PHYLOGENY AND CHARACTER POLARITY IN PLEUROCYSTITID RHOMBIFERANS (ECHINODERMATA)

A new cladistic analysis of Pleurocystitidae, a Paleozoic clade of flattened glyptocystitoid rhombiferans, is presented that uses species for most of the operational taxonomic units. Two outgroup coding schemes are tried, differing in the whether the outgroup codings have the presence of three pectinirhombs, a grooved internal respiratory structure, as plesiomorphic for, or derived within Pleurocystitidae. Included in the analysis is a new inflated pleurocystitid species from the Upper Ordovician Benbolt Formation of Tennessee, shedding light on the evolution of pleurocystitids with highly arched dorsal sides. A highly arched dorsal side has evolved at least twice, and maybe up to three times within the clade. Additional material from the Benbolt Formation, tentatively assigned to Amecystis raymondi, clearly shows a single, highly reduced pectinirhomb and reveals the intermediate evolutionary steps that result in other rhombless species of Amecystis. The analysis recovers all pleurocystitids species with three pectinirhombs as a monophyletic group, regardless of the outgroup choice. If having three pectinirhombs is plesiomorphic for the Pleurocystitidae, then a clade of Pleurocystitinae with fewer than three pectinirhombs is retrieved, otherwise these form a paraphyletic grade. The genus Pleurocystites is not retrieved as monphyletic. Although Pleurocystitidae range from the Middle Ordovician to the Middle Devonian, no pleurocystid has been retrieved from Silurian rocks. Records of Devonian taxa include Regulaecystis from Germany and Great Britain, Henicocystis from Australia, and Turgidicystis from Oklahoma. Hillicystis from the Devonian of Australia was not included in this analysis.

While not all relationships are resolved in the analysis, this phylogenetic analysis indicates that more than one ghost lineage extends through the Silurian.