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Heteromorph

I found this Phlycticrioceras trinodosum heteromorph specimen in June of 2018 whilst hunting the middle/upper Coniacian Atco formation. It is the largest fragment of this species that I am aware of, having a whorl height of 51 mm as opposed to 47 mm of the largest fragment I've seen published. This genus is a bigger, rarer, and (mostly) younger cousin of Allocrioceras. I sent pictures of it to Keith Minor and he pointed out that there was also an echinoid sticking out of the specimen, something which I had totally missed! With much of the echinoid still stuck in the living chamber it is hard to get a definitive ID. But because it has such a shallow anterior ambulacra, which gives the anterior end a more smooth rather than definitive heart shape, he ruled out both Mecaster texanus and batensis. He suggested Micraster since the site has a strong European component in both the bivalve and ammonite faunas, and because the periproct side has the right shape. From finding other, although not as well preserved specimens that show similar morphology he appears to be right.

 

I have yet to confirm this ID with Andrew Smith, but either way I think the piece is worth showing. And reading this thread got me thinking about how this could have happened and what effect it could have had on the echinoid's preservation. My thought is that because irregular echinoids lived and today still live most of their lives burrowing in the sediment it is unlikely that it would have crawled into the living chamber, but instead that it was blown into it post-mortem via currents that had dredged it out of the sediment.  I already know that this site was a high energy environment from my other finds here so this seems the most likely possibility to me. But because of the fact there is still at least one spine still attached to the specimen it could not have been swept up from the sediment too long after death or all of its hairlike spines would have blown away. I do, however, find it interesting that it is positioned anterior first with its posterior towards the aperture, the position I would expect to see it in if it had indeed crawled into the shell.   

 

The specimen is also the best preserved echinoid from this site so far. Despite the ammonites being generally well preserved and not too crushed, most of the echinoids that I have from the site are terribly crushed, flakey, and often infested with rotting pyrite. I think being encapsulated in the chamber very much reduced those effects. Even though the ammonite and the echinoid are a bit crushed, the echinoid would have probably been worse off otherwise. 

 

The heteromorph fragment length is 70 mm and the whorl breadth, being a bit crushed, is 13 mm.  I would think that this specimen, with its open planispiral coiling, would would have been at least over a foot in diameter when complete. It is the robust (female) morph of the species with a rib index of 5½. For comparison in Fig. 1 I pictured it with my most complete P. trinodosum specimen. From the part of the echinoid that is exposed I can measure 25 mm in length, 25 in width, and a thickness of 8 mm. 

 

I have also found abundant yet scattered fish remains at the site, so perhaps one day an ammonite-fish will come my way. But until then, anyone else got ammonite-echinoids to show?

 

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Fig. 1

 

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Fig. 2.

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Heteromorph

fullsizeoutput_4bf7.thumb.jpeg.8d1935fc8d9d3b348acf66a3521ebfef.jpeg

Fig. 3.

 

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Fig. 4.

 

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Fig. 5. View of the dorsum.

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Heteromorph

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Fig. 6. View showing abapical whorl section.

 

IMG_7117.thumb.JPG.ad618add378a20dc4003277ac68202ce.JPG

Fig 7.

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Heteromorph

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Fig 8. Angled view showing many tubercles. A spine is directly above the missing test. 

 

IMG_7153.thumb.JPG.3e9456d6da193430702a5b14c6696394.JPG

Fig. 9.

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IMG_7154.thumb.JPG.e78ddd1b959088b54b57793251d922fb.JPG

Fig. 10.

 

IMG_7150.thumb.JPG.bbde7d789ef89ce5734f043d3841ba22.JPG

Fig. 11.

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Heteromorph

IMG_7166.thumb.JPG.18d1ac16203da0d2c940011096ffbb9e.JPG

Fig. 12.

 

IMG_7141.thumb.JPG.8b513275088c4d21b4039f6992085f91.JPG

Fig. 13.

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IMG_7175.thumb.JPG.d6065902e8229c50fdae897d7aa3141d.JPG

Fig. 14.

 

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Fig. 15. This and the next picture show the minute ridges on the spine and its connection to the tubercle. 

 

IMG_7165.thumb.JPG.fd3d2e4ca5858cdcb28450e2a318f165.JPG

Fig. 16.

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IMG_7129.thumb.JPG.faa457f638e14e204d4f8f87df2d18b8.JPG

Fig. 17.

 

fullsizeoutput_4a71.thumb.jpeg.b78700398af2dead2bbda5e2018398da.jpeg

Fig. 18.

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IMG_7123.thumb.JPG.dd77c9baa2331498948f342d38492b15.JPG

Fig. 19.

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Wrangellian

I don't have any ammonites with echinoids to show but since your fauna and mine are similar in age and membership, it's interesting. I have seen recent dead echinoids washed up on the beach that still have some spines on them, but not sure how this works under the sea, and I've got fossil echs similar to yours with spines on them from my local collecting site also, but not sure how long they were dead before they were buried. I only recently noticed the spines, as they're not very obvious.

I do have one chunk of shale with a heteromorph and an echinoid on it nearby, which is the closest thing I have to what you're showing, but it's broken and I need to reassemble it. Maybe I can get a pic of it dry-fitted anyway.

I think this is the first time I've seen a proper pic of your little complete(ish) trinodosum specimen other than in your avatar. That's a nice fossil.

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