A Bundle of Brachiopods - Zygospira

The other day when Allison and I were out in the field, we made a quick stop near Dillsboro, IN to look at an outcrop of what was probably Mount Auburn Member (of the Grant Lake Fm) or the Arnheim Formation. We knew we were not any younger than that because we came across some rather large Platystrophia ponderosa (Vinlandostrophia ponderosa) which are known the make through the Arnheim, yet disappearing before the Waynesville Formation. However we didn't see any of the true diagnostic fossils like Lapteana  or Retrosirostra carleyi. 

Outcrop of Grant Lake or Arnheim Formation near Dillsboro, Indiana

We did come across this rather interesting pair of rocks in the float that showed some sort of groove or gutter carved into what must have been a semi-harden marine floor and in this groove was dozens of small Zygospira brachiopods.

Groove or gutter filled with the small brachiopod Zygospira

Another accumulation of Zygospira from the same locality. The large brachiopod on the bottom right is a Rafinesquina.

Zygospira are known for adhering themselves to whatever hard substrate they can find. In a paper by Micheal Sandy (1996), the author describes a specimen from the Waynesville Formation of Ohio where an articulated crinoid stem is thoroughly encrusted by Zygospira. There have also been reports by local trilobite collectors that they have found Zygospira concentrated around fragments of Isotelus in mudstone deposits colloquially referred to as butter shales (more on these next week).

A closer look at one of the accumulations of Zygospira

Lastly, an interesting note is that on at least one of the blocks we see the dumbbell shaped trace fossil Diplocraterion generally thought to be the dwelling trace of a crustacean or worm.

Arrow is pointing to the Diplocraterion trace fossils associated with one of these Zygospira accumulations.