A generic revision of the Stylasteridae (Coelenterata: Hydrozoa). Part 2. Phylogenetic analysis

A phylogcnelic analysis was performed on the 23 genera ofstylasterid corals. Hydractinia, a genus of athecate hydroid, was chosen as the out-group based primarily on morphological homology and secondarily on ontogeny, fossil record and advocacy. The evolutionary polarities of the 19 characters used in the analysis were established by out-group comparison and transformation series of mullislate characters were ordered by apparent structural complexity and the process of reciprocal illumination. Several equally parsimonious cladograms are discussed and the justifications for choosing one in preference to the others are given. The interrelationships of the genera arc discussed; l^pidopora is considered to be the most plesiomorphic genus, Pseudocryptheiia the most apomorphic. The final cladogram is compared to the evolutionary tree proposed by Mosclcy (1881), Within the context of the final cladogram, the relative value of the characters and degree of homoplasy are discussed. The stylasterids are considered as a family oFathecate hydroids and the subfamilial designations are recommended to be abolished. Stylasterid corals arc fragile, usually small, uniplanar to slightly arborescent colonial hydrozoans of the phylum Coelenterata. Their calcium carbonate skeletons are often brightly pigmented orange, red, blue or violet. The approximately 185 known species (Cairns, 1983b) occur in all ocean basins from continental Antarctica to the Arctic Circle at depths between 0-2,800 m. They are most diverse and abundant at depths of 200-500 m. They arc known from the Paleocene to the Recent. Opinion is divided as to whether they should be considered a separate order in the Hydrozoa or simply a family of calcified hydroids in the Hydroida. This analysis is based on the redescription of the 23 stylasterid genera as revised by Cairns (1983b). Ideally, a phylogenetic analysis should be based on out-group comparison, supplemented by evidence derived from ontogeny (Stevens, 1980). Unfortunately, the ontogeny of stylasterids is virtually unknown and the out-group chosen for this analysis is a genus of uncalcified athecate hydroids. All characters used in the classification of stylasterids at all taxonomic levels are based on the calcium carbonate skeleton, which makes comparison to an uncalcified out-group difficult. Nonetheless, certain characters can be polarized from the out-group, and those that could not were ordered into transformation series by their apparent structural complexity and by the process of reciprocal illumination, a method of testing hypotheses of character state series against one another (discussed later). The 43 taxa analyzed represent 23 presumably monophyletic genera (Cairns, 1983b). I Some generalized references on phylogenetic analysis, particularly on how to ' determine polarity and order multistate characters are: Eldredge and Cracraft { (1980), Watrous and Wheeler (1981) and Michcvich (1983). ' This is the second application of phylogenetic systematic methods to a coelentcrate group. The first was by Schmidt (1972; 1974), concerning the ordinal classification of the class Anthozoa.