Opabinia regalis is an extinct, stem group arthropod found in the Middle Cambrian Burgess Shale Lagerstätte (505 million years ago) of British Columbia.[1] Opabinia was a soft-bodied animal, measuring up to 7 cm in body length, and its segmented trunk had flaps along the sides and a fan-shaped tail. The head shows unusual features: five eyes, a mouth under the head and facing backwards, and a clawed proboscis that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food.[2] Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed, where they constitute less than 0.1% of the community.[3]
When the first thorough examination of Opabinia in 1975 revealed its unusual features, it was thought to be unrelated to any known phylum,[4] or perhaps a relative of arthropod and annelid ancestors.[2] However, later studies since late 1990s consistently support its affinity as a member of basal arthropods, alongside the closely-related radiodonts (Anomalocaris and relatives) and gilled lobopodians (Kerygmachela and Pambdelurion).[5][6][7][8][1][9][10]
In the 1970s, there was an ongoing debate about whether multi-celled animals appeared suddenly during the Early Cambrian, in an event called the Cambrian explosion, or had arisen earlier but without leaving fossils. At first Opabinia was regarded as strong evidence for the "explosive" hypothesis.[4] Later the discovery of a whole series of similar lobopodian animals, some with closer resemblances to arthropods, and the development of the idea of stem groups suggested that the Early Cambrian was a time of relatively fast evolution but one that could be understood without assuming any unique evolutionary processes.[11]
In 1911, Charles Doolittle Walcott found in the Burgess Shale nine almost complete fossils of Opabinia regalis and a few of what he classified as Opabinia? media, and published a description of all of these in 1912.[12] The generic name is derived from Opabin pass between Mount Hungabee and Mount Biddle, southeast of Lake O'Hara, British Columbia, Canada.[12] In 1966–1967, Harry B. Whittington found another good specimen,[13] and in 1975 he published a detailed description based on very thorough dissection of some specimens and photographs of these specimens lit from a variety of angles. Whittington's analysis did not cover Opabinia? media: Walcott's specimens of this species could not be identified in his collection.[2] In 1960 Russian paleontologists described specimens they found in the Norilsky region of Siberia and labelled Opabinia norilica,[14] but these fossils were poorly preserved, and Whittington did not feel they provided enough information to be classified as members of the genus Opabinia.[2]
All the recognized Opabinia specimens found so far come from the "Phyllopod bed" of the Burgess Shale, in the Canadian Rockies of British Columbia.[2]
In 1997, Briggs and Nedin reported from South Australia Emu Bay Shale a new specimen of Myoscolex that was much better preserved than previous specimens, leading them to conclude that it was a close relative of Opabinia[15]—although this interpretation was later questioned by Dzik, who instead concluded that Myoscolex was an annelid worm.[16]