one-eyed trilobites

[Joe Botting]

...but what an eye!

I was packing away some graptolites yesterday (Didymograptus, from the upper murchisoni shales), when one of the slabs started to split. Prsing it open, out leapt a wonderful cyclopygid trilobite. These are very streamlined beasties, where the eyes have been enlarged to a ridiculous degree. In this case, they've fused at the front to form a single huge eye around most of the head. It appears to be a thing called Microparia lusca

The specimen isn't perfect, and it's in a few pieces, but here's a drawing of the species:

photobucket.com/albums/y159/joseph00/?action=view¤t=Microparialusca.jpg

(the ridge at the back of the head is probably an artefact related to compression of the head onto the thoracic segments... sorry about that!)

This is, to my knowledge, the first cyclopygid found in the Builth Inlier from outside the teretiusculus Biozone. Not bad for an accident in the office, eh?

Joe

[Joe Botting]

Ok, so no-one bit. Next question: <i>why</i> is it the only example from outside the teretiusculus Zone?

The basic idea is that these pelagic trilobites lived in relatively deep water, perhaps around the edge of the continental shelf, a few hundred meters down. If you go towards Dyfed, there are trilobite assemblages dominated by these cyclopygids, and also blind benthic (bottom-living) species. If they live in deep water, they don't generally appear on the shelf.

The post-volcanic Builth sequence is bascically the record of erosion and subsidence of the volcanic island. The sediment gets finer and blacker on average as the rocks get younger - although the blackest, finest shales are in the immediately overlying murchisoni Zone, and the gracilis Zone at the top of the sequence. Much of the lower part of the teretiusculus Zone is grey siltstone, which lokks like it should have been laid down in shallower water, and yet it's this interval that, according to the trilobites, was deposited in the deepest water.
If the sequence was subsiding (and coinciding with a rise in sea level around the gracilis Zone), the deepest water should be in these youngest beds.

If it was, where are the cyclopygids?

If it wasn't, why not?

Joe

[hallucygenia]

That's a problem. I suppose one answer for why so few have been found in the murchisoni Zone is that hardly anyone looks there! There are some teretiusculus Zone localities that are well known for trilobites, so trilobite workers tend to go there. As to why there aren't any cyclopygids in the gracilis Zone - no idea. There are certainly lots of blind bottom-dwelling trilobites at that time, so there's no obvious reason why there shouldn' be cyclopygids too. Hasn't someone suggested that the gracilis Zone was a more restricted environment? Or am I confusing that with something else?

A minor point: I think you've got slightly confused on your zonal sequence. murchisoni is the oldest zone, followed by teretiusculus, then gracilis.

[Joe Botting]

The order of biozones is hardly a minor point - but by 'overlying' I was referring to the murchisoni Zone overlying the volcanic sequence. Sorry for the confusion.

Right. Yes, there are lots of blind trilobites in the gracilis zone, but they're all trinucleids and raphiophorids, and the trinucleids at least occur in much shallower water as well. I don't think the presence of blind trilobites to that extent tells us much, since all the others had perfectly good eyes.
Restricted environment? Are you thinking of Peter Sheldon's suggestion that it shows the development of a silled basin, with circulation with the rest of the Welsh Basin restricted by uplifted fault blocks? If so, does that really stop pelagic trilobites getting swept in and establishing a population in the region during the gracilis Zone - or even having their original population split into two?

Joe

[hallucygenia]

I see what you mean about the biozones now - next time I'll have a cup of coffee before I post!

I probably am thinking of Peter Sheldon's idea about restricted environments - it's one of these things I've heard, but I'm not sure where. You're right, it still doesn't solve the problem, as cyclopygids could still be swept in. Unless the restricted area was simply too small for them to maintain a viable population size in? Cyclopygids are noticeably rare in the Builth Inlier, although I'm not sure about other places. Of course, rarity as preserved fossils doesn't necessarily translate to rarity during life.

[Joe Botting]

Hmmm... population size has got to have something to do with it, plus there's the aspect of just how rare something has to be (in life) in order to make it highly unlikely that we would find one (as a fossil). My own feeling is that Llanfawr beds in particular have been collected so intensively that any cyclopygids there should have shown up, if there was a viable population. Even if one or two specimens turn up (perhaps like our murchisoni Zone one), my hunch is that they don't represent a stable population, but rather a random individual in the wrong place. But that is a guess, I freely admit.

The other thing is that modern pelagic/planktonic creatures have a habit of vertical diurnal migration - they go up and down on a daily basis, often as much as a kilometre. This is at least partly down to the modern ocean ecology (I think - do you agree?), as an avoidance of the intense predation by surface-dwelling fish during the day. If there were reason to assume the Ordovician biotas did the same, then there should be loads of cyclopygids at Llanfawr, assuming even a severe topographic barrier - but then we'd also not expect to be able to define their biofacies so neatly, so it probably doesn't work.

Personally, I think what it boils down to is that we need some way, not to exclude cyclopygids, but to prevent them forming a stable population once they're in. Comparisons with other areas would be really useful here. Bill, or anyone else, any hints as to what it's like at Shelve?

Joe