Kimberella

Kimberella is an extinct genus of marine bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the gastropods, although its affinity with this group is contentious.

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 are hotly debated. Paleontologists initially classified Kimberella as a type of Cubozoan, 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 the scientific understanding of the Cambrian explosion; if it was a mollusc, or at least a protostome, this would mean that the two dominant Nephrozoan lineages would have diverged significantly before 555 million years ago, and if it was at least bilaterian, its age would indicate that animals were diversifying well before the start of the Cambrian.

The genus is named in honour of Mr. John Kimber, student, teacher, and collector; who lost his life during an expedition to Central Australia in 1964. Originally, the genus was given the name Kimberia, however, the name Kimberia was already in use as a subgenus of Turritella (Gastropoda), according to Dr. N. H. Ludbrook; and a new genus, Kimberella, was proposed by Mary Wade in 1972.

Kimberella has been found in the Ediacara Hills of South Australia, in the Ust' Pinega Formation in the White Sea region of northwest Russia and the Kushk Series of central Iran. In 2014, Kimberella fossils were found in Brazil, but later studies showed fossils of other Silurian animals. The White Sea fossils are often associated with the Ediacaran "animals" Tribrachidium and Dickinsonia, as well as 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 both older and younger than this precisely dated range. The fossils from the Ediacara Hills have not been dated precisely.^{[citation needed]}

Over 1,000 specimens, representing organisms of all stages of maturity, have now been found in the White Sea area at the bottom of fine-grained sandstone layers. The large number of specimens, the small grain-size of the sediments and the variety of circumstances in which specimens were preserved provide detailed information about Kimberella's external form, internal anatomy, locomotion and feeding style.

All of the fossils are oval in outline. Elongated specimens illustrate that the organism was capable of stretching in an anterior-posterior direction, perhaps by as much as a factor of two. Like many other specimens found in the White Sea, the most common type of symmetry observed appears to be bilateral; with little to no sign of any of the kinds of radial symmetry found in Cnidarians, the group that includes jellyfish, sea anemones and hydras. The Australian fossils were originally described as a type of jellyfish, but this is inconsistent with the bilateral symmetry observed in the fossils. The White Sea fossils and the surrounding sediments also show that Kimberella lived on the surface of the sea-floor.

Kimberella had a dorsal covering that has been described as a (non-mineralized) "soft shell"; in larger specimens, this reached up to 15 cm in length, 5–7 cm in width, and 3–4 cm in height; with a minimum length of 2–3 mm.