Some More Silurian Finds

[EMP]

I had the opportunity to visit another Silurian site in the northern Georgia/southern Tennessee area. This is now the third such site I've visited, but the first in the Rockwood Formation as opposed to the Red Mountain Formation. As far as I can tell there's very little different between the two lithologically and paleontologically, with the Rockwood and Red Mountain occupying pretty much the same stratigraphic position. The difference seems to be that the TGS prefers to use the term "Rockwood" to describe it's Niagaran Silurian system and the GGS and AGS prefer the term "Red Mountain", mostly because the unit is more differentiable in Tennessee whereas in southern NW Georgia and Alabama it is less differentiable. However, the GGS does use the term Rockwood in some of it's reports, and there are some lithological differences between the more southerly and easterly exposures and the more northerly and westerly ones (most notably in the thickness of the hematite beds), so I'm going with Rockwood Formation for these specimens. 

 

I had some difficulty in telling the age of the rocks at first. The geologic map I was using wasn't very accurate, and had both the Rockwood Formation and some upper Ordovician units within close proximity to each other. As you'll see with some of the fossils, there are some forms more associated with the Ordovician, such as Isotelus and Vinlandostrophia (Platystrophia), but at the same time I couldn't help but notice there were some similar characteristics between this fauna and the Rose Hill Formation, such as the calymenid molt fragments, and prevalence of Leptaena, which I did not find in the nearby upper Ordovician rocks (not to say it doesn't occur in the Ordovician, just that I didn't find it). Combined with the presence of Eospirifer and the thin beds of iron-rich sandstone and ferruginous limestone, this site is most likely in the Silurian Rockwood Formation. However, in my opinion, it appears to host an earlier, transitionary fauna than the Rose Hill. 

 

The collecting itself was pretty easy. The weather was nice, the site not too hard to explore, and the fossils easily extracted from the rock. Most of the exposure was unfossiliferous, however every now and then I'd come across a little part where there'd be a densely packed assemblage, with some loose specimens scattered about. The shale and thin limestone were the most fossiliferous. 

 

The first up are some of the brachiopods. 

 

 

A couple of different species it looks like, both tentatively of the genus Dalmanella sp. The report I'm basing this on is pretty old so that name probably no longer applies, however. 

 

 

These are pretty good examples of the more Ordovician forms present at this site. Although Vinlandostrophia (Platystrophia) sp. does occur in the Silurian as late as the Waldron Shale, it is definitely more common in the Ordovician, where V. ponderosa forms veritable coquinas in the upper Leipers Formation. It's a bit hard to see in the bottom one, but it does have a sulcus, unlike most of the Dalmanella sp. present. 

 

A few Leptaena sp. Although it is known from the upper Ordovician, I have not found it in the Ordovician rocks nearby and have found several in the Silurian Rose Hill Formation. It is interesting to note that there are such similarities between the two early Silurian fauna across such distances. 

 

 

One characteristic I quickly noticed was that a lot of the specimens in this area occur as loose shells, whereas in Maryland and Pennsylvania they're often internal molds wedged in rock. It makes for easier collecting, and more photogenic fossils! I'm going to go out on a limb and say this is Dalmanella elegantula(?)

 

 

Pretty good preservation on this one, but I'm not quite sure about it's ID. Maybe some kind of Chonetes (?) sp.

 

 

Eospirifer sp. with crystalline preservation. 

 

 

[EMP]

Continuing with the non-brachiopod finds. 

 

 

A cross section of a thoracic segment from a Calymene sp. trilobite. Trilobites weren't as common in the Rockwood as the Rose Hill, but still, any trilobite is better than no trilobite!

 

 

Another unknown. I'm tentatively calling this a trilobite molt fragment of some kind, possibly the genal spine/free cheek area of a Calymene sp. ?

 

 

This is the main fossil that was giving me some trouble. It's a trilobite pygidium, and a large one at that (the largest I've found). Judging from it's shape, seeming lack of lobes and ribbing, and the "rim" around the base of the pygidium, I first thought Isotelus sp. which would make this Ordovician in age. However everything else is Silurian, and so I was debating if it weren't a Dipleura sp. I went back through the GGS report and they mentioned the occurrence of Isotelus brachycephalus from the Red Mountain Formation, so this is the very tentative ID I'm going with. I thought Isotelus was restricted to the Ordovician, so maybe it's some other asaphida? They died out in the Silurian, so it'd be a pretty late one. Note the little bit of shell material still preserved, and the many brachiopods above it. 

 

 

A nice little horn coral, Campophyllum sp. I haven't found any so far in the Rose Hill, so it's interesting to note their presence in the early Silurian further south. 

 

 

An interesting bryozoan. 

 

This is a slab of hash material, including a few different types of brachiopods, a trilobite molt fragment, and crinoid pieces. Personally I really enjoy looking at these Paleozoic hash plates and see how busy the seafloor was back then. 

 

 

Another hash plate crowded with brachiopods, crinoids, and other things. 

[Kane]

I don’t see Leptaena much outside Devonian deposits. Interesting to seem them in Silurian deposits. Possibly perform some quick abrasion on the suspected trilobite for more certainty, if you can.

 

Nice array of brachs (which is not something I find myself saying often  ). 

[EMP]

21 hours ago, Kane said:

I don’t see Leptaena much outside Devonian deposits. Interesting to seem them in Silurian deposits. Possibly perform some quick abrasion on the suspected trilobite for more certainty, if you can.

 

Nice array of brachs (which is not something I find myself saying often  ). 

 

Haha, thanks! Yeah I'm usually not the biggest fan of brachiopods, but I've found myself really liking the ones from around here. Something about finding a big Vinlandostrophia weathered out of a rock makes it much better than a Chonetes mold in a block of shale. 

 

Unfortunately, the rock it's in is very fragile, and the black shell material towards the top is also very thin and fragile. Most of it's a mold so it'd be hard to really expose more as it's not like it's covered in dirt. Hopefully the photo captured the rim area at the rear of the pygidium well enough, because it looks a lot like an Isotelus' pygidium but it's from the Silurian. Dipleura has lobes, so maybe it's a Bumastus? 

 

 

Bumastus niagarensis for reference.

Edited July 27, 2021 by EMP