CROWN-GROUP CNIDARIANS FROM THE CAMBRIAN OF UTAH

Numerous reports exist of purported Ediacaran and Cambrian cnidarian medusae, but many of these are considered controversial, in large part due to the absence of distinctive soft-part characters, including tentacles. Here we describe new beautifully preserved cnidarian medusae collected from the Middle Cambrian of Utah. The superb preservation of these specimens allow them to be confidently placed in the crown-group of three extant medusozoan cnidarian classes: the Cubozoa; Scyphozoa; and Hydrozoa. These new specimens can be assigned to extant orders and in one case an extant genus. For example, one of the new taxa bears strong morphological similarity to a modern cubozoan genus: given this assignment it suggests complex visual and sperm transfer structures may have evolved in the Cnidaria soon after the Cambrian radiation. The material also includes representatives of modern hydrozoan and scyphozoan orders. The oldest previously identified, definitive cubozoan and scyphozoan fossils were from the Pennsylvanian Essex fauna of Illinois. Reports of purported chondrophorine (“by-the-wind sailor”) hydrozoan fossils from Cambrian and younger strata have been published; however, no non-chondrophorine hydrozoan fossils from the Cambrian have been previously reported.

The new medusoid fossils provide tangible evidence that there had been significant cladogenesis within the crown groups (even at the generic level) of three major medusozoan clades by the Middle Cambrian. This implies either a tremendously rapid period of latest Neoproterozoic/Early Cambrian diversification or a very lengthy Proterozoic history for the cnidarians. Minimally our results suggest that essentially for all of the Phanerozoic the pelagic realm was occupied by these important predators; extant medusozoan cnidarians could have been important predators in the pelagic realm well back into the Proterozoic. Irrespective, this may have important ecological implications.