Fossil bivalves with periostracum???

[Max-fossils]

Hi all,

 

Found these two shells in Antwerp, Belgium. It was in a place with lots of sand, and the sea was rather far away. There are tons of Pliocene shells there to be found. I also found these two bivalves. What I find really weird is that the periostracum (the brown layer) is still preserved!? How could that be? Nearly all fossil shells lose it when fossilizing, yet these two seem to have kept it. 

 

What do you think is the answer to this mystery? Fossils, with the periostracum preserved, or modern (but how did they get here?)? (Or maybe this isn't a periostracum at all. But what is it then?)

 

I think that the species are Mytilus edulis and Spisula subtruncata (although that's by far the biggest Spisula I have ever seen) (both present modern in the North Sea, and occur as fossils in Belgium). 

 

I'm greatly anticipating your thoughts on this!

 

Max

 

[Max-fossils]

[Spongy Joe]

I'll have a stab at this. I think the answer to this is time-averaging: not everything in a deposit is the same age. Those Pliocene shells were reworked and deposited by more recent rivers, if I remember correctly about the geology of that area? These deposits can contain Ordovician and Silurian sponges in the Netherlands, originally deposited in the Baltic area.

   If so, then I suspect these shells with periostracum were living in the river that deposited the rest: they're freshwater species. Freshwater molluscs have much thicker periostracum than most marine species anyway, because the water is more acidic. There are some very large freshwater clams, and there certainly are freshwater mussels (e.g. Perna) as well - although I haven't seen any that quite match these in the UK, so we may be looking at some warmer interglacial episode, when the local faunas were a little different.

 

Yes, I'm guessing... but hopefully someone out there can confirm, or otherwise!

[Max-fossils]

8 minutes ago, Spongy Joe said:

I'll have a stab at this. I think the answer to this is time-averaging: not everything in a deposit is the same age. Those Pliocene shells were reworked and deposited by more recent rivers, if I remember correctly about the geology of that area? These deposits can contain Ordovician and Silurian sponges in the Netherlands, originally deposited in the Baltic area.

   If so, then I suspect these shells with periostracum were living in the river that deposited the rest: they're freshwater species. Freshwater molluscs have much thicker periostracum than most marine species anyway, because the water is more acidic. There are some very large freshwater clams, and there certainly are freshwater mussels (e.g. Perna) as well - although I haven't seen any that quite match these in the UK, so we may be looking at some warmer interglacial episode, when the local faunas were a little different.

 

Yes, I'm guessing... but hopefully someone out there can confirm, or otherwise!

Well that's the thing... both species here are saltwater species from the North Sea. Not freshwater. I had considered rivers, but the fact that these are saltwater rules that theory out.

It is true that there is a whole bunch of different layers in Antwerp (which is in Belgium, not the Netherlands ), but as far as I am aware there isn't any Paleozoic (nor Mesozoic) material here. The sand layers here come straight from the Esco river. The sand was dredged out in order to create space to expand the large Antwerp harbor, and then that sand is simply dumped in huge piles in several areas, namely the one I was hunting at. 

 

There are several different layers of sand (each with different fossils; some containing many shells, some more rich in shark teeth) in the Esco, but I think all are either Miocene or Pliocene. So my saltwater species here above have either been dredged with the Esco sands (meaning that they arrived in the river beforehand. But seeing that they are saltwater and not freshwater the same question arises: how did they get in that river?) 

OR

they got to the Antwerp sand dump after the dredgings (I found the shells among lots of other clearly Pliocene shells at the top, no digging). But again, how did they get there (over 10 meters altitude and several kilometres away from the sea)?

 

I do appreciate your help, so thanks a lot!

[Spongy Joe]

Hi Max, and thanks for the detailed rebuttal!

    OK... I am way out of my area here, and don't want to go querying your identifications, but I will anyway!  How certain are you of the species? In both I can see significant differences from the species you suggest, especially in the details of the hinge area, and also to some extent the outlines. (Also, does the pallial sinus match for Spisula? I can't quite see because of the reflection. ) This may be intraspecific variation, but it seems to be somewhat too much for comfort... but as I say, this isn't my area!

 

    If the species are indeed good, then yes, that totally scuppers the freshwater idea. In that case, I'm going to throw another mad idea at you: lunch. I imagine the large clam would be very edible, as well as the mussels, and perhaps someone had a seafood picnic there many moons ago. Perhaps even the workmen doing the dredging - shellfish were cheap, after all.

 

Edit: I'm now going to scupper that idea too: they're worn, including the inner surfaces, so were washed around on a beach for a little while (but not long enough to get rid of the periostracum). So, no - not lunch. Urgh. Aliens? That usually works...

[doushantuo]

can recommend:

 

 

(below: Contributions to the knowledge of marine molluscs in Pliocene West European basins)

(Dutch Geol.Survey)

 

 

 

 

Heering's 1959 in the MRGD treatise I can't find anywhere 

these two are too large too post,but are informative in this context

 

[Max-fossils]

On 5/22/2018 at 4:22 PM, Spongy Joe said:

Hi Max, and thanks for the detailed rebuttal!

    OK... I am way out of my area here, and don't want to go querying your identifications, but I will anyway!  How certain are you of the species? In both I can see significant differences from the species you suggest, especially in the details of the hinge area, and also to some extent the outlines. (Also, does the pallial sinus match for Spisula? I can't quite see because of the reflection. ) This may be intraspecific variation, but it seems to be somewhat too much for comfort... but as I say, this isn't my area!

 

    If the species are indeed good, then yes, that totally scuppers the freshwater idea. In that case, I'm going to throw another mad idea at you: lunch. I imagine the large clam would be very edible, as well as the mussels, and perhaps someone had a seafood picnic there many moons ago. Perhaps even the workmen doing the dredging - shellfish were cheap, after all.

 

Edit: I'm now going to scupper that idea too: they're worn, including the inner surfaces, so were washed around on a beach for a little while (but not long enough to get rid of the periostracum). So, no - not lunch. Urgh. Aliens? That usually works...

Aliens seem to be the most scientifically accurate option for now. And I wish I could say that with sarcasm...  

 

I'm not too sure about the ID's. I'm about 90% sure that the first one is a Mytilus species, but not definitely Mytilus edulis. 

For the second one, Spisula subtruncata is the closest I could come up with. But I do agree that it looks slightly different, and mostly MUCH BIGGER (than any of the specimens I have seen in large quantities here on the Dutch coast). I'll try and get a more definite ID (I have a contact which is very good with modern North Sea shells, he'll likely know). 

 

About the lunch idea, in addition to them being worn as you said, another thing that rules that out is that the mussel is way too small to have contained anything worthwhile eating. According to Wiki, Spisula aequilateris and S. solidissima are edible (among others within the Mactridae), so it is probable that S. subtruncata also is. Then again I have never heard of it being eaten, and I wonder if it is even ever sold as food. But it's definitely not a popular food source (if it is one at all), so I would be surprised if people had eaten this species. 

 

Thanks though!

[Max-fossils]

On 5/22/2018 at 4:35 PM, doushantuo said:

 

 

can recommend:

 

 

 

(below: Contributions to the knowledge of marine molluscs in Pliocene West European basins)

(Dutch Geol.Survey)

 

 

 

 

 

 

 

Heering's 1959 in the MRGD treatise I can't find anywhere 

these two are too large too post,but are informative in this context

 

 

4

Thanks 

[DPS Ammonite]

Don't forget about the non human animals that may have moved their lunch to a better spot. I have seen recent shells a mile or so from the California coast: birds?

[abyssunder]

Both bivalves seem to have bioerosional signs on the inner and outer surface of the shell, borings similar to Caulostrepsis, so the bivalves were bored in saltwater.
The first one looks like a Mytiloid / Mytilid (don't know which genus/species) mussel.
The second one might be Mactrid, possibly Mactra (Spisula).
They look like they were tumbled in the sea wash for a decent time, but it's hard to say if they are fossils or not, I'm afraid.

[Max-fossils]

On 5/25/2018 at 9:31 PM, abyssunder said:

Both bivalves seem to have bioerosional signs on the inner and outer surface of the shell, borings similar to Caulostrepsis, so the bivalves were bored in saltwater.
The first one looks like a Mytiloid / Mytilid (don't know which genus/species) mussel.
The second one might be Mactrid, possibly Mactra (Spisula).
They look like they were tumbled in the sea wash for a decent time, but it's hard to say if they are fossils or not, I'm afraid.

I agree with your ID's, but mine are more precise (although still fitting in yours). I also agree with the signs that they have likely been tumbling in the sea for a while. 

I'm leaning to the non-fossil side for now, but I am really confused as to how they could've gotten here...

 

On 5/25/2018 at 7:15 PM, DPS Ammonite said:

Don't forget about the non human animals that may have moved their lunch to a better spot. I have seen recent shells a mile or so from the California coast: birds?

Well I looked on a map and we're about 60 or so kilometers from the open North Sea (I'd be surprised if a seagull would fly that far with a shell in its mouth). Plus, the signs show that the shells have been tumbling in the sea for a while, meaning they were already dead (and empty) at that point. And I don't see why a bird would bother to bring an empty shell all the way to here. 

 

But the estuary of the Scheldt river does go further into the land, up to a point where it is only 5 or so kilometers from the location. But I don't think that the water is still salty that far into the land, it's probably sweet. I do think that the two species here are able to live in brackish water, so if we can find out where in the Scheldt the water is salty to brackish, we can more easily calculate how far the minimum distance from their original habitat is. 

 

Thanks for the help though guys!

[Bathollovian]

On 22/5/2018 at 4:35 PM, doushantuo said:

 

 

What is the complete reference of this paper please ?

[abyssunder]

1 hour ago, Bathollovian said:

What is the complete reference of this paper please ?

F. C. Wählisch et al. 2014. Surviving the surf: The tribomechanical properties of the periostracum of Mytilus sp. Acta Biomaterialia 10(9): 3978-3985

link

[Bathollovian]

Thanks, really nice paper!

[doushantuo]

you're welcome,Bathohollovian PM me if you want good (tips on) periostracum/molluscan biomineralization literature

[Bathollovian]

Thanks doushantuo I wouldn't hesitate!