Opabinia
Encyclopedia
Opabinia is an animal genus found in Cambrian
fossil
deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale
of British Columbia
, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed
, where they comprise less than 0.1% of the community. Opabinia was a soft-bodied animal of modest size, and its segmented body had lobes along the sides and a fan-shaped tail. The head shows unusual features: five eye
s, a mouth under the head and facing backwards, and a proboscis
that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food.
When the first thorough examination of Opabinia in 1975 revealed its unusual features, it was thought to be unrelated to any known phylum
, although possibly related to a hypothetical ancestor of arthropod
s and of annelid
worms. However other finds, most notably Anomalocaris
, suggested that it belonged to a group of animals that were closely related to the ancestors of arthropods and of which the living animals onychophorans and tardigrade
s may also be members.
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. Later the discovery of a whole series of similar lobopod animals, some with closer resemblances to arthropods, and the development of the idea of stem groups suggested that the Early Cambrian was a time of relatively fast evolution
but one that could be understood without assuming any unique evolutionary processes.
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, and in 1975 he published a detailed description based on 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. In 1960 Russian paleontologists described specimens which they
found in the Norilsky region of Siberia
and labelled Opabinia norilica, but these fossils were poorly preserved, and Whittington did not feel they provided enough information to be classified as members of the genus
Opabinia.
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 a new specimen of Myoscolex
which was much better preserved than previous specimens, leading them to conclude that it was a close relative of Opabinia - although this interpretation was later questioned by Dzik, who instead concluded that Myoscolex was an annelid
worm.
, whose total length was about one third of the body's and which projected down from under the head and then curved forwards and upwards. The proboscis was striated
like a vacuum cleaner
's hose and probably flexible, and it ended with a claw-like structure whose inner edges bore spines that projected inwards and forwards. The head bore five eyes: two on stalks near the front and fairly close to the middle of the head, pointing upwards and forwards; two larger eyes, also stalked, near the rear and outer edges of the head, pointing upwards and sideways; and a single eye with a shorter stalk between the larger pair of stalked eyes, pointing upwards. It has been assumed that the eyes were all compound, like arthropod
s' lateral eyes, but this reconstruction, which is not backed up by any evidence, is "somewhat fanciful". 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 appeared sufficiently long and flexible to reach the mouth.
The main part of the body was typically about 5 millimetre (0.196850393700787 in) wide and had 15 segments, on each of which there was a pair of lobes (flaps) pointing downwards and outwards. The lobes 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 lobes 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 exoskeleton
s like those of insects, their bodies were flattened as they were buried and fossilized, and smaller or internal features appear as markings within the outlines of the fossils.
Whittington interpreted as gills some markings on the upper surfaces of all lobes except the first on each side, and thought that these gills were flat underneath, had overlapping layers on top, and attached to the bases of the lobes. Budd
(1996, 2011) thought they consisted of separate "blades" attached along the front edges on the undersides of the lobes. He also found marks inside the lobes' 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. On the other hand Bergström (1986) interpreted both the "gills" and the lobes as part of a covering over the upper surface of the body.
Whittington found evidence of near-triangular features along the body, and concluded that they were internal structures, most likely sideways extensions of the gut. Chen et al. (1994) interpreted them as contained within the lobes along the sides. Budd thought the "triangles" were too wide to fit within Opabinia’s slender body, and that cross-section
views showed they were attached separately from and lower than the lobes, and extended below the body. He later found specimens that appeared to preserve the legs' exterior cuticle. He therefore interpreted the "triangles" as short, fleshy, conical legs. He also found small mineralized patches at the tips of some, and interpreted these as claws. Zhang and Briggs
(2007) analyzed the chemical composition of the "triangles", and concluded that they had the same composition as the gut, and therefore agreed with Whittington that they were part of the digestive system. Instead they regarded Opabinia′s lobe+gill arrangement as an early form of the biramous limbs which are seen in trilobite
s and crustacean
s and which may be the original form in all arthropod
s. However, this similar chemical composition is not only associated with the digestive tract; Budd and Daley suggest that it represents mineralization forming within fluid-filled cavities within the body (consistent with hollow lobopod legs).
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. Whittington, 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 "Mexican waves". On the other hand he thought the body was not flexible enough to allow fish-like undulations of the whole body.
Considering how paleontologists' reconstructions of Opabinia differ, it is not surprising that the animal's classification is still debated. Walcott, the original describer, considered it to be an anostracan crustacean
, while Leif Størmer
, following earlier work by Percy Raymond, thought that it belonged to the so-called "trilobitoids". After his thorough analysis Whittington concluded that Opabinia was no arthropod
, as he found no evidence for arthropodan jointed limbs, and 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 lobes with gills persuaded him that it may have been a representative of the ancestral stock
from which both the annelid
s and arthropods arose.
In 1985, Derek Briggs
and Whittington published a description of Anomalocaris
, also from the Burgess Shale. Swedish palaeontologist Jan Bergström suggested that the two animals were related, as they shared lateral flaps with gills, stalked eyes and other features; and he classified them as primitive arthropods, although he considered that arthropods are not a single phylum.
including Hallucigenia
and Microdictyon
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Simplified summary of Budd's "broad-scale" cladogram (1996)Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
deposits. Its sole species, Opabinia regalis, is known from the Middle Cambrian Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, Canada. Fewer than twenty good specimens have been described; 3 specimens of Opabinia are known from the Greater Phyllopod bed
Phyllopod bed
The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess shale fossil lagerstatte. It was quarried by Charles Walcott from 1911–1917, and was the source of 95% of the fossils he collected during this time;tens of thousands of...
, where they comprise less than 0.1% of the community. Opabinia was a soft-bodied animal of modest size, and its segmented body had lobes along the sides and a fan-shaped tail. The head shows unusual features: five eye
Eye
Eyes are organs that detect light and convert it into electro-chemical impulses in neurons. The simplest photoreceptors in conscious vision connect light to movement...
s, a mouth under the head and facing backwards, and a proboscis
Proboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...
that probably passed food to the mouth. Opabinia probably lived on the seafloor, using the proboscis to seek out small, soft food.
When the first thorough examination of Opabinia in 1975 revealed its unusual features, it was thought to be unrelated to any known phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
, although possibly related to a hypothetical ancestor of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s and of annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
worms. However other finds, most notably Anomalocaris
Anomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...
, suggested that it belonged to a group of animals that were closely related to the ancestors of arthropods and of which the living animals onychophorans and tardigrade
Tardigrade
Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...
s may also be members.
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
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
, or had arisen earlier but without leaving fossils. At first Opabinia was regarded as strong evidence for the "explosive" hypothesis. Later the discovery of a whole series of similar lobopod animals, some with closer resemblances to arthropods, and the development of the idea of stem groups suggested that the Early Cambrian was a time of relatively fast evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
but one that could be understood without assuming any unique evolutionary processes.
History of discovery
Charles Doolittle Walcott
Charles Doolittle Walcott was an American invertebrate paleontologist. He became known for his discovery in 1909 of well-preserved fossils in the Burgess Shale of British Columbia, Canada.-Early life:...
found in the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
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
Mount Hungabee
Mount Hungabee, officially Hungabee Mountain, is a mountain located on the boundaries of Banff National Park and Yoho National Park on the Continental Divide at the head of Paradise Valley. The peak was named in 1894 by Samuel Allen after the Stoney Indian word for "chieftain" as the mountain is...
and Mount Biddle, southeast of Lake O'Hara
Lake o'hara
Lake O'Hara a lake at an elevation of 2,115 m in the alpine area of Yoho National Park, in the province of British Columbia, on the western side of the Great Divide with the province of Alberta and Banff National Park to the east...
, British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. In 1966-1967 Harry B. Whittington
Harry B. Whittington
Harry Blackmore Whittington FRS was a British paleontologist based at the Department of Earth Sciences, Cambridge, and was affiliated to Sidney Sussex College. He attended Handsworth Grammar School in Birmingham, followed by a degree and Ph.D in geology from the University of Birmingham...
found another good specimen, and in 1975 he published a detailed description based on very thorough dissection
Dissection
Dissection is usually the process of disassembling and observing something to determine its internal structure and as an aid to discerning the functions and relationships of its components....
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. In 1960 Russian paleontologists described specimens which they
found in the Norilsky region of Siberia
Siberia
Siberia is an extensive region constituting almost all of Northern Asia. Comprising the central and eastern portion of the Russian Federation, it was part of the Soviet Union from its beginning, as its predecessor states, the Tsardom of Russia and the Russian Empire, conquered it during the 16th...
and labelled Opabinia norilica, but these fossils were poorly preserved, and Whittington did not feel they provided enough information to be classified as members of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Opabinia.
Occurrence
Phyllopod bed
The Phyllopod bed, designated by USNM locality number 35k, is the most famous fossil-bearing member of the Burgess shale fossil lagerstatte. It was quarried by Charles Walcott from 1911–1917, and was the source of 95% of the fossils he collected during this time;tens of thousands of...
" of the Burgess Shale, in the Canadian Rockies
Canadian Rockies
The Canadian Rockies comprise the Canadian segment of the North American Rocky Mountains range. They are the eastern part of the Canadian Cordillera, extending from the Interior Plains of Alberta to the Rocky Mountain Trench of British Columbia. The southern end borders Idaho and Montana of the USA...
of British Columbia
British Columbia
British Columbia is the westernmost of Canada's provinces and is known for its natural beauty, as reflected in its Latin motto, Splendor sine occasu . Its name was chosen by Queen Victoria in 1858...
.
In 1997 Briggs and Nedin reported from South Australia a new specimen of Myoscolex
Myoscolex
Myoscolex is a problematic fossil from the Cambrian Emu Bay Shale that, originally interpreted as an annelid, seems to be an arthropod; the possible presence of an Opabinia-like nozzle suggests a plausible relationship with that animal....
which was much better preserved than previous specimens, leading them to conclude that it was a close relative of Opabinia - although this interpretation was later questioned by Dzik, who instead concluded that Myoscolex was an annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
worm.
Description
Opabinia looked so strange that the audience at the first presentation of Whittington's analysis laughed. The length of Opabinia regalis from head to tail ranged between 4 centimetres (1.6 in) and 7 centimetres (2.8 in). The animal also had a hollow proboscisProboscis
A proboscis is an elongated appendage from the head of an animal, either a vertebrate or an invertebrate. In simpler terms, a proboscis is the straw-like mouth found in several varieties of species.-Etymology:...
, whose total length was about one third of the body's and which projected down from under the head and then curved forwards and upwards. The proboscis was striated
Striation
Striations means a series of ridges, furrows or linear marks, and are used in several ways* Glacial striation* Striation , a striation as a result of a geological fault* In medicine, striated muscle...
like a vacuum cleaner
Vacuum cleaner
A vacuum cleaner, commonly referred to as a "vacuum," is a device that uses an air pump to create a partial vacuum to suck up dust and dirt, usually from floors, and optionally from other surfaces as well. The dirt is collected by either a dustbag or a cyclone for later disposal...
's hose and probably flexible, and it ended with a claw-like structure whose inner edges bore spines that projected inwards and forwards. The head bore five eyes: two on stalks near the front and fairly close to the middle of the head, pointing upwards and forwards; two larger eyes, also stalked, near the rear and outer edges of the head, pointing upwards and sideways; and a single eye with a shorter stalk between the larger pair of stalked eyes, pointing upwards. It has been assumed that the eyes were all compound, like arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s' lateral eyes, but this reconstruction, which is not backed up by any evidence, is "somewhat fanciful". 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 appeared sufficiently long and flexible to reach the mouth.
The main part of the body was typically about 5 millimetre (0.196850393700787 in) wide and had 15 segments, on each of which there was a pair of lobes (flaps) pointing downwards and outwards. The lobes 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 lobes 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 exoskeleton
Exoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
s like those of insects, their bodies were flattened as they were buried and fossilized, and smaller or internal features appear as markings within the outlines of the fossils.
Whittington interpreted as gills some markings on the upper surfaces of all lobes except the first on each side, and thought that these gills were flat underneath, had overlapping layers on top, and attached to the bases of the lobes. Budd
Graham Budd
Graham Edward Budd is a British palaeontologist, Professor of paleobiology at Uppsala University.Budd’s research primarily has focused on the anatomy and evolutionary significance of Paleozoic arthropods and in the integration of palaeontology into evolutionary developmental biology...
(1996, 2011) thought they consisted of separate "blades" attached along the front edges on the undersides of the lobes. He also found marks inside the lobes' 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. On the other hand Bergström (1986) interpreted both the "gills" and the lobes as part of a covering over the upper surface of the body.
Whittington found evidence of near-triangular features along the body, and concluded that they were internal structures, most likely sideways extensions of the gut. Chen et al. (1994) interpreted them as contained within the lobes along the sides. Budd thought the "triangles" were too wide to fit within Opabinia’s slender body, and that cross-section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...
views showed they were attached separately from and lower than the lobes, and extended below the body. He later found specimens that appeared to preserve the legs' exterior cuticle. He therefore interpreted the "triangles" as short, fleshy, conical legs. He also found small mineralized patches at the tips of some, and interpreted these as claws. Zhang and Briggs
Derek Briggs
Derek Ernest Gilmor Briggs is an Irish paleontologist and taphonomist based at Yale University. Briggs is one of three paleontologists who were key in the reinterpretation of the fossils of the Burgess Shale.-Professional achievements:...
(2007) analyzed the chemical composition of the "triangles", and concluded that they had the same composition as the gut, and therefore agreed with Whittington that they were part of the digestive system. Instead they regarded Opabinia′s lobe+gill arrangement as an early form of the biramous limbs which are seen in trilobite
Trilobite
Trilobites are a well-known fossil group of extinct marine arthropods that form the class Trilobita. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period , and they flourished throughout the lower Paleozoic era before...
s and crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
s and which may be the original form in all arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s. However, this similar chemical composition is not only associated with the digestive tract; Budd and Daley suggest that it represents mineralization forming within fluid-filled cavities within the body (consistent with hollow lobopod legs).
Lifestyle
The way in which the Burgess ShaleBurgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
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. Whittington, 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 "Mexican waves". On the other hand he thought the body was not flexible enough to allow fish-like undulations of the whole body.
Classification
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...
, while Leif Størmer
Leif Størmer
Leif Størmer was a Norwegian paleontologist and geologist. He was professor of historical geology at the University of Oslo from 1946 to 1975. His father was the famous mathematician Carl Størmer.-Early life and education:...
, following earlier work by Percy Raymond, thought that it belonged to the so-called "trilobitoids". After his thorough analysis Whittington concluded that Opabinia was no arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
, as he found no evidence for arthropodan jointed limbs, and nothing like the flexible, probably fluid-filled proboscis was known in arthropods. Although he left Opabinia′s classification above the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
level open, the annulated but not articulated body and the unusual lateral lobes with gills persuaded him that it may have been a representative of the ancestral stock
Articulata Hypothesis
The Articulata Hypothesis is the grouping in a higher taxon of animals with segmented bodies, consisting of Annelida and Panarthropoda. This theory states that these groups are descended from a common segmented ancestor...
from which both the annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
s and arthropods arose.
In 1985, Derek Briggs
Derek Briggs
Derek Ernest Gilmor Briggs is an Irish paleontologist and taphonomist based at Yale University. Briggs is one of three paleontologists who were key in the reinterpretation of the fossils of the Burgess Shale.-Professional achievements:...
and Whittington published a description of Anomalocaris
Anomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...
, also from the Burgess Shale. Swedish palaeontologist Jan Bergström suggested that the two animals were related, as they shared lateral flaps with gills, stalked eyes and other features; and he classified them as primitive arthropods, although he considered that arthropods are not a single phylum.
Hallucigenia
Hallucigenia is an extinct genus of animal found as fossils in the Middle Cambrian-aged Burgess Shale formation of British Columbia, Canada, represented by the species H. sparsa, and in the Lower Cambrian Maotianshan shale of China, represented by the species H. fortis...
and Microdictyon
Microdictyon
Microdictyon is an extinct "armored worm" coated with dot-likescleritic scales, known from the Early CambrianMaotianshan shale of Yunnan China. Microdictyon is sometimes included...
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In 1996 Graham Budd
Graham Budd
Graham Edward Budd is a British palaeontologist, Professor of paleobiology at Uppsala University.Budd’s research primarily has focused on the anatomy and evolutionary significance of Paleozoic arthropods and in the integration of palaeontology into evolutionary developmental biology...
found what he considered evidence of short, un-jointed legs in Opabinia. His examination of the anomalocarid
Anomalocarid
AnomalocarididsNeolatin compound word from Greek ἀνώμαλος anomalos and καρίς karis , meaning "strange shrimp". Note that while "Anomalocarid" is a widely used alternative spelling, the double "id" at the end is technically the correct form, for the reasons given in...
Kerygmachela from the Sirius Passet
Sirius Passet
Sirius Passet is a Cambrian Lagerstätte in Greenland. The Sirius Passet Lagerstätte was named after the Sirius sledge patrol that operates in North Greenland. It comprises six localities located on the eastern shore of J.P. Koch Fjord in the far north of Greenland. It was discovered in 1984 by A....
lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
, about and over 10M years older than the Burgess Shale, convinced him that this had similar legs. He considered the legs of these two genera
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
very similar to those of the Burgess Shale Aysheaia
Aysheaia
Aysheaia was a genus of soft-bodied, caterpillar-shaped organisms average body length of 1–6 cm. The genus name commemorates a mountain peak named "Ayesha" due north of the Wapta Glacier. This peak was originally named Aysha in the 1904 maps of the region, and was re-named Ayesha after the heroine...
and the modern onychophorans, which are regarded as closely related to the ancestors of arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
s. After examining several sets of features shared by these and similar lobopods he drew up a "broad-scale reconstruction of the arthropod stem-group", in other words of arthropods and what he considered to be their evolutionary "aunts" and "cousins". One striking feature of this family tree is that modern tardigrades may be Opabinia’s closest living evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary relatives.
Although Zhang and Briggs (2007) disagreed with Budd's diagnosis that Opabinia’s "triangles" were legs, the resemblance they saw between Opabinia’s 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.
Theoretical significance
Opabinia made it clear how little was known about soft-bodied animals, which do not usually leave fossils. When Whittington described it in the mid-1970s, there was already a vigorous debate about the early evolution of animalAnimal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...
s. Preston Cloud
Preston Cloud
Preston Ercelle Cloud, Jr. was an American paleontologist, geographer, and professor. He was best-known for his work on the geologic time scale and the origin of life on Earth.-Early life:...
argued in 1948 and 1968 that the process was "explosive", and in the early 1970s Niles Eldredge
Niles Eldredge
Niles Eldredge is an American paleontologist, who, along with Stephen Jay Gould, proposed the theory of punctuated equilibrium in 1972.-Education:...
and Stephen Jay Gould
Stephen Jay Gould
Stephen Jay Gould was an American paleontologist, evolutionary biologist, and historian of science. He was also one of the most influential and widely read writers of popular science of his generation....
developed their theory of punctuated equilibrium
Punctuated equilibrium
Punctuated equilibrium is a theory in evolutionary biology which proposes that most species will exhibit little net evolutionary change for most of their geological history, remaining in an extended state called stasis...
, 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
Martin Glaessner
Martin Fritz Glaessner AM was a geologist and palaeontologist. Born and educated in Austro-Hungarian Empire, he spent the majority of his life in working for oil companies in Russia, and studying the geology of the South Pacific in Australia...
both argued that the animal kingdom had a long Proterozoic
Proterozoic
The Proterozoic is a geological eon representing a period before the first abundant complex life on Earth. The name Proterozoic comes from the Greek "earlier life"...
history that was hidden by the lack of fossils. Whittington (1975) concluded that Opabinia, and other taxa such as Marrella and Yohoia
Yohoia
Yohoia is a tiny, extinct animal from the Cambrian period that has been found as fossils in the Burgess Shale formation of British Columbia, Canada. It has been placed among the arachnomorphs, a group of arthropods that includes the chelicerates and trilobites. Their sizes range from 7 to 23 mm...
, cannot be accommodated in modern groups. This was one of the primary reasons why Gould in his book on the Burgess Shale
Burgess Shale
The Burgess Shale Formation, located in the Canadian Rockies of British Columbia, is one of the world's most celebrated fossil fields, and the best of its kind. It is famous for the exceptional preservation of the soft parts of its fossils...
, Wonderful Life
Wonderful Life (book)
Wonderful Life: The Burgess Shale and the Nature of History is a book on the evolution of Cambrian fauna by Harvard paleontologist Stephen Jay Gould...
, considered that Early Cambrian life was much more diverse and "experimental" than any later set of animals and that the Cambrian explosion
Cambrian explosion
The Cambrian explosion or Cambrian radiation was the relatively rapid appearance, around , of most major phyla, as demonstrated in the fossil record, accompanied by major diversification of other organisms, including animals, phytoplankton, and calcimicrobes...
was a truly dramatic event, possibly driven by unusual evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...
ary 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
Anomalocaris
Anomalocaris is an extinct genus of anomalocaridid, which are, in turn, thought to be closely related to the arthropods. The first fossils of Anomalocaris were discovered in the Ogygopsis Shale by Joseph Frederick Whiteaves, with more examples found by Charles Doolittle Walcott in the famed...
from the Burgess Shale and Kerygmachela from Sirius Passet
Sirius Passet
Sirius Passet is a Cambrian Lagerstätte in Greenland. The Sirius Passet Lagerstätte was named after the Sirius sledge patrol that operates in North Greenland. It comprises six localities located on the eastern shore of J.P. Koch Fjord in the far north of Greenland. It was discovered in 1984 by A....
. Another Burgess Shale animal, Aysheaia
Aysheaia
Aysheaia was a genus of soft-bodied, caterpillar-shaped organisms average body length of 1–6 cm. The genus name commemorates a mountain peak named "Ayesha" due north of the Wapta Glacier. This peak was originally named Aysha in the 1904 maps of the region, and was re-named Ayesha after the heroine...
, was considered very similar to modern Onychophora, which are regarded as close relatives of arthropods. Paleontologists defined a group called lobopods to include animals that are thought to be close relatives of arthropods but which lack jointed limbs. There is still debate about whether lobopods are monophyletic, i.e. whether they include all and only the descendants of a single common ancestor, and about whether arthropods are a sub-group of lobopods or a sister-group.
While this discussion about specific fossils such as Opabinia and Anomalocaris was going on, the concept of stem groups was introduced to cover evolutionary "aunts" and "cousins". A crown group
Crown group
A crown group is a group consisting of living representatives, their ancestors back to the most recent common ancestor of that group, and all of that ancestor's descendants. The name was given by Willi Hennig, the formulator of phylogenetic systematics, as a way of classifying living organisms...
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 tardigrade
Tardigrade
Tardigrades form the phylum Tardigrada, part of the superphylum Ecdysozoa. They are microscopic, water-dwelling, segmented animals with eight legs. Tardigrades were first described by Johann August Ephraim Goeze in 1773...
s are living animals which form a crown group in their own right, but Budd (1996) regarded them also as being a stem group relative to the arthropods. 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.