3D animation of Opabinia regalis.
ANIMATION BY PHLESCH BUBBLE © ROYAL ONTARIO MUSEUM
Opabinia’s bizarre morphology has made its affinity challenging to resolve. Although it was initially considered a crustacean by Walcott (1912), later researchers beginning with Whittington (1975) suggested a more distant relationship with arthropods. Collins (1996) proposed the name Dinocarida for a group including radiodontans (Anomalocaris and its relatives) and Opabinia, though this has not been generally supported. The flexible frontal “nozzle” and possible lobopodous appendages suggest a connection with lobopodians, implying that Opabinia might have diverged before the evolutionary split between radiodontans and true arthropods. On the other hand, Opabinia has a backward facing mouth and five eyes, which could suggest a position closer to the some of the earliest true arthropods which share these features. Both placements have been found in recent analyses (Lerosey-Aubril and Pates 2018; Aria et al. 2020; Zeng et al. 2020; Pates et al. 2021; Moysiuk and Caron 2022). Several less well-preserved fossils have been proposed to share close affinities with Opabinia, including Utaurora comosa (Pates et al. 2021) and Myoscolex ateles (Briggs and Nedin 1997).
Genus – 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.
species – regalis – from the Latin regalis, “royal, or regal.”
Burgess Shale and vicinity: none.
Other deposits: none.
The Walcott and Raymond Quarries on Fossil Ridge.
Opabinia regalis was first described by Walcott (1912) as an anostracan crustacean. 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 early reconstruction even shows the animal swimming upside down like a living anostracan (Hutchinson 1930). It wasn’t until the detailed redescription by Whittington (1975) that Opabinia was revealed to be 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! 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. Later 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 true arthropods, and also suggested the animal had lobopodous limbs in addition to lateral swimming flaps, although this idea was contested by Zhang and Briggs (2007). The issue of whether Opabinia had lobopodous limbs remains controversial. Budd and Daley (2012) figured additional material and provided further arguments in support of this hypothesis. However, other authors have noted similar features in other Burgess Shale arthropods and in decay experiments which appear to represent decayed remains of internal tissues or body cavities extending into the appendages rather than distinct appendages in themselves (Aria and Caron 2015; Butler et al. 2015; Mayers et al. 2018; Moysiuk and Caron 2022). While the morphology of Opabinia remains unique, several recent discoveries have shown that some of its unusual traits are shared by other Cambrian arthropods. Anomalocaris canadensis from the Burgess Shale possessed a similar tail fan (Collins 1996). Several radiodontans, including Stanleycaris and Peytoia appear to share the presence of a large median eye with Opabinia (Moysiuk and Caron 2022). Another animal, Kylinxia zhangi from the Chengjiang deposit, has an even more similar cluster of five large eyes, while also possessing traits of true arthropods like biramous limbs and a headshield (Zeng et al. 2020).
Opabinia has five eyes, a frontal “nozzle,” or proboscis, a body with segmentally 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 a cluster of four bulbous eyes on short stalks and a large fifth eye positioned centrally 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 bands 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 trunk ends in a pair of tiny, rigid spines. The gut begins with a U-shaped bend near the rearward opening ventral mouth end extends to the body termination. Paired, segmentally arranged digestive glands flank the gut. There are also controversial triangular features in the central region of the body, connected to a dark stain surrounding the gut, which have alternatively been interpreted as lobopod-like walking limbs (Budd 1996; Budd and Daley 2012), or extensions of the gut or haemolymph cavities into the flaps (Whittington 1975; Zhang and Briggs 2007; Aria and Caron 2015; Moysiuk and Caron 2022).
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. As for Anomalocaris (Usami 2006; Sheppard et al. 2018), undulatory waves along its lateral flaps could have propelled it forward, while it used its tail fan to steer. Opabinia probably used the claws at the tip of its flexible nozzle to grasp food items and carry them towards its ventral mouth. The spinous morphology of the claws, the large eyes, and the paired digestive glands suggest that Opabinia was an active predator (Whittington 1975; Vannier et al. 2014).
http://paleobiology.si.edu/burgess/opabinia.html