious little morsels. They’re one of the most abundant large animals on the planet, where some authors say that they make up nearly 90% of the deep marine animal biomass. They also have many clever abilities, such as buoyancy control and burrowing ability, which is all quite impressive for something with no brain.

One of talents they don’t have, though, is being fossilised. There are very few examples of complete fossil holothurians, with the oldest until now being the Late Silurian of Australia, 50 million years later. There are some isolated, microscopic sclerites and plates from the same sort of age as ours, but until now, no whole ones. At this stage, I should explain what’s important about the Middle Ordovician, regarding echinoderms. There are numerous extinct groups of echinoderms, but the living ones all appear in the Ordovician (except the concentricycloids, but they’re just weird, and are probably merely a strange group of starfish). The crinoids appear in the earliest Ordovician, the starfish (in the form of the extinct somasteroids) around the same time, followed by the true starfish and brittle-stars, and then finally the sea urchins and holothurians. The latter two appear for the first time in the Middle Ordovician; in fact, we’ve got the oldest sea urchin too, but that’s not published yet. They are also believed to be very close relatives, so the earliest ones should show features that are somewhere between those of the two groups…

Until now, that really wasn’t true – and in a way, it still isn’t. There’s a truly bizarre extinct class of echinoderms called the ophiocystioids that might be close to ancestral holothurians, and the bothriocidaroids appear to be an early branch off the sea urchin lineage, but there’s really nothing that shows anything like a clear transition between the two groups. Ours helps a little bit, in that it’s got traces of a sea urchin-like skeleton in the form of reduced ambulacrae – the lines down the side of an urchin, where all the tube feet go. These skeletal ambulacrae are completely lost in modern holothurians, which makes the feature particularly important, and it actually ties in with some predictions made years ago by a German researcher. Isn’t it nice when science actually works?

There’s still a fundamental question remaining, though – where did they come from? Both urchins and cucumbers appear in the fossil record in the Middle Ordovician, in obviously primitive but still recognisable forms. There’s nothing obviously related before them, and there’s nothing (except ours) that’s clearly intermediate. We now have diverse Early Ordovician faunas like the Fezouata Biota, replete with abundant echinoderms, but nothing there that could be closely related to urchins – although there they do find somasteroids in abundance, along with edrioasteroids, which might be the ancestors of them all. The fossil record is pushing us towards thinking that these groups really did evolve in an extremely short space of time…

The thing to remember, though, is that the fossil record lies. It hides things. It gives you glimpses, and then pretends that the rest isn’t really there. Anything living in shallow water, for example, tends to get obliterated before it even has a chance to be fossilised. Our undescribed urchin is from Llandegley Rocks – conveniently, a shallow-water deposit preserved due to being on the side of a subsiding volcano. Coincidence? Maybe. Or maybe the urchins and cucumbers really did evolve in the Early Ordovician, but in a place where we can’t – yet – see them. Time to get the hammers out again, then.

No comments:

Post a Comment