Opabinia

Opabinia is a fossil animal found in Cambrian fossil deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale (505 million years ago) of British Columbia. The discoverer of Opabinia, Charles Doolittle Walcott, named it after a local mountain, Opabin Peak in the Canadian Rockies. Thirty specimens of Opabinia are known and each ranges in size from 40 to 70 mm.

Whittington found evidence of near-triangular features along the body, saying they were internal structures, most likely sideways extensions of the gut (diverticula). He also thought Opabinia had no legs, so he said that it crawled on its lobes and that it could also have swum slowly by flapping the lobes, especially if it timed the movements to create a wave with the metachronal movement of its lobes.

Later studies since the late 1990s support its affinity as a member of basal arthropods, alongside the Radiodonts (Anomalocaris and relatives) and Lobopodians (Kerygmachela and Pambdelurion).

 Description 
Opabinia is a strange animal. It looked so weird that the audience at the first presentation of Whittington's analysis broke out in laughter. The length of Opabinia regalis from head (excluding proboscis) to tail end ranged between 4 centimetres (1.6 in) and 7 centimetres (2.8 in).

 Anatomy 
The most intriguing and unusual feature of Opabinia are its five eyes found on the dorsal surface of the head. These eyes were used to search for food. It also had a clawed proboscis that likely passed food to its mouth. Opabinia lived on the seafloor, using the proboscis to seek for tiny food. Because of its flexible body it is not known whether Opabinia was pelagic or benthic.

Additionally, Opabinia swam using flaps on its sides and a fan-shaped tail. The proboscis is quite long and flexible enough to reach the mouth.

Interpretations of other things in Opabinia fossils differ. Since the animals did not have mineralized armor their bodies were flattened as they were buried and fossilized, and smaller or internal features appear as markings within the outlines of the fossils.

 Taxonomy 
The classification of Opabinia was highly debated during the 20th century.

Charles Doolittle Walcott thought it would be an anostracan in 1912. The idea was then followed by G. Evelyn Hutchinson in 1930 with the first reconstruction of Opabinia as an anostracan swimming upside down. In 1986, Swedish palaeontologist Jan Bergström, noting the similarity of Anomalocaris and Opabinia, said the two were related, as they shared many features (lateral flaps, gill blades, stalked eyes, and frontal appendages).

 References 
Category:Fossils
Category:Cambrian
Category:Arthropods
Category:Extinct species

 Further reading 
 Bergström, J. (1986). Opabinia and Anomalocaris, unique Cambrian arthropods. Lethaia 19, 241–246.
 Bergström, J. (1987). The Cambrian Opabinia and Anomalocaris. Lethaia 20, 187–188.
 Briggs, D. E. G. & Whittington, H. B. (1987). The affinities of the Cambrian animals Anomalocaris and Opabinia. Lethaia 20, 185–186.
 Budd, G. E. (1996). The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group. Lethaia 29, 1–14.
 Whittington, H. B. (1975). The enigmatic animal Opabinia regalis, Middle Cambrian Burgess Shale, British Columbia. Philosophical Transactions of the Royal Society of London B 271, 1–43.
 Zhang, X. & Briggs, D. E. G. (2007). The nature and significance of the appendages of Opabinia from the Middle Cambrian Burgess Shale. Lethaia 40, 161–173.

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