ARCHAEOCYATHS: A HISTORY OF PHYLOGENETIC INTERPRETATION

Archaeocyaths are conical, calcareous, Cambrian fossils with a long history of phylogenetic uncertainty and changing interpretations. There is now a strong consensus among currently active workers that archaeocyaths are sponges. However, at different times in the 19th and 20th centuries there were five distinct schools of thought about their phylogeny. Changes in taxonomic assignment were compelled by one or more of the following phenomena: (1) the discovery of revealing new specimens, (2) a paradigm shift within paleontology, (3) a technological innovation.

Late 19th century and early 20th century paleontologists worked within a paradigm of inexorably increasing diversity through time, and they did not believe in the concept of extinct phyla. Consequently, prior to about 1950, archaeocyaths were bounced around from coelenterates to sponges to algae. By the 1930s all workers agreed that archaeocyaths were sponges of one type or another. In the mid-20th century a significant paradigm shift occurred in paleontology, allowing the viability of the concept of a phylum with no extant species. Correspondingly, two new schools of thought emerged regarding archaeocyathan phylogeny. One of these schools placed them in their own phylum, and the other placed them in a separate kingdom.

The Phylum Archaeocyatha school was the mainstream interpretation from the 1950s through the 1980s. However, the widespread use of SCUBA beginning in the 1960s ultimately led to the rejection of the interpretation that archaeocyaths belong in a separate phylum. SCUBA allowed biologists to study deep fore-reef and submarine cave environments, leading to the discovery of living calcareous sponges, including one aspiculate species that is morphologically similar to archaeocyaths. These discoveries in the 1960s and 1970s stimulated a re-examination of sponge phylogeny generally, and comparisons between archaeocyaths and sponges in particular. The result was the abandonment of the Phylum Archaeocyatha school in the 1990s by virtually all specialists. Present consensus is that archaeocyaths represent both a clade and a grade--Class Archaeocyatha and the archaeocyathan morphological grade--within Phylum Porifera.