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Pygidium Spine From Ceraurus?


JeanB

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Hi,

I spent some time with my magnifying glass on a nice piece shale with many brachiopods (possibly Leptaena sp. or Rafinesquina sp.). This rock is from Trenton group, Ordovician (Quebec). I came across some kind of curved spine (not segmented), about 1.5 cm long next to a lovely brach. I excluded crinoid. Since I found several Flexicalymene sp. pygidium from the same site, I wondered if it could be a trilobite part. I searched for genera that could match. I found that Ceraurus sp. have long pygidium spines about 1.5 cm in length. Ceraurus pleurexanthemus is a species found in Trenton, Quebec.

Two figures are presented below (I have moved the flash to bring out more 3D details).

Is that plausible?

Thanks!

P.S. If I am completely wrong, at least it was a lot of fun and I am on a learning curve :)

Jean

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post-11707-0-59853800-1398821767_thumb.jpg

JeanB

Montreal, QC, Canada

Ordovician, Trenton group

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Well, I think you're right that this is a piece of a trilobite. However, I would identify it as the underside of an Isotelus cephalon. The wrinkled flattened surface is characteristic of where the exoskeleton folds underneath the animal (a region called the "doublure"). We can exclude it being a Cerarus pygidium, because the area the "spine" is attached to is pointed in front (it should be more or less straight where it would join the thorax) and lacks an axis and pleural ribs.

Don

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Wow thanks Don! Isotelus crossed my mind also. But the cephalon spines seem to curve outward toward the end and I decided to exclude it on that basis. However, the «tip» of the genal spine is missing in my sample (after a closer look), so it may have bend outward... In addition to all the details you provided on the attachment to the cephalon. Fantastic!

There's a peculiar external «outline» on the sample (on the up «side»), as if there was some kind of shield on the dorsal side of the spine. I included a high resolution photo (the arrow point to the «outline» structure). Is that an artefact?

I know all these details must be very technical, but can you point me to some documentation on the structure of these trilobites? I find all this very interesting.

Jean

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Edited by JeanB

JeanB

Montreal, QC, Canada

Ordovician, Trenton group

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