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Trilobites, Brachiopods, and Crinoids From Georgia's Silurian


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I had the opportunity to collect in the Red Mountain Formation recently, and considering the seeming lack of accessible sites in the area (RIP Tibb's Bridge) I thought it'd be good to show some of my finds here and say there is some stuff out there. At first I thought the site was in the Mississippian Lavender or Floyd Shales, which was my initial reason for venturing out to it as I didn't have the opportunity to collect in marine Mississippian units closer to home. I can say now with almost 100% certainty it's actually within the Red Mountain Formation, an early Silurian unit that is also a prominent ridge former in northwestern Georgia. This is based on the trilobites I found. 

 

I am more familiar with the Rose Hill Formation of central Appalachia, which bears some similarities with the Red Mountain. Not only do the two begin with "R" (and consist of two words XD), but more importantly both represent roughly similar stratigraphic sections and depositional environments. This was something I instantly noticed at this site, as some of the fossils, their mode of preservation, and the rock lithology is strikingly similar to the Rose Hill further north. Both the Red Mountain and the Rose Hill consist primarily of clastic units, particularly iron-rich sandstones and silty-shales. They are both dominated by shelly invertebrate fauna, and contain many of the same genera. There are some differences, however, namely that the Rose Hill contains a much more diverse fauna, is more abundantly fossiliferous, has many ostracodes and some tentaculitids (which the Red Mountain lacked), and contains more limestone and calcareous layers, whereas the Red Mountain appears to be mostly coarser-grained clastics. Likewise the Red Mountain Formation has more abundant crinoid remains, is a ridge-forming unit, and does not appear to have a Rochester Shale equivalent, at least from what I've seen, which is interesting. 

 

Of course these are just some quick observations I noted from a couple of outcrops, so take all of that with a heavy grain of salt. I know the Red Mountain does contain limestone and calcareous shale layers in more complete sections at Birmingham, and the exposure I went to was obviously not complete. Elsewhere the Red Mountain has produced a more varied and abundant fauna, and conversely the Rose Hill is largely devoid of fossils in many sections. 

 

Anyways, here's what everyone's been waiting for:

 

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Calymene sp. (?) 

 

I've seen a couple of threads about the Red Mountain Formation in Georgia, but none mentioned trilobites from it. According to the Georgia Department of Mines, Mining, and Geology, the Red Mountain has a a few species of trilobites. This is a plate containing a pygidium and part of the lower thorax of an individual, and a possible fragmentary glabella/cephalon. These are likely molt fragments. This was also my very first find of the day, so a great way to start it off XD. There are probably more specimens at the site, as the exposure was actually fairly large (for what I've seen in this state) and exposed a decent section of the interbedded shales and siltstones, but it was very overgrown and I couldn't reach it. This was laying on the soil. 

 

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I'm going to go on a limb and say Anoplotheca hemispherica ?

 

A plate with a couple of brachiopod shells on it. The Red Mountain's shale layers are said to be more fossiliferous than the sandstones, which is what I found to be the case at this site. Even then fossils weren't that common in even the most fossiliferous layers. 

 

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A well worn plate containing abundant crinoid columnals and brachiopod fragments. I'm going to go out on a limb again and say the larger shell fragment is a Dalmanella (?) sp. , but that is a guess. 

 

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A plate with a Dalmanella (??) sp. and crinoid fragments. The report I'm basing this off of is old, so a lot of these names probably no longer apply. 

 

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Lots of crinoid pieces in this rock. 

 

All in all fairly successful. Any day you find a trilobite (even fragmentary) is a good day in my opinion XD

 

 For those of you interested, the site is near Summerville, near an evidently popular swimming location. The fossils actually came from several different "exposures", albeit all within close proximity to one another. If you want to visit I would highly recommend going in winter, when the plants will all be dead and the snakes/spiders down to a minimum. Thankfully I didn't come across any snakes, but I did find an alarmingly large piece of shed skin. 

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Thomas.Dodson

Very interesting, thanks for sharing. I collected a little bit of Red Mountain Formation in the general area when I was in Georgia but didn't get to spend too much time in it.

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