Dinocarida (Order: Radiodonta, stem group arthropods)
Opabinia is an anomalocaridid. Anomalocaridids have been variously regarded as basal stem-lineage euarthropods (e.g., Budd, 1996; Zhang and Briggs, 2007, Daley et al., 2009), basal members of the arthropod group Chelicerata (e.g., Chen et al., 2004), and as a sister group to the arthropods (e.g., Hou et al., 2006)
Opabinia – from Opabin Pass (2,606 m) between Mount Hungabee and Mount Biddle in Yoho National Park. From the Stoney First Nation Nakoda word for “rocky,” a descriptive name for the pass given by Samuel Allen in 1894.
regalis – from the Latin regalis, “royal, or regal.”
Lectotype –USNM57683 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott and Raymond Quarries on Fossil Ridge.
Brief history of research:
Opabinia regalis was first described by Walcott (1912) as the most primitive of all Burgess Shale arthropods. Owing to its unique morphology with a bizarre frontal “nozzle,” Opabinia became a flagship fossil for the Burgess Shale, leading to much speculation on its affinity and lifestyle. One famous reconstruction shows the animal swimming upside down as an anostracan crustacean (Hutchinson, 1930).
It wasn’t until the major redescription by Whittington (1975) that the morphology of Opabinia was revealed to be truly one of the most enigmatic of all fossils. It was so unusual, in fact, that when Whittington showed an early version of his reconstruction in a meeting of palaeontologists in 1972, the whole room burst out laughing!
Further work by Bergström (1986) identified similarities between Opabinia and the recently discovered whole-body specimens of Anomalocaris (Whittington and Briggs, 1985), and updated the morphology of the gills and frontal proboscis. Budd (1996) was the first to place Opabinia in the stem lineage of the euarthropods (just below the anomalocaridids), and also suggested the animal had trunk limbs, though this idea was contested by Zhang and Briggs (2007). The issue of whether Opabinia had trunk limbs remains controversial (Budd and Daley, 2011).
Opabinia has five eyes, a frontal “nozzle,” or proboscis, a body with serially repeated lateral lobes and gills, and a prominent tail fan. The whole body length ranges between 4.3 and 7.0 cm (excluding proboscis). The head has a rounded anterior margin, with five bulbous compound eyes on short stalks clustered on the dorsal surface of the head. The annulated frontal proboscis is four times longer than the head. It is highly flexible, and has a fused pair of appendages at the distal end, consisting of two opposing claws with five or six spines each. The mouth was ventral and faced to the rear.
The trunk was divided into 15 segments, each bearing a pair of lateral lobes in association with gill structures consisting of a series of lanceolate blades. There is some controversy as to the exact location of the gills (dorsal, ventral or posterior) relative to the lobes. The tail fan consists of three pairs of upward-directed flaps. The central region of the body shows an outline of the main body cavity, and a dark line representing a trace of the gut runs along the length of the body, starting with a U-shaped bend near the rearward opening ventral mouth. Paired spherical structures next to the alimentary canal could represent gut glands. There are also controversial triangular features in the central region of the body, which have alternatively been interpreted as lobopod-like walking limbs (Budd, 1996), or as undifferentiated diverticula or extensions of the gut (Whittington, 1975; Zhang and Briggs, 2007).
Opabinia is rare, with only 42 specimens known from all collections. In the Walcott Quarry, Opabinia represents only 0.006% of the community (Caron and Jackson, 2008).
Opabinia was a swimmer. Undulatory waves along its lateral lobes propelled it forward, while it used its tail fan to steer. Opabinia probably employed the distal claws on its flexible nozzle to grasp soft food items and carry them towards its ventral mouth.