| Archeocyathids in a chunk of limestone from the Lower Cambrian Harkless Formation, Gold Point fossil area, Esmeralda County, Nevada. The archeocyathids are the orange-brown conical specimens embedded on the matrix. 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. |