Opabinia

History of discovery

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. The generic name is derived from Opabin pass between Mount Hungabee and Mount Biddle, southeast of Lake O'Hara, British Columbia, Canada. In 1966–1967, Harry B. Whittington found another good specimen,{{cite book

Occurrence

All the recognized Opabinia specimens found so far come from the "Phyllopod bed" of the Burgess Shale, in the Canadian Rockies of British Columbia.

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{{cite journal

Morphology

File:20191108 Opabinia regalis.png|Restoration File:20210222 Opabinia size.png|Size estimation

Opabinia looked so strange that the audience at the first presentation of Whittington's analysis laughed. The length of Opabinia regalis from head (excluding proboscis) to tail end ranged between 4 cm and 7 cm. One of the most distinctive characters of Opabinia is the hollow proboscis, whose total length was about one-third that of the body, and projected down from under the head. The proboscis was striated like a vacuum cleaner's hose and flexible, and it ended with a claw-like structure whose terminal edges bore 5 spines that projected inwards and forwards. The bilateral symmetry and lateral (instead of vertical as reconstructed by Whittington 1975) arrangement of the claw suggest it represents a pair of fused frontal appendages, comparable to those of radiodonts and gilled lobopodians. 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, but this reconstruction, which is not backed up by any evidence, is "somewhat fanciful".{{Cite journal | doi-access = free

The main part of the body was typically about 5 mm wide and had 15 segments, on each of which there were pairs of flaps (lobes) pointing downwards and outwards. The flaps overlapped so that the front of each was covered by the rear edge of the one ahead of it. The body ended with what looked like a single conical segment bearing three pairs of overlapping tail fan blades that pointed up and out, forming a tail like a V-shaped double fan.

Interpretations of other features of Opabinia fossils differ. Since the animals did not have mineralized armor nor even tough organic exoskeletons like those of other arthropods, their bodies were flattened as they were buried and fossilized, and smaller or internal features appear as markings within the outlines of the fossils.

File:20210809 Opabinia regalis flap gill interpretation.png|Various interpretations on the flap and gill structures of Opabinia regalis A: Whittington (1975), B: Bergström (1986), C: Budd (1996), D: Zhang & Briggs (2007), E: Budd & Daley (2011) File:20210807 Opabinia regalis trunk cross section.png|Opabinia cross-section based on Budd and Daley (2011)

Whittington (1975) interpreted the gills as paired extensions attached dorsally to the bases of all but the first flaps on each side, and thought that these gills were flat underneath, had overlapping layers on top. Bergström (1986) revealed the "overlapping layers" were rows of individual blades, interpreted the flaps as part of dorsal coverings (tergite) over the upper surface of the body, with blades attached underneath each of them.{{cite journal Whittington (1975) found evidence of near-triangular features along the body, and concluded that they were internal structures, most likely sideways extensions of the gut (diverticula). Chen et al. (1994) interpreted them as contained within the lobes along the sides.{{cite journal

Lifestyle

The way in which the Burgess Shale animals were buried, by a mudslide or a sediment-laden current that acted as a sandstorm, suggests they lived on the surface of the seafloor. Opabinia probably used its proboscis to search the sediment for food particles and pass them to its mouth. Since there is no sign of anything that might function as jaws, its food was presumably small and soft. The paired gut diverticula may increase the efficiency of food digestion and intake of nutrition. Whittington (1975) believing that Opabinia had no legs, thought 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. On the other hand, he thought the body was not flexible enough to allow fish-like undulations of the whole body.

Classification

Considering how paleontologists' reconstructions of Opabinia differ, it is not surprising that the animal's classification was highly debated during the 20th century. Charles Doolittle Walcott, the original describer, considered it to be an anostracan crustacean in 1912. The idea was followed by G. Evelyn Hutchinson in 1930, providing the first reconstruction of Opabinia as an anostracan swimming upside down. 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 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. Although he left ''Opabinia'''s classification above the family level open, the annulated but not articulated body and the unusual lateral flaps with gills persuaded him that it may have been a representative of the ancestral stock from the origin of annelids and arthropods, two distinct animal phyla (Lophotrochozoan and Ecdysozoan, respectively) which were still thought to be close relatives (united under Articulata) at that time.

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.

In 1996, Graham Budd found what he considered evidence of short, un-jointed legs in Opabinia. His examination of the gilled lobopodian Kerygmachela from the Sirius Passet lagerstätte, about and over 10M years older than the Burgess Shale, convinced him that this specimen had similar legs.{{cite journal | editor-last =Fortey | editor-first =R. A. | editor-link = Richard Fortey | editor2-last =Thomas | editor2-first =R. H.

Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that Opabinias "triangles" were legs, the resemblance they saw between Opabinias lobe+gill arrangement and arthropods' biramous limbs led them to conclude that Opabinia was very closely related to arthropods. In fact they presented a family tree very similar to Budd's except that theirs did not mention tardigrades. Regardless of the different morphological interpretations, all major restudies since 1980s similarly concluded that the resemblance between Opabinia and arthropods (e.g., stalked eyes, dorsal segmentation, posterior mouth, fused appendages, gill-like limb branches) are taxonomically significant.

Since the 2010s, the suggested close relationship between Opabinia and tardigrades/cycloneuralians is no longer supported, while the affinity of Opabinia as a stem-group arthropod alongside Radiodonta (a clade that includes Anomalocaris and its relatives) and gilled lobopodians is widely accepted, as consistently shown by multiple phylogenetic analyses, as well as new discoveries such as the presence of arthropod-like gut glands and the intermediate taxon Kylinxia.

In 2022, Paleontologists described a similar looking animal which was discovered in Cambrian-aged rocks of Utah. The fossil was named Utaurora comosa, and was found within the Wheeler Shale. The stem-arthropod was actually first described in 2008, but at the time it was originally considered a specimen of Anomalocaris. This discovery could suggest there were other animals that looked like Opabinia, and its family may have been more diverse.

Theoretical significance

Main article: Cambrian explosion

However, other discoveries and analyses soon followed, revealing similar-looking animals such as Anomalocaris from the Burgess Shale and Kerygmachela from Sirius Passet. Another Burgess Shale animal, Aysheaia, was considered very similar to modern Onychophora, 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.

| | | | | | While this discussion about specific fossils such as Opabinia and Anomalocaris was going on in the late 20th century, the concept of stem groups was introduced to cover evolutionary "aunts" and "cousins". A crown group is a group of closely related living animals plus their last common ancestor plus all its descendants. A stem group contains offshoots from members of the lineage earlier than the last common ancestor of the crown group; it is a relative concept, for example tardigrades are living animals that form a crown group in their own right, but Budd (1996) regarded them also as being a stem group relative to the arthropods.{{cite journal |author1=Craske, A. J. |author2=Jefferies, R. P. S. |year=1989 |title=A new mitrate from the Upper Ordovician of Norway, and a new approach to subdividing a plesion |journal=Palaeontology |volume=32 |pages=69–99

References

References

1. Whittington, H. B.. (June 1975). "The enigmatic animal ''Opabinia regalis'', Middle Cambrian Burgess Shale, British Columbia". [[Philosophical Transactions of the Royal Society B]].
2. (October 2006). "Taphonomy of the Greater Phyllopod Bed community, Burgess Shale". PALAIOS.
3. Budd, Graham E.. (1998). "The morphology and phylogenetic significance of Kerygmachela kierkegaardi Budd (Buen Formation, Lower Cambrian, N Greenland)". Earth and Environmental Science Transactions of the Royal Society of Edinburgh.
4. Briggs, Derek E. G.. (2015-04-19). "Extraordinary fossils reveal the nature of Cambrian life: a commentary on Whittington (1975) 'The enigmatic animal Opabinia regalis, Middle Cambrian, Burgess Shale, British Columbia'". Philosophical Transactions of the Royal Society B: Biological Sciences.
5. Ortega-Hernández, Javier. (2016). "Making sense of 'lower' and 'upper' stem-group Euarthropoda, with comments on the strict use of the name Arthropoda von Siebold, 1848". Biological Reviews.
6. (2017-05-01). "Origin and evolution of the panarthropod head – A palaeobiological and developmental perspective". Arthropod Structure & Development.
7. WALCOTT, C. D. 1912. [https://repository.si.edu/bitstream/handle/10088/23430/SMC_57_Walcott_1910_6_145-245.pdf?sequence=1&isAllowed=y Middle Cambrian Branchiopoda, Malacostraca, Trilobita and Merostomata]. Smithsonian Miscellaneous Collections, 57: 145-228.
8. Miroshnikov, L. D. and Krawzov, A. G. (1960). Rare paleontological remains and traces of life in late Cambrian deposits of the northwestern Siberian platform. Palaeontology and biostratigraphy of the Soviet Arctic, 3, pp. 28–41.
9. (2006). "Originations, Radiations and Biodiversity Changes – evidences from the Chinese fossil record.".
10. Chipman, Ariel D.. (2015-12-18). "An embryological perspective on the early arthropod fossil record". BMC Evolutionary Biology.
11. (2011). "The lobes and lobopods of ''Opabinia regalis'' from the middle Cambrian Burgess Shale". Lethaia.
12. Hutchinson, George Evelyn (1930). [https://repository.si.edu/handle/10088/15851 Restudy of some Burgess shale fossils]. ''Proceedings of the United States National Museum'' '''78''' (2854): 1–24. doi:10.5479/si.00963801.78-2854.1.
13. Simonetta AM. 1970 Studies on non trilobite arthropods of the Burgess Shale (Middle Cambrian). Palaeontogr. Ital. 66, 35–45.
14. Edgecombe, Gregory D.. (2009). "Palaeontological and Molecular Evidence Linking Arthropods, Onychophorans, and other Ecdysozoa". Evolution: Education and Outreach.
15. (1985-05-14). "The largest Cambrian animal, Anomalocaris, Burgess Shale, British-Columbia". Philosophical Transactions of the Royal Society of London. B, Biological Sciences.
16. (1995-09-01). "Anomalocaris and other large animals in the lower Cambrian Chengjiang fauna of southwest China". GFF.
17. Collins, Desmond. (1996). "The "evolution" of Anomalocaris and its classification in the arthropod class Dinocarida (nov.) and order Radiodonta (nov.)". Journal of Paleontology.
18. (2012-12-07). "Cambrian bivalved arthropod reveals origin of arthrodization". Proceedings of the Royal Society B: Biological Sciences.
19. Legg, David. (2013). "Multi-Segmented Arthropods from the Middle Cambrian of British Columbia (Canada)". Journal of Paleontology.
20. (2013). "The affinities of the cosmopolitan arthropod ''Isoxys'' and its implications for the origin of arthropods". Lethaia.
21. (2013-09-30). "Arthropod fossil data increase congruence of morphological and molecular phylogenies". Nature Communications.
22. (2014). "Hallucigenia's onychophoran-like claws and the case for Tactopoda". Nature.
23. (2015). "Anomalocaridid trunk limb homology revealed by a giant filter-feeder with paired flaps". Nature.
24. (2015). "Hallucigenia's head and the pharyngeal armature of early ecdysozoans". Nature.
25. (2017-01-31). "Cambrian suspension-feeding lobopodians and the early radiation of panarthropods". BMC Evolutionary Biology.
26. (2018). "A three-dimensionally preserved lobopodian from the Herefordshire (Silurian) Lagerstätte, UK". Royal Society Open Science.
27. (2020). "An early Cambrian euarthropod with radiodont-like raptorial appendages". Nature.
28. (2021). "Stranger than a scorpion: a reassessment of ''Parioscorpio venator'', a problematic arthropod from the Llandoverian Waukesha Lagerstätte". Palaeontology.
29. (2020). "Exceptional multifunctionality in the feeding apparatus of a mid-Cambrian radiodont". Paleobiology.
30. (2014-05-02). "Sophisticated digestive systems in early arthropods". Nature Communications.
31. (2022-02-09). "New opabiniid diversifies the weirdest wonders of the euarthropod stem group". Proceedings of the Royal Society B: Biological Sciences.
32. (2008). "Middle Cambrian arthropods from Utah". Journal of Paleontology.
33. Bengtson, Stefan. (2004). "Neoproterozoic-Cambrian Biological Revolutions: Presented as a Paleontological Society Short Course at the Annual Meeting of the Geological Society of America, Denver, Colorado, November 6, 2004". Yale University Reprographics & Imaging Service; [[Paleontological Society]].
34. Gould, S. J.. (1989). "Wonderful Life". Hutchinson Radius.
35. Knoll, A. H.. (2004). "The First Three Billion Years of Evolution on Earth". Princeton University Press.
36. Robison, R. A.. (January 1985). "Affinities of ''Aysheaia'' (Onychophora), with Description of a New Cambrian Species". Journal of Paleontology.
37. (2000). "Molluscan engrailed Expression, Serial Organization, and Shell Evolution". Evolution and Development.
38. Budd, G. E.. (2003). "The Cambrian Fossil Record and the Origin of the Phyla". Integrative and Comparative Biology.