Many of the late Neoproterozoic “Ediacaran fossils” have been referred to the Cnidaria, often on the basis of vague or poorly known features. However, representatives of the living Chondrophorina (=Porpitidae, Hydrozoa), Pennatulacea (Anthozoa), and Coronatae and/or Stauromedusae (Scyphozoa) have all been identified in Ediacaran biotas, based on specific morphological features preserved in a number of specimens. These three cnidarian groups have plausible Paleozoic representatives as well, but many of their Paleozoic fossils are also somewhat problematic. We test these systematic hypotheses by using them to calibrate divergence dates across the Cnidaria, based on an extensive molecular phylogeny of extant cnidarians. In this reductio ad absurdum approach, if a calibration based on one interpretation of a problematic fossil yields a glaringly inconsistent age for a better-known clade, that interpretation is likely to be mistaken. We find that assuming the existence of Pennatulacea and Scyphozoa in the “Ediacara biota” places the root of the Cnidaria between 800 and 1,000 Ma, a figure which is, at least, not out of line with other molecular clock estimates. However, assuming the existence of the Chondrophorina in the Neoproterozoic, or anywhere in the Paleozoic, pushes the root of the Cnidaria back to between 1,500 and 2,000 Ga, which is considerably older than the oldest previous estimates for the origin of the Cnidaria. We suggest that the likeliest explanation is that chondrophorines were not present in the late Precambrian or Paleozoic. The Ediacaran and Paleozoic fossils previously interpreted as chondrophorines probably represent other taxa.