Origin and early diversification of the phylum <scp>C</scp>nidaria <scp>V</scp>errill: major developments in the analysis of the taxon's <scp>P</scp>roterozoic–<scp>C</scp>ambrian history

Abstract

Abstract Diploblastic eumetazoans of the phylum C nidaria originated during the N eoproterozoic E ra, possibly during the C ryogenian Period. The oldest known fossil cnidarians occur in strata of E diacaran age and consist of polypoid forms that were either nonbiomineralizing or weakly so. The oldest possible anthozoans, including the genus R amitubus , may be related to tabulate corals and occur in the D oushantuo L agerstätte (upper D oushantuo F ormation, S outh C hina), the age of which is poorly constrained (approximately 585 Ma?). Conulariid scyphozoans may first appear as early as 635–577 Ma ( L antian F ormation, S outh C hina). A definite conulariid, most similar to Palaeozoic species assigned to the genus P araconularia , occurs in association with the possible scyphozoan, C orumbella werneri , in the latest Ediacaran ( c . 543 Ma) Tamengo Formation of Brazil. Basal C ambrian ( c . 540 Ma) phosphorites in the upper Kuanchuanpu Formation (South China) yield solitary polyps of the oldest probable anthozoan ( E olympia pediculata ), which appears to have been a stem hexacorallian. This same formation contains fossils interpreted by some authors as pentaradial cubozoan polyps; however, both the oldest known cubozoans and the oldest hydrozoans, all medusae, may actually occur in the Cambrian (Series 3, c . 505 Ma) Marjum Formation (Utah, USA ). Although these recently published palaeontological data tend to corroborate the hypothesis that Cnidaria has a relatively deep Neoproterozoic history, the timing of major internal branching events remains poorly constrained, with, for example, the results of some molecular clock analyses indicating that the two cnidarian subphyla ( A nthozoaria and M edusozoa) may have originated as many as one billion years ago. Further progress towards elucidating the evolution and early fossil record of cnidarians may accrue from: (1) an intensive search for phosphatized soft parts in possible anthozoans from the E diacaran D oushantuo F ormation; (2) an expanded search for E diacaran conulariids; and (3) additional detailed analyses of the taphonomy and preservation of E diacaran and C ambrian cnidarians, including possible pentaradial cubozoan polyps from the Fortunian upper Kuanchuanpu Formation.