Michigan favositid genus/species ID?

[LisaL]

Hi, all,

 

A friend over on the Facebook group "Great Lakes Rocks and Minerals" recently posted this little silicified pebble she found along the shore of Lake Michigan, northern lower peninsula. The tiny pores got several of us on the group curious about whether we could narrow down a possible ID. Someone initially suggested heliolitid, but I think we ruled that out because there doesn't seem to be enough room between corallites for coenenchyme. We decided it must be some species of small-celled favositid, but is it possible to narrow beyond that? (My gut says probably not, since we don't have thin sections or better views!) I was curious about Astrocerium (referred to as Favosites venustus in older descriptions). It's described from the Silurian (Niagaran) of Michigan, which seems to fit with this style of silicification. Also, I think I see what might be interpreted as spinules in some of the corallites, but maybe this is just an effect of silicification. Plus, I guess spinules are pretty common in favositid species, so it could be any number of small-celled species of Favosites or Emmonsia.

 

If anyone has examples of Astrocerium from their collection, I would love to see photos!  

 

P.S. Hope it's okay to tag you two, @TqB

and @FossilDAWG -- wasn't sure of other coral experts to ask, though I know they are around! 

Thanks much!
Lisa