Dave, over at Views Of The Mahantango, put a post up yesterday describing some sponges(stromatoporoids) that he had found from Devonian rocks of New York and Kentucky, and some Silurian rocks of Pennsylvania.
This post will add to his description.
Stromatoporoids are peculiar, and poorly understood, sponges(Porifera) that evolved in either the Cambrian or Ordovician. Some experts classify some Cambrian archaeocyathids as stromatoporoids, but that classification is tenuous. They became extinct during either the Devonian or Mississippian… then re-appear in the Mesozoic forming reef building communities, again.
The Paleozoic forms lacked spicules-common to other types of Porifera.
I said that they were peculiar.
As Dave noted in his post, they formed massive reefs in the Devonian. Sometimes, tens of meters thick. Though, in the Ordovician, the reefs are less massive. Also, they were restricted to warm tropical waters near the equator. The Late Ordovician glaciation on Gondwana appears to have substantially reduced the numbers; since they lived in the shallow tropical seas, their habitat was decimated by the regression associated with the ice advances.
Sometime around Devonian/Mississippian time, they became extinct… but then re-appeared in the Jurassic with spicules.
Confused? Well, that makes two of us.
Most of the uncertainty is the result of very little research on stromatoporoids other than their paleoecological relationships. For the most part, their phylogenetics is unknown.
A few photos of some Ordovician stromotoporoids. All are from the Millersburg member of the Lexington Limestone, and found in Fayette Co. Kentucky(though, from two different localities). As always, click on image for bigger version.
Another peculiar sponge that I find in the local rocks is Solenopora. Like the stromatoporoids, it was also mis-classified for years. Originally, the chaetetids were classified as tabulates.
Recently, Solenopora was re-classifed as a chaetetid, again no spicules. Solenopora aren’t common in the rocks of the Lexington, so when I find them, they find a new home in the cabinets(too, they are smaller).
Again, these were collected from the Millerburg mb of the Lexington Lm in Fayette Co. Ky.
To learn more about these peculiar critters, here are some good links.
Paleos Metazoa: Porifera -follow the links.
Systematics of Porifera– again, follow the links.
Or, you could ask Mr. Google.