Kimberella is a 555-558 million-year-old fossil that was originally classified as a jellyfish but is now considered a stem-group mollusk due to its anatomical features. It is the oldest well-documented triploblastic bilateral animal in the fossil record and was a carpet grazer that lived alongside other simple organisms during the Late Ediacaran period.
Kimberella is a very unusual and important Late Ediacaran fossil, dated to between 555 and 558 million years ago, 13 million years before the start of the Cambrian period. Kimberella, originally classified as a jellyfish and then a box jellyfish, had its classification changed in 1997 to that of a stem-group mollusk or mollusk-like creature. This was due to anatomical features (such as a non-mineralized dorsal shell) and the presence of nearby scratches suggesting that the animal had a radula, the universal chitinous tongue for all molluscs.
Due to Kimberella’s age and classification, it is the oldest well-documented triploblastic (three layers of tissue) bilateral (bilaterally symmetrical) animal in the fossil record. Another possible older bilateran is Vernanimacula, dated to 580-600 million years ago, but some scientists argue that these 0.1 mm objects are just filled bubbles, not fossils of organic origin. In any case, it shows that the earliest known bilateral animal was a mollusk or at least mollusk-like, making it a likely member of the protostomes, one of the two main groups of non-cnidarians (jellyfish, corals, etc.) or ctenophores ( jelly box) animals.
During Kimberella’s age, there was little multicellular life, and the ocean floors were dominated by microbial mats: layers of bacteria and archaea several millimeters thick. Kimberella was a “carpet grazer” — an animal that specialized in sliding along the top layer of microbial carpet and scraping off bits of microbes with its chitinous radula. The largest specimens of this animal measure 15 cm (6 inches) long, 5 to 7 cm (2-3 inches) wide and 3 to 4 cm (1-1.3 inches) high, although the smallest are only 2 to 3 mm long. It had a soft “skirt” that it could use for breathing, and could be pulled back into its shell in case of danger.
Kimberella’s time, the Late Ediacaran, had other simple organisms that serve as forerunners of the complexity that would have evolved at the dawn of the Cambrian, just 13 million years later. It lived alongside small, trisymmetric, button-shaped organisms called trilobozoans, simple jellyfish footprints that belonged to early cnidarians (relatives of jellyfish), frond-shaped filter feeders like Charniodiscus, and mysterious ribbed ovates from a phylum called Proarcticulata. None of these other organisms have left any definitive traces, but Kimberella has.
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