Kimberella, a slug-like early bilaterian

Kimberella quadrata was an oval, slug-like animal about 7 to 15 centimetres long, with a tough, bulging upper surface and a muscular foot. Its fossils come from the White Sea region of Russia, where rocks about 555 million years old preserve it, and from South Australia, where it was first described by Martin Glaessner and Mary Wade in 1966 and named for the Kimberley region. The paleontologists Mikhail Fedonkin, from Russia, and Benjamin Waggoner, from the United States, described its feeding traces in 1997.¹²

Around many specimens lie sets of fine, repeated scratch marks, called Radulichnus, in the microbial mat covering the seafloor. They look like the marks a snail's rasping tongue makes while grazing algae, which suggests Kimberella fed by scraping the mat with a retractable organ. That points to a bilaterian, an animal with left and right sides, a front and back, and a gut, rather than a simple sponge or jellyfish relative. Whether Kimberella was a true mollusc, a close cousin of molluscs or an early branch of bilaterians is debated. In any case it is among the oldest signs of the body plan that later animals used to eat, burrow and hunt.¹²