Mississippian sponge, heteractinid?, from UK

[TqB]

From the Great Limestone, Pendleian (Upper Mississippian) of County Durham, UK.

 

One for @Spongy Joe and any other sponge experts out there.

 

There appear to be no sponges (apart from Chaetetes) recorded from this well researched limestone but I've collected over thirty over the past few years. There are several different types, generally fossilised as broken fragments though these can be quite large (several inches across).

 

This one is a curved sheet, like part of a vase or dish, about 10 - 15mm thick.

 

The outer (convex) layer contains a good proportion of spicules showing five (and a few six) rays so could it be a heteractinid of some sort?

 

The preservation is mostly calcite, perhaps original.

 

Scale bar is 1cm long.

 

Vertical section, showing disposition of tangential sections below.

(specimen no. Sp. 13)

 

Tangential section, cutting through outer surface on right and bottom (the orange layer, with smaller spicules than the internal ones).

 

Closeups of tangential sections. Several five rayed spicules are visible along the edges (i.e. in the outer layer).

 

 

 

[TqB]

Another closeup, with at least one apparent six-rayed spicule:

Sp. 13

 

 

Weathered vertical section, as found:

Sp.13

[KCMOfossil]

5 minutes ago, TqB said:

a couple of apparent six-rayed spicules:

I am new to sponges.  Could you circle the spicules so I can see what you mean?  Thanks.

[TqB]

36 minutes ago, KCMOfossil said:

I am new to sponges.  Could you circle the spicules so I can see what you mean?  Thanks.

 

Sure, it takes a while to get your eye in with these! All the dark blobby bits are spicules of one sort or another but the five rayed ones I'm talking about look like this:

 

[KCMOfossil]

4 hours ago, TqB said:

All the dark blobby bits are spicules

Thanks.  I see them.  Now I know more what to look for thanks to tff.edu

 

Russ

[Spongy Joe]

Ooooh yes. That's a heteractinid alright. The shallow bowl shape matches something like Gondekia, but these are really not that well known, and it's probably a new species. Rigby says that Gondekia spicules were all hexaradiate, but he said that about Eiffelia as well...

 

There's one thing here that really grabs my attention. You say that the preservation is 'mostly calcite', and it looks as though many of the spicules show a different refractive index in the centre. Could that be quartz, by any chance? According to a random recent paper by a couple of nutters called Botting & Muir, at least some of the heteractiinids should be in the early stem lineage of Calcarea, and have biminerallic spicules - silica in the middle, calcite around it. Most biologists don't accept this is possible. Notwithstanding that, it's been shown for Eiffelia, but not yet seen in anything else - probably because of recrystallisation. Any beautifully preserved heteractinids sould be looked at very carefully indeed...

 

[TqB]

7 hours ago, Spongy Joe said:

Ooooh yes. That's a heteractinid alright. The shallow bowl shape matches something like Gondekia, but these are really not that well known, and it's probably a new species. Rigby says that Gondekia spicules were all hexaradiate, but he said that about Eiffelia as well...

 

There's one thing here that really grabs my attention. You say that the preservation is 'mostly calcite', and it looks as though many of the spicules show a different refractive index in the centre. Could that be quartz, by any chance? According to a random recent paper by a couple of nutters called Botting & Muir, at least some of the heteractiinids should be in the early stem lineage of Calcarea, and have biminerallic spicules - silica in the middle, calcite around it. Most biologists don't accept this is possible. Notwithstanding that, it's been shown for Eiffelia, but not yet seen in anything else - probably because of recrystallisation. Any beautifully preserved heteractinids sould be looked at very carefully indeed...

 

 

Many thanks, Joe! All fascinating stuff and I'll see if I can identify the possible silica axes.  A simple etch test maybe? - I don't have facilities to make thin sections. I'll try it anyway - there is quite a bit of diagenetic silica around to complicate things.

 

I have several specimens that seem to be the same so plenty to work with - I want to see what the concave surface looks like too. And I'm going to need a better microscope...

 

(As a minor aside, are you pro-sponge for Chancelloria?  )

[Spongy Joe]

Etch test might well do the job; if you've got several bits then it's worth having a go, certainly. I've made enquiries through the NMW (Cardiff) as to whether I can get thin sections there (I've got honorary stauts, so it might be doable!). And yes, I also want to see the upper surface if you can get it crack around there!

 

Chancelloriids... yes, indeed. I used to be a bit on the fence, but it turns out that all of their weirdly non-spongy features are shared with derived protomonaxonids like Lenica, Halichondrites and especially Pirania - which even has the weird epidermis-like texture. So, yes - I think that's probably case closed, especially now Peiyun Cong and Tom Harvey have been analysing the growth patterns of them and matching them to primitive modern calcareans. I'm sure others will have more words to say on this, of course (Stefan Bengtson tells me he is pondering his response, if there's to be one..!), but I'm now fairly confident on this one.

[TqB]

Thanks again, Joe, I'll get onto it soon. I think the pentaradiates/hexaradiates are larger on the top surface.

 

 I'm very pleased to hear about chancelloriids.

 

Another specimen, weathered section, showing larger pentaradiates (the large one at the top, left of centre, is about 3mm) on the concave (top) region.

( @Spongy Joe you've seen this one before, when you first mentioned heteractinid as a probability).

Sp.11

Sp.11

 

 

[Spongy Joe]

I thought these looked familiar!

 

Now, that top picture is very interesting.... it doesn't seem to show the same two-phase construction of the spicules, but look at that normal four-rayed spicule in the lower middle. That appears to have a trace of the axial canal, which is what you get in silicean (hexactinellid and demosponge) spicules because they're secreted onto an organic filament. Calcarean spicules don't have an axial filament, because they're secreted inside an outer organic sheath. Ergo, this is a hexactinellid. Except that hexactinellids don't have pentaradiate or hexaradiate spicules...

 

Dying to hear what the etch test tells us..!  

[doushantuo]

great find,Tarq!

[TqB]

On 13/05/2018 at 5:20 AM, doushantuo said:

great find,Tarq!

Thanks, Ben!

 

On 13/05/2018 at 1:43 AM, Spongy Joe said:

I thought these looked familiar!

 

Now, that top picture is very interesting.... it doesn't seem to show the same two-phase construction of the spicules, but look at that normal four-rayed spicule in the lower middle. That appears to have a trace of the axial canal, which is what you get in silicean (hexactinellid and demosponge) spicules because they're secreted onto an organic filament. Calcarean spicules don't have an axial filament, because they're secreted inside an outer organic sheath. Ergo, this is a hexactinellid. Except that hexactinellids don't have pentaradiate or hexaradiate spicules...

 

Dying to hear what the etch test tells us..!  

I'm very intrigued by all that and have done an etch on the first specimen. I'm afraid it only seems to show that the spicules are made of interlocking calcite crystals (so maybe replacements), no sign of a silica centre as far as I can see. I think any white spots and lineations on the first photo are just reflections... (These blow up quite a lot if you click.)

Sp. 13

Sp. 13

 

[doushantuo]

Lovely detailed pic,Tarq.Awesome!

[Spongy Joe]

Well done - yes, that really picks out the structure. As you say, it's blatantly recrystallised, this one. Makes me wonder what I was seeing in the first post, then... but I guess it was just random.

 

Could you check the other specimens to see whether any look 'different'? I suspect not, but there is often surprising variability in preservation over short distances, after all (even on one bedding surface, sometimes). It's still a really interesting species, especially with that 'ghost' of an axial structure in the weathered one. Don't etch that one, eh?

 

 

[TqB]

3 hours ago, doushantuo said:

Lovely detailed pic,Tarq.Awesome!

Thanks again, Ben, down to a recently acquired 95mm macro lens.

16 minutes ago, Spongy Joe said:

Well done - yes, that really picks out the structure. As you say, it's blatantly recrystallised, this one. Makes me wonder what I was seeing in the first post, then... but I guess it was just random.

 

Could you check the other specimens to see whether any look 'different'? I suspect not, but there is often surprising variability in preservation over short distances, after all (even on one bedding surface, sometimes). It's still a really interesting species, especially with that 'ghost' of an axial structure in the weathered one. Don't etch that one, eh?

 

 

Thanks, Joe, will do. And I'll try to get some good views of the top surface.

[TqB]

This is a little bit of the weathered specimen sliced just at the dorsal surface - nothing really new but there's a nice large hexaradiate and pentaradiate, about 3.5mm. So they do tend to be a lot larger than the ventral ones.

Sp. 13

Sp. 13

 

And on closer examination, this break in the same specimen that I showed before is actually a very oblique section and is also very close and parallel to the dorsal surface.

Sp. 11

 

[Spongy Joe]

Interesting! And a cute little gastropod, just to prove it's close to the surface.

 

I'm trying to see whether the skeleton is fused, but it doesn't seem to be - except, perhaps, by secondary cement between the spicules, but that's really hard to separate from the matrix with the shells in.

 

p.s. There should be no problem getting a few thin sections made from this fauna!

[Spongy Joe]

Another query: what's the preservation like in the hexactinellids or lithistids from this level? Any silica preserved there, or does it look the same?

[TqB]

On 20/05/2018 at 6:48 AM, Spongy Joe said:

Another query: what's the preservation like in the hexactinellids or lithistids from this level? Any silica preserved there, or does it look the same?

I was thinking that too. There's a lot of silica replacement of calcite around there, usually partial (e.g. in corals and Chaetetes).

 

I guess we can assume this probable Haplistion (ID'd by you a while back) was siliceous originally - it is now partly silicified but is that original? The silica (white) looks blobby, as if it's replacing secondary calcite...

 

Weathered, as found:

Sp. 5

 

Same specimen polished, under water, silicified around top half:

Sp. 5

 

Closeup, showing granular silica:

Sp. 5

[TqB]

And the now calcitic bottom left quadrant of the same Haplistion, showing a lot of odd spicules but less clear on the mesh:

[Spongy Joe]

Thanks, Tarquin! Yes, this is partly the original siliceous skeleton, coarsely recrystallised in situ. Where it's been calcified as a secondary process, the structure has really gone to pot.

 

What this means (assuming broadly consistent processes across the deposit, etc. etc...) is that the nice clear calcite spicules in the heteractinid are original calcite spicules. Given it's got some hexactins in it, this is significant - it rules out convergence going the other way, from a hexactinellid to looking like a heteractinid. It also suggests that if there had been replacement of silica by calcite in the cores of the spicules, we'd see a trace of that in the form of really grotty microcrystalline calcite. The large crystals look like in situ crystallisation. All things being equal, this puts your sponge somewhere off the branch between 6 and 7 (likely between 6 and Astraeospongium) on the evolutionary tree, and shows that silica seems to have been lost before hexactins were. That's actually rather neat.

 

Of course, that's a lot of interpretation based on limited information; call it an improved hypothesis, rather than an answer! Anyhow, assuming it holds up in comparisons with other sponges from there (got any hexactinellids to compare with?) then yes, this is probably publishable - even without getting extra useful stuff from the inside of the spicules.

[TqB]

"got any hexactinellids to compare with?"

 

@Spongy Joe  Great stuff! - Nothing mainstream that I've seen yet but you said that this one is hexactinellid-like, with thick hexactines, cf. Malumispongium.

It's a cylinder, up to about 2cm across and at least 9cm long. Seems to be all silica.

 

Sp.3

 

Sp.3

[Tidgy's Dad]

Very interesting and informative thread, chaps. 

I like the way this is going.

[Spongy Joe]

Glad you're enjoying it, Adam! Me too.

 

OK... I think I'm going to backtrack a little on the "cf. Malumispongium" comment. This is actually pretty weird. There aren't many obvious hexactins there, but there are a lot of weird branchings going on. I'm also not sure how much is a spicule network rather than isolated spicules; it's hard to tell from a section.

 

As for the mineralogy, what makes you say it's all silica? I can see similar preservation to the Haplistion forming a coating around the edges, but the spicule cross-sections look much more like the calcite spicules in the heteractinid. The axial areas being different is plausible for a replacement fabric, since the axial canals in siliceans are often widened by dissolution. However... there are sometimes several bits of this 'axial' region within a single spicule, implying some sort of fused network structure, with the outer part of each 'spicule' being a peculiar overgrowth.

 

Siliceous fusion of spicules does happen in some hexactinellids (but not in demosponges), but then the network tends to be very regular... just like this isn't. There are various forms of calcitic overgrowth in a range of demosponges and calcareans, including some heteractinids, such as Mehl & Reitner's description of Astraeospongium meniscum (see below), which is rather similar at upper right. I wonder whether this is actually another heteractinid... but if so, the silicification around the edges should be secondary. If it's silica all the way through, despite appearances, then I'll have a major rethink!

[TqB]

@Spongy Joe - You're absolutely right about the preservation, I was fooled by the white colour and the strong relief weathering of the original exposed surface. There's a fair bit of silica around, some of it overgrowth, but the original all seems to be calcite, including the dark axes. (I etched it - pics if you want but I don't want to overload this post.)

Sorry about that, I'm a bad mineralogist. 

 

And you seem to be right to suggest another heteractinid - I've done two more transverse sections and found nice pentaradiates in the central area.

 

In fact, I'm seeing pentaradiates in most of my other specimens now so most of them may be the same heteractinid species, with various degrees of diagenetic silicification making them look like stiodermatids or whatever.

 

Anyway, here are the new sections of this one (Sp. 3 - I'm adding specimen numbers to the photos).

 

Sp. 3 b

 

Sp. 3 c