Posts Tagged ‘isotelus’

Isotelus Molts

February 5, 2011

Working an outcrop in the Clays Ferry Formation that is in the same bed, and close to, another outcrop that I have collected extensively(see here, here, and here), I came across some curious molts of an Isotelus gigas.

The Clays Ferry Fm was deposited in a relatively shallow subtidal environment, and throughout most of the area, the formation straddles the Mohawkian/Cincinnatian boundary(early Late Ordovician). Due to deposition below wave base in typical weather, most of the fossil are complete and a diverse assemblage can be found in the column.

The Bed form which this piece comes is composed primarily of orthoconic cephalopods, Isotelus, a few bivalves, and an occasion Flexicalymene trilobite.

What I find curious about the molts is found on pygidium and thorax of the smaller dorsal specimen. I have annotated it with a white line to highlight the area.

There is a crack and indentation that runs from the anterior posterior(Jeeez, I just caught that) margin through the pygidium, and from there it appears as if it continues on through the pleurons. As there is no suture along that area of the thorax, I would assume that the distal portions of the pleurons would be adjacent to the rest of the critter, but they are not there, So that has left me wondering if the structure is pre-molt, or if it is an artifact of lithification. Any Ideals?


August 20, 2010

Year ago while working an outcrop that has produced some stunning asasphids, I found this little Isotelus gigas. It appears that the little guy had a bite taken out of his cephalon, enrolled and died. I found it in a bed of the Clays Ferry Fm(early Late Ordovician) in Anderson Co. Kentucky. The bed is comprised of, almost entirely, I. gigas molts and orthocerid cephalopods.

EDIT: the bar at the bottom is 1 inch(2.54cm)

Trilobite Molts-Up Is Down, And Down Is Up

July 1, 2010

A while back, Chris over at Ediacaran put up a post in his “Paleoporn” series describing the environment and sedimentology of two sites, in the same formation, where the same Cambrian trilobite is found. His post is a description of the orientation of the molts in the two different environments. In the comments, we discussed the curious habit of finding inverted cephalons, ie, ventral up cephalons while the rest of the critter is found in its normal position(dorsal up).

In a thin bed of the Clays Ferry Formation in Anderson Co. Kentucky, I find numerous partial molts from the trilobite Isotelus gigas. Some of the bedding planes are almost entirely composed thoracic segments, cephalons and cranidiums, and pygidiums from this large asasphid. Occasionally, one comes across the curious preservation described above.

During ecdysis, the cephalic sutures of the trilobites rupture allowing the critter to escape from the front of its carapace. Sometimes during the egress, the critter will push the cephalon upside down. Hence, what we have here.

When I found this one, it didn’t appear that much was there.

But, I brought home anyway. While hitting it with a scribe, I noticed the ventral, but I thought that it was trash and nearly blew through it… as more became apparent, I slowed up a bit(word to the wise).

I have found several more from the same locality, but those are all that I have photographed. However, here is a nice juvenile that appears to have had a bite taken out it.

More on that later.