The wonderful world of crinoids
Morning all. It's about time we did some more fossils, so here's something a bit different. You're all familiar with starfish and sea urchins, right? Of course you are. Most of you might even know that they're closely related. They are both defined as classes within the subphylum Eleutherozoa, along with sea cucumbers, brittle-stars and concentricycloids. I’ll be impressed if many of you have heard of the Eleutherozoa, but you might well have come across the echinoderms – that’s the even broader group that they all belong to. It’s a marvellous phylum – lots of unique features, like pentaradial symmetry and a hydrostatic support system. The six modern classes cover a lot of ground in terms of variety, but they’re nothing compared with their ancestors.
At some point I’ll post photos of an edrioasteroid that Lucy brought back from Morocco for me – imagine a starfish on a cushion and… actually you’ll be nowhere near, but it’s a start. Anyhow, for today I wanted to introduce the other living class, and some other Moroccan fossils: the crinoids. Otherwise known as sea-lilies (when they’re known at all), these are delicate-looking animals consisting of a long stalk, a cup-like body (calyx), and an array of arms. The modern ones have in many cases lost the stalk, and swim around like a cross between a brittle-star and a jellyfish. (I must stop these daft analogies – I once told an undergrad to think of these as being like starfish on stalks, and one of my fellow students has never forgiven me…).
Crinoids have a pretty ancient history, going back to the base of the Ordovician. For a long time, the oldest one was Ramseyocrinus, from Ramsey Island in Pembrokeshire, but it’s been superseded by some amazing finds in the US. These rare discoveries are what we have to rely on, though – crinoids fall apart very rapidly when they die, so complete fossils rely on live burial (like so much else that we study). This means, of course, that our view of the early crinoid record is very sketchy indeed, with a few spectacular faunas, and a lot of gaps. Which brings us to Morocco.
A few years back I was invited to write up the crinoids from the Late Ordovician rocks of Morocco (these rocks have very famous echinoderm faunas that are being described en masse, but crinoids are relatively rare components of the fauna). I duly accepted, and now the time for submission is long overdue, so I’m desperately trying to finish them off. Just for once, it isn’t just me being lazy that’s caused the delay – in this case the number of crinoid specimens has been extremely low, and I’ve literally been waiting for specimens to describe. I had a few from the beginning, but most of them only as single specimens... which isn’t really enough to describe, as you need to see a lot of details for a meaningful description. Another problem is that they’re preserved in sandstone, with the grain size so coarse that is makes working out the arrangement of plates on the calyx very tricky, or just plain impossible. I’ve been making latex casts, cleaning and preparing, but in some cases there just isn’t enough information.
Finally, though, the end is in sight. Lucy came back from Morocco in the spring with a bunch of new specimens, some from a new site, and the preservation is just gorgeous. These I could do something with… and promptly set to work doing so. What I didn’t expect, though, was just how stonkingly, befuddlingly weird they all are. It has taken ages just to understand the things, work out what families they’re related to, and so on. There are some that simply don’t fit in known families, or have combinations of features from a whole stack of different ones. So how about the beautiful pair of specimens in the picture? Well, they’re clearly a new species of Euptychocrinus, a monobathrid camerate (honest). The problem is that they appear to be otherwise closest to an odd group of diplobathrids called the Opsiocrinidae – i.e. in a different suborder. This is the sort of result that makes you wonder whether the distinction between the suborders is reliable at all… which opens a whole can of worms about whether we’ve got the classification system right.
It’s never easy, eh? Every time you think you understand it a little bit, it just gets weirder. The fauna is a combination of groups known otherwise from North America, the UK, and elsewhere, and from well-known families or truly obscure genera. Some appear to be extremely primitive, and others most closely resemble things from the Devonian. What’s most surprising, though, is how diverse the fauna is. Although there are many, many fewer specimens of crinoids than of other groups (brittle-stars, cystoids), the diversity is (I think, although the formal descriptions aren’t out yet) even higher. There are no species with lots of specimens, and most of them are represented so far only by one or two. We see the same thing in the Builth Inlier in Wales, but in reverse: there the crinoids are relatively abundant, and it is the starfish that are extremely rare… but so far, every starfish specimen seems to be a different species. What is this telling us about ecology? No idea, but patterns like this are always interesting to ponder. Besides, it distracts me a bit from the taxonomic headaches that these amazing specimens generate every time I open the drawers.
Saturday, 4 August 2012
The wonderful world of crinoids