Phyllocarid valve?

[Mainefossils]

This morning I split this shale (technically it fell apart on me), and found this interesting little fossil. I was thinking that there was a possibility of it being a phyllocarid valve, but I have never seen one. This also raises a question that I have been wondering - how do you differentiate between a phyllocarid and a bivalve valve when the tail is absent? What raised my suspicions on this specimen are the raised bumps on the external mold and the depressions on the cast.  

 

The pictures below are of the specimen. The first shows the cast/internal mold, and the second the external mold. It is from the Leighton Formation, Maine; which is Pridoli, Silurian. 

 

Thanks in advance for your help everyone! 

 

@Fossildude19 @mikeymig

 

 

[Rockwood]

I nearly called the texture off, but at that scale, I think you may be on to something.

[Fossildude19]

Texture is a helpful indicator for differentiation between bivalve and phyllocarid.

Bumpy texture is usually indicative of an arthropod. Also, there are sometimes growth lines visible along the edge of the carapace.

I think you have a phyllocarid valve there.

 

Devonian Rhinocarids, below :

 

 

 

 

[Mainefossils]

Thanks @Rockwood and @Fossildude19 for your opinions! I'm glad I posted this - it nearly got discarded as a Nuculites bivalve. Do you think that there is a possibility of identifying it to genus, or does it lack the details? Thanks for your time! 

[Fossildude19]

Asher, 

Are there any faunal lists for the Leighton Formation? 

That would be the place to check for the genus/species that may have been found there before. 

There is a chance that it could just be some odd preservation of a bivalve, but, my gut says this is probably phyllocarid. 

 

 

EDIT: Looks like Ceratiocaris is known from the Eastport Formation, so, maybe a possibility? 

[Mainefossils]

 

Thanks @Fossildude19 for the info! If this is a phyllocarid, it would be the first from this formation. You are right though, there is a Ceratiocaris sp. from the Eastport formation. It was mentioned in a correspondence from a R. H. Denison to Olcott Gates in 1968, but I cannot find any records or literature on it. It is also listed as part of the fauna of the Eastport formation by Churchill-Dickson(2004), but the specimen is not figured.  

 

As to the faunal list of the Leighton fm, I was going to do a detailed report on this in the near future, based on literature and an excellent book I have. I would be more than happy to PM it to you early, if you are interested. 

 

I will continue to research possible IDs for this phyllocarid, and will keep you all posted. Thank you all for your time! 

[mikeymig]

Nice Phyllocarid. I have only found one Silurian Phyllocarid and supplied you with a pic of it. Cool find. 

[Mainefossils]

A little bit of an update, based on some literature I have been reviewing. 

 

I have found that the phyllocarid genus Gonatocaris matches pretty well in the valve shape and size, but not in the texture. Do you think that the pits could have been imprinted? I have not found any Silurian phyllocarids that have this texture on the shell. If the pits were imprinted from foreign particulate, would you still think that this is a phyllocarid? What do you guys think? @Fossildude19 @mikeymig

 

Below is a plate of the carapace of this genus, from Collette, J. H., Plotnick, R. E. (2020) Redescription, paleogeography, and experimental paleoecology of the Silurian phyllocarid Gonatocaris. Journal of Paleontology, 94(5). 

 

 

And a two more plates from Collette, J. H., Hagadorn, J. W. (2010) Early Evolution of Phyllocarid Arthropods: Phylogeny and Systematics of Cambrian-Devonian Archaeostracans. Journal of Paleontology, 84(5). In this paper, Gonatocaris is called Emmelezoe, just a bit of a heads up. 

 

                                                                     

 

                                                                     

 

I also examined the surface of both the external mold and cast of my specimen, and think that the imprints have a high probability of being produced afterwards. It seems that both the cast and the external mold have the pits, instead of just the cast - possibly meaning foreign particulate during or after the fossil was made. Of course, it is really hard to tell, as the pits are very shallow, so I could easily be mistaken. One argument against them being imprinted afterwards is the rather regular pattern of them.

 

Of course, if they are actual pits, I would be at a loss to identify it. If they are not actual pits, they could be a bivalve. I am inclined, though, to think they are a phyllocarid of some sort - I have not found a bivalve that looks like this from the Leighton formation. The closest is Nuculites corrugatus, but the absence of a hinge, the rather pointed right side, and the growth rings on the edge of the valve of my specimen are against this identification. 

 

I think that it may be worth contacting the authors of the above papers. Thoughts? 

 

Thanks so much for reading this everyone! Sorry it was rather long-winded. 

[Fossildude19]

Asher, 

 

The pits could be an artifact of preservation.  Hard to say. 

I think it is always worthwhile to reach out to pro paleontologists when you have questions. 

Some are more helpful than others, but you don't know if you don't try.   

[Rockwood]

55 minutes ago, Fossildude19 said:

The pits could be an artifact of preservation.  Hard to say.

It was my first impression. The size convinced me that the texture is genuine, but there could be weathering too.