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Archaeocyaths--A history of phylogenetic interpretation

Journal of Paleontology,  Nov 2001  by Rowland, Stephen M

ABSTRACT-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.

INTRODUCTION

ARCHAEOCYATHS ARE an extinct group of sessile marine organisms known only from the Cambrian System, and nearly exclusively from the Lower Cambrian. The basic archaeocyathan skeleton consists of two nested, porous, calcareous cones, forming an outer wall and an inner wall (Fig. 1). Various panels, curved plates, rods, and tubes may occur between the walls, in some cases forming a complex network of skeletal elements (Fig. 2). Two informal groups of archaeocyaths are commonly recognized: regulars (Fig. 2) and irregulars (Fig. 3). The morphology of the irregulars is more complex than that of the regulars, and some irregulars have no inner wall at all. Archaeocyathan gross morphology ranges from cylindrical to discoid, sometimes with exothecal outgrowths projecting from the outer wall. In most species the maximum diameter of the cup is 1-5 cm; however some are considerably larger. The most recent Treatise on Invertebrate Paleontology volume on archaeocyaths (Hill, 1972) provided a detailed summary of archaeocyathan anatomy, although some of the anatomical terms in use when it was written have since been replaced in the interest of using the same terms for homologous structures in closely related organisms (Debrenne and Zhuravleva, 1992). Taxonomically, the Treatise (Hill, 1972) is out of date, not least because it places archaeocyaths in a separate phylum, but also because many genera have been redefined. For current taxonomy and anatomical terminology, see Debrenne et al. (1990) and Debrenne and Zhuravlev (1992).

Archaeocyths first appear in the fossil record at the base of the Tommotian Stage of the Lower Cambrian Series; they diversified steadily through the Atdabanian, reaching a peak diversity of about 170 genera in the Botomian Stage (Fig. 4) (Zhuravlev, 1986a; Debrenne, 1992). Two extinction events, the Sinsk event in the Botomian (Zhuravlev and Debrenne, 1996) and another extinction in the Toyonian, took heavy tolls on archaeocyathan diversity, the latter event virtually wiping them out (Debrenne, 1991, 1992; Wood, 1999). Two post-Early Cambrian genera have been described from Antarctica, one from the Middle Cambrian and the other from the Upper Cambrian (Debrenne et al., 1984; Wood et al., 1992a).

Since their discovery in the mid-nineteenth century, archaeocyaths have endured a century and a half of wildly diverse assignments to higher level taxa-from algae to foraminifera to corals to sponges to a separate phylum of their own to members of a new kingdom. During much of this time, and particularly in the 1970s and 1980s, there were co-existing conflicting opinions among specialists about where to place archaeocyaths, causing non-specialists to avoid them. For example, Sepkoski (1979), in modeling taxonomic diversity of early Phanerozoic metazoan families, chose not to include archaeocyaths in his data set because there seemed to be good reasons to doubt that they were metazoans. Following Sepkoski's (1979) lead, several other studies of the Cambrian fauna published in the 1980s ignored archaeocyaths altogether (Sepkoski, 1981; Bambach, 1983, 1985; Sepkoski and Miller, 1985). Recently, Stanley and Hardie (1998, 1999) excluded archaeocyaths from their analysis of the influence of tectonically-forced changes in seawater chemistry on Phanerozoic reef-building because, in their opinion, they are "taxonomically problematical" (Stanley and Hardie, 1999, p. 4).