Jump to content
andyrice11

G. celebra trilobite with two distinct mineralizations

Recommended Posts

andyrice11

This post is about a well preserved Gravicalymene celebra molt I recently found in the Laurel member of the Salamonie formation of Southeastern Indiana. It is quite a peculiar specimen since it appears to have two very distinct mineral compositions. Most of the trilobite is composed of dolomite as is typical for fossils found in the Laurel. However, I initially noticed what appeared to be white calcitic pieces of the cephalon partially exposed at the anterior end of the specimen. The matrix surrounding these pieces was very easy to remove, having a fine sand like consistency. After some prep work, I was able to uncover a good portion of the glabella and concluded that these white pieces did indeed belong to the same specimen. My initial thought was that they are composed entirely of calcite, but I haven't been able to make that conclusion so I decided to post some detailed pictures in order to see what you all think.

 

Figures 1&2. Specimen in ventral and lateral views.

5aebc883e8e3d_ventralview.thumb.jpg.9cbb60cc43b1003a67adbaefe1f52670.jpg

(Before prep work, the white rostral plate and lateral border (cephalic doublure?) which are quite obvious in the above picture, were only partially exposed.

Initially my professor suggested that they likely belonged to a separate fossil specimen, perhaps a bryozoan.)

5aebc92dec7d9_lateralviewanteriorfacingtotheright.thumb.jpg.d60fa5813cf326b53199c7827462db85.jpgAnterior end

 

Figure 3. Anterior view showing the left and right lateral borders (cephalic doublures?), rostral plate and patrially exposed glabella.

 5aebca4c04e80_freecheeksrostralplateandglabella.thumb.jpg.7ca4dc27a8ab3ed790ade9e48c250a17.jpg

 

Figures 4&5. Magnified images of the rostral plate displaying uniform bumpy texture on the surface.

5aebcd43a96dd_anothercloseuponrostralplate.thumb.jpg.0fe48c2684e1240da46bbfdb069e49d2.jpg5aebcd3711c58_closeupofbumpsonrostralplate.thumb.jpg.9230a809e82b4abcc7e7c9f5dbb32f14.jpg\

 

Figure 6. Magnified image of dolomitized lateral border (cephalic doublure?). Note the absence of 

the bumpy texture seen in the previous images.

5aebcda233380_dolomitizedlibrigena.thumb.jpg.a5be0522bdaba7f886a4106fb6e5f235.jpg

 

 

So essentially my main questions are:

1. Could this white colored mineral be calcite, or something else?

2. Are the long narrow pieces considered cephalic doublures or just lateral borders? (In my research, I haven't been able to find a detailed description of Calymenid cephalic anatomy)

3. What exactly are the uniform bumps found on the white pieces?

4. Is double mineralization of a single specimen a rare occurrence, or has anyone seen something like this before?

5. What could this mineralization mean in terms of the taphonomic interpretation of this specimen. 

 

 

An interesting side note:

A few weeks later I was once again fossil hunting in the spot at which I found the specimen described above. Along with some nice brachs and another full trilobite, I found a partial mold of a G.celebra thorax. I looked and looked for the specimen it may have once been attached to, but was unable to find anything. After returning to my lab, I noticed something quite interesting. It turns out that the mold belonged to the specimen I had collected just a few weeks before! I was glad to have found this mold, since it shows the morphology of part of the specimens posterior half which has been weathered away.

 

Figure 7. The partial mold 

 mold.thumb.jpg.bebe693e23d0b8e636af4fbfee6312bf.jpg

 

Figure 8. Specimen and mold side by side.

5aebd057532c6_moldandspecimen.thumb.jpg.1ae1b807456e570b9ddb9afdfb07848e.jpg

 

Figure 9. Reunited and it feels so good! :)

5aebd0ccdc3a6_moldandspecimenpiecedtogether.thumb.jpg.59cc7c51b032f8a0bc07e0dfd6f7f47c.jpg

Share this post


Link to post
Share on other sites
doushantuo

Nadine Wilmot published some great stuff on the original mineralogy and structure of the trilobite exoskeleton ,and has commented on the effects of diagenesis

Share this post


Link to post
Share on other sites
andyrice11

UPDATE:

It just occurred to me that I haven't yet to tested the effects of HCl on this specimen, so I just conducted a test. I put a drop of acid on the dolomitized portion of the specimen as a control and saw no reaction. Then I tested a small portion of the supposed calcitic doublure/lateral border and witnessed a very intense reaction! So it appears as though the mineral composition is indeed calcitic!

Share this post


Link to post
Share on other sites
doushantuo

While fully realizing that the scale is rather smallerB):ninja:

(McAlllister et al,source publication indicated)

egudgeslkkifernakristlanthc.jpg

Share this post


Link to post
Share on other sites
doushantuo

From Wilmot's thesis thesis:taphonomy of terrace lines

eudgesllifernakristlanthc.jpg

Share this post


Link to post
Share on other sites
Ludwigia
1 hour ago, andyrice11 said:

UPDATE:

It just occurred to me that I haven't yet to tested the effects of HCl on this specimen, so I just conducted a test. I put a drop of acid on the dolomitized portion of the specimen as a control and saw no reaction. Then I tested a small portion of the supposed calcitic doublure/lateral border and witnessed a very intense reaction! So it appears as though the mineral composition is indeed calcitic!

I was about to suggest that, but you beat me to it. At least that question is settled now. I'm thinking that the white pieces were originally composed of a substance which didn't allow the magnesium in. Either that, or for some reason it was removed at a later stage. Just thinking out loud. Paragenesis and diagenesis are very complicated subjects.

Share this post


Link to post
Share on other sites
andyrice11
14 hours ago, piranha said:

 

figures from:...

 

Thanks piranha! I knew I could count on you, haha. Any idea what the bumps along the surface of the calcite pieces might be? I find it odd that they are so uniformly spaced from one another.

EDIT: I see what the bumps are now after actually reading everyones replies in depth :doh!:

Share this post


Link to post
Share on other sites
andyrice11
16 hours ago, doushantuo said:

Nadine Wilmot published some great stuff on the original mineralogy and structure of the trilobite exoskeleton ,and has commented on the effects of diagenesis

Thank you! I will definitely check out some of Wilmot's work and see if I'm able to make any interpretations from it! :)

Share this post


Link to post
Share on other sites
andyrice11
14 hours ago, doushantuo said:

From Wilmot's thesis thesis:taphonomy of terrace lines

eudgesllifernakristlanthc.jpg

 

Ahh, i just now realized the infilled canals referred to on image "C" are those evenly spaced bumps. Those look identical to the bumps on my specimen. Awesome! It's remarkable how much detail is being preserved in these specimens!

Share this post


Link to post
Share on other sites
doushantuo

 

tral.jpg

Share this post


Link to post
Share on other sites
Lmshoemaker
On 5/3/2018 at 11:42 PM, andyrice11 said:

This post is about a well preserved Gravicalymene celebra molt I recently found in the Laurel member of the Salamonie formation of Southeastern Indiana. It is quite a peculiar specimen since it appears to have two very distinct mineral compositions. Most of the trilobite is composed of dolomite as is typical for fossils found in the Laurel. However, I initially noticed what appeared to be white calcitic pieces of the cephalon partially exposed at the anterior end of the specimen. The matrix surrounding these pieces was very easy to remove, having a fine sand like consistency. After some prep work, I was able to uncover a good portion of the glabella and concluded that these white pieces did indeed belong to the same specimen. My initial thought was that they are composed entirely of calcite, but I haven't been able to make that conclusion so I decided to post some detailed pictures in order to see what you all think.

 

Figures 1&2. Specimen in ventral and lateral views.

5aebc883e8e3d_ventralview.thumb.jpg.9cbb60cc43b1003a67adbaefe1f52670.jpg

(Before prep work, the white rostral plate and lateral border (cephalic doublure?) which are quite obvious in the above picture, were only partially exposed.

Initially my professor suggested that they likely belonged to a separate fossil specimen, perhaps a bryozoan.)

5aebc92dec7d9_lateralviewanteriorfacingtotheright.thumb.jpg.d60fa5813cf326b53199c7827462db85.jpgAnterior end

 

Figure 3. Anterior view showing the left and right lateral borders (cephalic doublures?), rostral plate and patrially exposed glabella.

 5aebca4c04e80_freecheeksrostralplateandglabella.thumb.jpg.7ca4dc27a8ab3ed790ade9e48c250a17.jpg

 

Figures 4&5. Magnified images of the rostral plate displaying uniform bumpy texture on the surface.

5aebcd43a96dd_anothercloseuponrostralplate.thumb.jpg.0fe48c2684e1240da46bbfdb069e49d2.jpg5aebcd3711c58_closeupofbumpsonrostralplate.thumb.jpg.9230a809e82b4abcc7e7c9f5dbb32f14.jpg\

 

Figure 6. Magnified image of dolomitized lateral border (cephalic doublure?). Note the absence of 

the bumpy texture seen in the previous images.

5aebcda233380_dolomitizedlibrigena.thumb.jpg.a5be0522bdaba7f886a4106fb6e5f235.jpg

 

 

So essentially my main questions are:

1. Could this white colored mineral be calcite, or something else?

2. Are the long narrow pieces considered cephalic doublures or just lateral borders? (In my research, I haven't been able to find a detailed description of Calymenid cephalic anatomy)

3. What exactly are the uniform bumps found on the white pieces?

4. Is double mineralization of a single specimen a rare occurrence, or has anyone seen something like this before?

5. What could this mineralization mean in terms of the taphonomic interpretation of this specimen. 

 

 

An interesting side note:

A few weeks later I was once again fossil hunting in the spot at which I found the specimen described above. Along with some nice brachs and another full trilobite, I found a partial mold of a G.celebra thorax. I looked and looked for the specimen it may have once been attached to, but was unable to find anything. After returning to my lab, I noticed something quite interesting. It turns out that the mold belonged to the specimen I had collected just a few weeks before! I was glad to have found this mold, since it shows the morphology of part of the specimens posterior half which has been weathered away.

 

Figure 7. The partial mold 

 mold.thumb.jpg.bebe693e23d0b8e636af4fbfee6312bf.jpg

 

Figure 8. Specimen and mold side by side.

5aebd057532c6_moldandspecimen.thumb.jpg.1ae1b807456e570b9ddb9afdfb07848e.jpg

 

Figure 9. Reunited and it feels so good! :)

5aebd0ccdc3a6_moldandspecimenpiecedtogether.thumb.jpg.59cc7c51b032f8a0bc07e0dfd6f7f47c.jpg

 

My best guess is that the specimen only experienced partial dolomitization? I hear the Laurel is mostly barren due to the pervasive dolomitization. If that is the case it could be possible  that the occurrence of this dual mineralization here is due to the minimal dolomitization. 

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Recently Browsing   0 members

    No registered users viewing this page.

×