Opabinia regalis is an extinct, stem group marine arthropod found in the Middle Cambrian Burgess Shale Lagerstätte (505 million years ago) of British Columbia. 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.

When the first thorough examination of Opabinia in 1975 revealed its unusual features, it was thought to be unrelated to any known phylum,

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. The generic name is derived from Opabin pass between Mount Hungabee and Mount Biddle, southeast of Lake O'Hara, British Columbia, Canada. and in 1975 he published a detailed description based on a 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. but these fossils were poorly preserved, and Whittington did not feel they provided enough information to be classified as members of the genus Opabinia.—although this interpretation was later questioned by Dzik, who instead concluded that Myoscolex was an unusual annelid worm similar to Pikaia.

Morphology

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Opabinia looked so strange that the audience at the first presentation of Whittington's analysis laughed. The head bore five stalked eyes: two near the front and fairly close to the middle of the head, pointing upwards and forwards; two larger eyes with longer stalks near the rear and outer edges of the head, pointing upwards and sideways; and a single eye between the larger pair of stalked eyes, pointing upwards. It has been assumed that the eyes were all compound, like other arthropods' lateral eyes, The mouth was under the head, behind the proboscis, and pointed backwards, so that the digestive tract formed a U-bend on its way towards the rear of the animal. The proboscis appears to have been sufficiently long and flexible to reach the mouth. Budd (1996) thought the gill blades attached along the front edges on the dorsal side of all except the first flaps. He also found marks inside the flaps' front edges that he interpreted as internal channels connecting the gills to the interior of the body, much as Whittington interpreted the mark along the proboscis as an internal channel. Zhang and Briggs (2007), however, interpreted all flaps have posterior spacing where the gill blades attached. Budd (1996) thought the "triangles" were too wide to fit within Opabinias slender body, and that cross-section views showed they were attached separately from and lower than the lobes, and extended below the body. However, this similar chemical composition is not only associated with the digestive tract; Budd and Daley (2011) suggest that it represents mineralization forming within fluid-filled cavities within the body, which is consistent with hollow lobopods as seen in unequivocal lobopodian fossils. They also clarify that the gut diverticula of Opabinia are a series of circular gut glands individualized from the "triangles". While they agreed on the absence of terminal claws, the presence of lobopods in Opabinia remains as a plausible interpretation. Alberto Simonetta provided a new reconstruction of Opabinia in 1970 very different to those of Hutchinson's, with lots of arthropod features (e.g. ,dorsal exoskeleton and jointed limbs) which are reminiscent of Yohoia and Leanchoilia. Leif Størmer, following earlier work by Percy Raymond, thought that Opabinia belonged to the so-called "trilobitoids" (trilobites and similar taxa). After his thorough analysis, Harry B. Whittington concluded that Opabinia was not an arthropod in 1975, as he found no evidence for arthropodan jointed limbs, and that nothing like the flexible, probably fluid-filled, proboscis was known in arthropods.

In 1985, Derek Briggs and Whittington published a major redescription of Anomalocaris, also from the Burgess Shale. Soon after that, Swedish palaeontologist Jan Bergström, noting in 1986 the similarity of Anomalocaris and Opabinia, suggested that the two animals were related, as they shared numerous features (e.g., lateral flaps, gill blades, stalked eyes, and specialized frontal appendages). He classified them as primitive arthropods, although he considered that arthropods are not a single phylum. He considered the legs of these two genera very similar to those of the Burgess Shale lobopodian Aysheaia and the modern onychophorans (velvet worms), which are regarded as the bearers of numerous ancestral traits shared by the ancestors of arthropods. After examining several sets of features shared by these and similar lobopodians he drew up a "broad-scale reconstruction of the arthropod stem-group", i.e., of arthropods and what he considered to be their evolutionary basal members.) and gilled lobopodians is widely accepted, as well as new discoveries such as the presence of arthropod-like gut glands and the intermediate taxon Kylinxia. The fossil was named Utaurora comosa, and was found within the Wheeler Shale. This discovery could suggest there were other animals that looked like Opabinia, and its family may have been more diverse. and in the early 1970s Niles Eldredge and Stephen Jay Gould developed their theory of punctuated equilibrium, which views evolution as long intervals of near-stasis "punctuated" by short periods of rapid change. On the other hand, around the same time, Wyatt Durham and Martin Glaessner both argued that the animal kingdom had a long Proterozoic history that was hidden by the lack of fossils. Whittington (1975) concluded that Opabinia, and other taxa such as Marrella and Yohoia, cannot be accommodated in modern groups. This was one of the primary reasons why Gould in his book on the Burgess Shale, Wonderful Life, considered that Early Cambrian life was much more disparate and "experimental" than any later set of animals and that the Cambrian explosion was a truly dramatic event, possibly driven by unusual evolutionary mechanisms. He regarded Opabinia as so important to understanding this phenomenon that he wanted to call his book Homage to Opabinia.

However, other discoveries and analyses soon followed, revealing similar-looking animals such as Anomalocaris from the Burgess Shale and Kerygmachela from Sirius Passet. which are regarded as close relatives of arthropods. Paleontologists defined a group called lobopodians to include fossil panarthropods that are thought to be close relatives of onychophorans, tardigrades, and arthropods but lack jointed limbs. This group was later widely accepted as a paraphyletic grade that led to the origin of extant panarthropod phyla.

thumb|Concept of [[stem groups Viewing strange-looking organisms like Opabinia in this way makes it possible to see that, while the Cambrian explosion was unusual, it can be understood in terms of normal evolutionary processes.

See also

References

Further reading

  • Smithsonian page on Opabinia, with photo of Burgess Shale fossil