Joe, thanks for yet another wonderfully detailed reply!
I'll start slicing some more specimens, I think I have pieces like the last one but I'm obviously not very good at telling! On the subject of possible diagenetic overgrowth,it's perhaps suggestive that other (calcite) fossils from here (corals and brachiopods mostly) are often silicified but not overgrown. Anyway, more sections - there are quite a few specimens where the preservation is a very indistinct grey or perhaps colourless on black limestone - might be worth trying some peels, I've got the materials but not sure how to photograph them - I'll look it up.
No problem - it's great to see these things.
Good luck with the peels - they can be a bit tricky, I think, but I've never tried it myself. You might also want to try UV on it - these crystalline things can sometimes react in useful ways to make them stand out better... Failing that, some thick sections might also do the trick - say 2-3 mm, at least to start with - so that you can make the most of the translucency contrast. If you've got enough bits to play around with then it's going to be worth experimenting.
Thanks! - just found a good chunk of a thick (1cm+) bowl that looks (at first glance!) like the last one - I've rough sawm it and it looks really clear so will get it polished over the next day.
Once you know they're there, the sponges start to leap out at you, never common though. I'm still surprised nobody's listed them before from the Great Limestone, even microscopic algae have had some attention...
Yes, must get a new UV lamp! - been meaning to for corals but hadn't thought of it for these.
Here's the 1cm thick bowl section that superficially has the same appearance as the last one, but it doesn't so obviously have the thick branching tubes.
Sp.6 sliced block:
weathered surface:
section - not highly polished yet, photographed wet:
Post by Joe Botting on Aug 28, 2014 6:04:21 GMT -5
This is different again, Tarquin!
One clear feature is that there's a discontinuity between the inner part of the skeleton, and the thin surface layer of smaller structures. Given that the skeleton is made of apparently isolated elements rather than continuous structures, I'd say this was more spicules, albeit rather inflated and lumpy ones. There are quite a few that appear to be hexactine-like, as I think you probably spotted (hence your uncertainty).
The distinct dermal spicule layer does help to narrow down the taxonomy, so I think we're looking at Stiodermatidae again, or a related family. The more diverse a fauna gets, the more complex the identification seems to become... and in that case the solution is more specimens, until you can start to see the divisions between species (and also, spot continua). I'd say what you've found already is worth writing up, but more will definitely improve it.
Last Edit: Aug 28, 2014 6:04:54 GMT -5 by Joe Botting
Post by Joe Botting on Aug 31, 2014 14:46:19 GMT -5
That one does indeed look very similar to sp. 6 - the differences could just be due to the earlier growth stage. This is promising, as it makes it look like there are at least some species where you'll have multiple specimens, and can therefore get somewhere with the taxonomy... Nice.
Last Edit: Aug 31, 2014 14:46:52 GMT -5 by Joe Botting
Thanks, Joe! I collected a couple of good specimens yesterday that I'd known about for some time - in large loose blocks that needed reducing with my heaviest hammer... Sections to come soon.
Also, after months of scouring quarry faces and stream sections I've at last found an in situ specimen (one of the bowl types). I'll leave it there, of course - good to have a horizon (high up in the Great Limestone where macrofossils are rare), and it fits the distribution of the loose material.
Post by Joe Botting on Sept 9, 2014 17:50:04 GMT -5
This one's more tricky, Tarquin. I can't see the detail of the skeleton very clearly, but it seems to be more of a network of spicules with the rays fused together (at least in places) - would you agree? If so, then it looks like a lithistid to me, probably one with tetractine spicules. The surfaces seem to be spiky in places, which would also fit something like rhizoclone spicules. That would be plausible for a branching, cylindrical body. I can't find anything that immediately reminds me of it, but lithistids are a bit of a grey area for me. This is one I'd have to squint at under a microscope for a while to work out what was going on, I suspect...
Thanks very much, Joe - I agree about the fused fuzziness of this one and, with the little I know, it's what I've been expecting a lithistid to look like. Mind you, I thought some of the others were until you started pointing out hexactines .
Well, on with the slicing then! - I'll get the rest done in a week or two and hopefully find a few repeats...