Archaeocyaths are calcareous, conical, Cambrian fossils with a long history of phylogenetic uncertainty and changing interpretations. The history of phylogenetic interpretation of archaeocyaths reveals five distinct schools of thought: the coelenterate school, the sponge school, the algae school, the Phylum Archaeocyatha school, and the Kingdom Archaeata school. Late nineteenth century and early twentieth 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, after considerable study, all workers agreed that archaeocyaths were sponges of one type or another. In the mid-twentieth 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 taxonomy. The Phylum Archaeocyatha school placed them in their own phylum, which was inferred to be closely related to Phylum Porifera within Subkingdom Parazoa. A second new school removed archaeocyaths and some other Paleozoic problematica from the animal kingdom and placed them in Kingdom Archaeata (later Kingdom Inferibionta). 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. Present consensus is that archaeocyaths represent both a clade and a grade—Class Archaeocyatha and the archaeocyathan morphological grade—within Phylum Porifera.
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