G. celebra trilobite with two distinct mineralizations

[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.

(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.)

Anterior end

 

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

 

 

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

\

 

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

the bumpy texture seen in the previous images.

 

 

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 

 

 

Figure 8. Specimen and mold side by side.

 

Figure 9. Reunited and it feels so good! 

[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

[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!

[doushantuo]

While fully realizing that the scale is rather smaller

(McAlllister et al,source publication indicated)

[doushantuo]

From Wilmot's thesis thesis:taphonomy of terrace lines

[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.

[piranha]

5 hours ago, andyrice11 said:

...(In my research, I haven't been able to find a detailed description of Calymenid cephalic anatomy)...

 

 

figures from:

 

Siveter, D.J. 1973
Trilobites of the family Calymenidae from the Ordovician, Silurian and Devonian systems of north-west Europe. 
PhD Thesis, University of Leicester, 382 pp. 92 plts.   PDF LINK

 

Kaesler, R.L., ed. 1997
Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, Revised. 
Geological Society of America, University of Kansas Press, 530 pp.

 

 

 

text from:

 

Whittington, H.B. 1971 

Silurian calymenid trilobites from the United States, Norway, and Sweden. 

Palaeontology, 14(3):455-477   PDF LINK

 

Gravicalymene (=Flexicalymene) celebra

 

Description. Occipital ring longest (sag. and exs.) behind median glabellar lobe, shorter and curving forward abaxially, with slight inflation at distal tip. Occipital furrow deep behind median lobe, narrower and extended into apodeme behind inner part of lateral lobe 1p, at extremity shallower. Lateral glabellar lobe 1p oval in outline, long axis longitudinal, inflated above adjacent part of median lobe. Lateral furrow 1p shallow abaxially, deepening where it curves around inner, anterior side of lobe 1p, on adaxial side of inner part a faintly inflated ovate area. Lateral lobe 2p about half length (exs.) of lobe 1p, similar in outline and inflation. Lateral lobe 3p a small inflation on the lateral slope of the glabella, furrow 3p a faint, short indentation. Axial furrow shallow beside occipital ring, beside lateral lobe 1p a deep, steep-sided trench, broader with steeply sloping sides in front of here; shallow anterior pit near anterior extremity. Preglabellar furrow narrow medially, widening abaxially; anterior border short (sag. and exs.), the slope from the preglabellar furrow flat, inclined gently upward and forward, on anterior side curving abruptly over to slope downward and backward. In anterior view border moderately arched. Eye lobe small, mid-point in transverse line with mid-point of lateral lobe 3p; anterior branch of suture directed straight inward and forward, turning more strongly inward as it crosses border, on anterior slope of border aligned with connective suture; rostral suture traverses along upper part of anterior slope. Mould of inner surface of border sector of rostral plate (PI. 87, fig. 10) suggests that this plate is similar to that of F. senaria (Evitt and Whittington 1953, pl. 9, figs. 3-6). Antero-lateral cephalic border moderately convex, curled under so that doublure slopes upward and outward, the inner edge ventral to the broad, shallow border furrow. Inner part of posterior border narrow (exs.) and convex, outer part broader (exs.) and less convex; blunt genal angle. Posterior branch of suture directed straight outward and slightly backward from eye lobe, then curving through an oblique angle and continuing to the border furrow; here it curves again and is directed exsagitally to the genal angle.

 

Thorax of thirteen segments, axial rings with conspicuous rounded inflation adjacent to axial furrow. Articulating furrow with deep, elongate (tr.) pit distally, the external expression of the apodeme. Inner part of pleura narrow (tr.), horizontal, outer part wider (tr.) and steeply curved down from fulcrum. The deep, slightly diagonal pleural furrow shallows at the margin of the large facet, the furrow continued a short distance on the facet by a much narrower and shallower groove. Posterior band of pleura convex, continued along the margin of the facet to the tip. Articulating boss on anterior band of pleura at fulcrum. Internal moulds (PI. 88, fig. 8) show a curved groove, deepest adaxially, extending along the anterior edge of the facet, abaxially the groove dies out and is continuous with the flat, narrow doublure which extends around the tip of the pleura and along the posterior margin of the facet. The groove on the internal mould is the mould of the ridge which forms a stop to enrollment (cf. Campbell 1967, pl. 10, fig. 15). In the internal mould slim pillars may be observed, connecting the ventrally facing outer tip of the facet to the mould of the inner surface. These are infillings of canals through the exoskeleton. Axis of pygidium divided into six rings and the low, rounded terminal portion; first ring furrow deepest and broadest (sag. and exs.) medially, successive furrows progressively shallower medially so that sixth furrow is impressed distally only. Pleural region with first pleural furrow deepest, extending to facet; succeeding four pleural furrows progressively shorter and shallower, impressed on inner pleural region only, and successively more strongly backwardly directed. There are thus four distinct pleural ribs, only the first two impressed adjacent to the axial furrow by the beginning of an interpleural furrow. A low fifth rib runs back beside the tip of the axis, and dies out on the border. The outer part of the pleural region is smooth, the doublure curled under, the exoskeleton traversed by canals, the infillings of which appear as incomplete pillars in the internal mould (PI. 87, fig. 13). External surface, except in the bottom of the deep, narrow furrows and on the facets, covered by a dense, fine granulation. Scattered larger tubercles are present on the glabella, cheek inside the border furrows, axial rings, posterior pleural bands and ribs of pygidium; the largest of these tubercles are on the glabella.

 

Discussion. Two additional specimens from localities north of Chicago and probably the Racine Dolomite, may belong to this species. One (PI. 87, fig. 5) shows the narrow (sag. and exs.) anterior border, the inner side of which slopes gently upward and forward, the lack of a buttress opposite lateral lobe 2p, and the sinuous course of the posterior branch of the suture. The second (PI. 89, figs. 8, 11) shows the internal surface of part of the cephalon and thorax, and has the hypostome in place. The hypostome is like that of Flexicalymene (Evitt and Whittington 1953, pi. 9, figs. 8, 9), shows the pit in the anterior wing (the external expression of the wing process), and that the process rests against the anterior slope of the anterior pit of the dorsal exoskeleton. The absence of papillation of glabellar lobes and corresponding buttresses, suggests that this species belongs in either Gravicalymene or Flexicalymene. In the present state of knowledge the form of the anterior border is decisive in distinguishing between these two genera. In Ordovician species of Gravicalymene (Dean 1962, 1963; Ross 1967b) the preglabellar furrow may be narrow or broad (sag. and exs.), the anterior border is relatively broad (sag. and exs.), stands well above the preglabellar furrow and may have a flattened upper surface. This surface may be approximately horizontal, or may slope forward and upward; at the posterior edge it joins the wall of the preglabellar furrow, at the anterior edge it curves abruptly over into the outer face of the border, which slopes downward and backward. I am not aware of any Silurian species referred to Gravicalymene, but a number of Lower Devonian species from Bohemia, North Africa, Turkey, Australia, and New Zealand have been referred to this genus (Shirley 1938, p. 487, pi. 44, fig. 17; Philip 1962, p. 231; Talent 1963, pp. 105-106; Strusz 1964, pp. 94-95; Talent 1965, p. 49; Haas 1968, pp. 101-103; Alberti 1969, pp. 413-414). In these species the preglabellar furrow is deep, the anterior border of moderate width (sag. and exs.) and semicircular in cross section, not flattened on the upper surface. Certain Australian authors have expressed doubt that these species are congeneric with Ordovician species, and this question remains open - the Devonian species may be derived from a different ancestor. 

 

The present species celebra has the anterior border rising in a flat slope upward and forward from the base of the preglabellar furrow, and at the anterior margin it turns abruptly over to slope downward and backward as the outer face of the border. This shape is typical of Ordovician species of Flexicalymene (Dean 1962, 1963; Whittington 1965; Ross 1967b), as is the lack of 2p papilla and buttress, though the outline of the glabella is slightly bell-shaped rather than parabolic. On balance I prefer to place it in this genus, and the hypostome attributed to it is like that of species of Flexicalymene rather than of Calymene (Campbell 1967, pi. 10, figs. 1-7; Haas 1968, pi. 29, figs. 6, 7, text-fig. 16b, d,f). Haas (1968, pp. 100-101) has recently described a high Llandovery to Wenlock aged species that he attributes to Flexicalymene, and Temple (1969, pp. 224-228) an early Llandovery form. The anterior border is not well preserved in Haas's material, and the cast figured by Temple (1969, pi. 6, fig. 1) has an anterior border apparently of Gravicalymene type. Thus celebra is the only Silurian species that I consider with some confidence may be placed in Flexicalymene.

 

 

 

***Note: "Many of these taxa have been revised over the past 170 years since the first specimens were reported. Interestingly, the best-known taxon, Gravicalymene celebra, has been the subject of the most name changes, including Calymene blumenbachii, Calymene niagarensis, Calymene celebra, Flexicalymene celebra, Apocalymene celebra, Sthenarocalymene celebra, and Gravicalymene celebra. Proposed revisions continue (see Chestnut, et. al., 2006, 2008)."

 

Kleffner, M.A., Cramer, B.D., Brett, C.E., Mikulic, D.G., Kluessendorf, J., & Johnson, T. 2012

Lower Silurian of western Ohio - The case of the disappearing Dayton, and unique Midwestern co-occurrence of pentamerid brachiopods with the Gravicalymene celebra Trilobite Association in the Springfield Formation. Geological Society of America, Field Guide, 27:1-18

[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 

[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!

[andyrice11]

14 hours ago, doushantuo said:

From Wilmot's thesis thesis:taphonomy of terrace lines

 

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!

[doushantuo]

 

[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.

(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.)

Anterior end

 

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

 

 

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

\

 

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

the bumpy texture seen in the previous images.

 

 

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 

 

 

Figure 8. Specimen and mold side by side.

 

Figure 9. Reunited and it feels so good! 

 

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.