| Archeocyathids in a chunk of brownish limestone from the Lower Cambrian Harkless Formation, Gold Point fossil area, Esmeralda County, Nevada. The archeocyathids are the pale-brown, slender, elongated specimens embedded on the darker-colored matrix.; the black "blob" at lower right corner is a recent lichen. They can probably be assigned to the genus Ethmophyllum. Ever since they were first described, archeocyathids have been assigned by paleontologists to many different animal groups, primarily the corals and Pleosponges; many researchers even referred to them to a distinct, separate Phylum--Archeocyatha. But more recent investigations have proved pretty conclusively that archeocyathids were an early experiment in the Phylum Porifera; they are now generally considered an extinct type of calcareous sponge. |