Kimberella

Kimberella is a genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized. Specimens were first found in Australia's Ediacara Hills, but recent research has concentrated on the numerous finds near the White Sea in Russia, which cover an interval of time from 555 to 558 million years ago. As with many fossils from this time, its evolutionary relationships to other organisms is hotly debated. Paleontologists initially classified Kimberella as a type of jellyfish, but since 1997 features of its anatomy and its association with scratch marks resembling those made by a radula have been interpreted as signs that it may have been a mollusc. Although some paleontologists dispute its classification as a mollusc, it is generally accepted as being at least a bilaterian.

The classification of Kimberella is important for scientific understanding of the Cambrian explosion: if it was a mollusc or at least a protostome, the protostome and deuterostome lineages must have diverged significantly before 555 million years ago. Even if it was a bilaterian but not a mollusc, its age would indicate that animals were diversifying well before the start of the Cambrian.

Occurence
Kimberella has been found both in the Ediacara Hills of South Australia and in the Ust’ Pinega Formation in the White Sea region of Russia. The White Sea fossils are often associated with the Ediacaran "animals" Tribrachidium and Dickinsonia; meandering trace fossil trails, possibly made by Kimberella itself; and algae. Beds in the White Sea succession have been dated to 555.3 ± 0.3 million years ago and 558 million years ago by radiometric dating, using uranium-lead ratios in zircons found in volcanic ash layers that are sandwiched between layers that contain Kimberella fossils. Kimberella fossils are also known from beds older and younger than this precisely dated range.[2] The fossils from the Ediacara Hills have not been dated precisely.

Preservation
All Kimberella fossils were preserved as depressions in the bases of beds, implying that the organism, although not mineralised, was firm enough to resist being crushed as sediment accumulated above it; as the soft parts of the organism decayed, the soft muds underneath would be squeezed up into the shell, preserving the shape of the organism.

Preservation of most specimens was made possible by the fast sedimentation which quickly cut the organism off from seawater; it may also have been enhanced by the decay products of the rotting organism, which could have helped the overlying sediment to mineralise and harden. It has been suggested that a mucus trail produced by the organism may have assisted its preservation, but experiments suggest that mucus disintegrates too easily to play a role in binding sediment together.