Kimberella - Classification

Classification

All the Kimberella fossils found so far are assigned to one species, K. quadrata. The first specimens were discovered in Australia in 1959. They were originally classified as jellyfish by Martin Glaessner and Mary Wade in 1966, and then as box jellyfish by Wade in 1972, a view that remained popular until the fossils of the White sea region were discovered; these prompted a reinterpretation. Research on these specimens by Mikhail A. Fedonkin, initially with Benjamin M. Waggoner in 1997, led to Kimberella being recognised as the oldest well-documented triploblastic bilaterian organism — not a jellyfish at all.

So far Kimberella fossils show no sign of a radula, the toothed chitinous "tongue" that is the diagnostic feature of modern molluscs, excluding bivalves. Since radulae are very rarely preserved in fossil molluscs, its absence does not necessarily mean that K. quadrata did not have one. The rocks in the immediate vicinity of Kimberella fossils bear scratch marks that are very similar those made by the radulae of molluscs as they graze on microbial mats. These traces, named Radulichnus, have been interpreted as circumstantial evidence for the presence of a radula. In conjunction with the univalve shell, this has been taken to indicate Kimberella was a mollusc or very closely related to molluscs. In 2001 and 2007 Fedonkin suggested that the feeding mechanism might be a retractable proboscis with hook-like organs at its end. Kimberella′s feeding apparatus appears to differ significantly from the typical mollusc radula, and this demonstrates that Kimberella is at best a stem-group mollusc.

However, sceptics feel that the available evidence is not enough to reliably identify Kimberella as a mollusc or near-mollusc, considering it presumptuous to call it anything more than a "possible" mollusc, or even just a "probable bilaterian". Nicholas J. Butterfield argues that Kimberella's association with Radulichnus marks is not strong evidence that it was a mollusc, as other groups of organisms bear structures capable of making similar marks. Indeed it has been argued that the shape of the feeding traces is incompatible with a radula, and that despite the molluscan body form, the lack of this synapomorphy places it well outside the molluscan crown group.