Amiskwia
Encyclopedia
Amiskwia is a large, soft-bodied invertebrate
of unknown affinity known from fossil
s of the Middle Cambrian
Lagerstätten both in the Burgess shale
formation in British Columbia
and the Maotianshan shales
of Yunnan
Province, China.
Very few specimens of this organism have been found, only eighteen in the Burgess shale — which may be a reflection of its genuine rarity, but is more likely to be due to taphonomic (preservational) or behavioural factors. The fossils reach 25 mm (0.984251968503937 in) in length. The head is rounded, tipped with two tentacles, and appears to contain a four-ganglion
brain; the body flattens out and broadens in the trunk, which appears to have been fairly muscular. Where the trunk meets the head there is a small tubular opening, which can be interpreted as the mouth; the gut terminates where the trunk narrows and meets the tail, which is broad and paddle shaped. The body morphology suggests a free energetic swimmer, which may be consistent with the dearth of fossils.
Amiskwia was originally categorized by paleontologist
Charles Walcott. Walcott thought he saw three buccal spines in the fossils, and therefore categorized Amiskwia as a chaetognath
worm (arrow worm). However, Amiskwia appears to lack the characteristic grasping spines and teeth of other Burgess fossil arrow worms. Later scientists suggested an affinity with the nemertea
ns (ribbon worms), but the evidence for this was somewhat inadequate. Conway Morris
, on re-examining of the Burgess Shale fauna in the 1970s, described it as being the single known species in an otherwise unknown phylum, given that it has two tentacles near its mouth, rather than the characteristic single tentacle of true nemerteans. (Nemerteans do not have a single tentacle. However, a pair of antero-lateral tentacles is present in 2 of the many genera of pelagic nemerteans. Nemerteans do have a single eversible - normally internal - proboscis, which when everted could resemble an anterior median tentacle if fossilized. Whether retracted or everted, the proboscis is the only structure in pelagic nemerteans likely to fossilize, as it is the only structure with substantial connective tissue and muscle. The body wall has almost no muscle or connective tissue and is exceedingly unlikely to fossilize; hence, a pelagic nemertean fossil would be only the proboscis). Butterfield implies from the appearance of the fossils that the organisms may have lacked a cuticle: whilst this is also true of the nemerteans, these organisms lack a coelom
and are thus unlikely to fossilise. He goes on to argue that the absence of cuticle is characteristic of the Chaetognaths; whilst teeth would be expected, a similar fossil, Wiwaxia
, only shows such structures in 10% of the expected instances, and Anamolocarid
s are often found detached from their mouthparts, so the absence may be taphonomic rather than genuine. The absence of spines could simply mean that the fossils represent young organisms — or that later chaetognath evolution involved paedomorphosis.
While more work on undescribed specimens is required before firm consensus is reached, the discovery of P. spinosa, which strongly resembles modern Chaetognaths, in sediments of a similar age in Chengjiang biota
places doubts on its position in the Chaetognath crown group. It has also been likened to the molluscs, and bears limited resemblance to unusual forms such as Pterotrachea
. Of course, many similarly enigmatic Cambrian fossils probably represent stem groups to living taxa, and there is no reason to assume that this organism must fall into an extant crown group at all.
The name Amiskwia sagittiformis derives from the nearby Amiskwia river, and its sagittiform shape.
Invertebrate
An invertebrate is an animal without a backbone. The group includes 97% of all animal species – all animals except those in the chordate subphylum Vertebrata .Invertebrates form a paraphyletic group...
of unknown affinity known from fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
s of the Middle Cambrian
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...
Lagerstätten both 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...
formation in 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...
and the Maotianshan shales
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
of Yunnan
Yunnan
Yunnan is a province of the People's Republic of China, located in the far southwest of the country spanning approximately and with a population of 45.7 million . The capital of the province is Kunming. The province borders Burma, Laos, and Vietnam.Yunnan is situated in a mountainous area, with...
Province, China.
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....
brain; the body flattens out and broadens in the trunk, which appears to have been fairly muscular. Where the trunk meets the head there is a small tubular opening, which can be interpreted as the mouth; the gut terminates where the trunk narrows and meets the tail, which is broad and paddle shaped. The body morphology suggests a free energetic swimmer, which may be consistent with the dearth of fossils.
Amiskwia was originally categorized by paleontologist
Paleontology
Paleontology "old, ancient", ὄν, ὀντ- "being, creature", and λόγος "speech, thought") is the study of prehistoric life. It includes the study of fossils to determine organisms' evolution and interactions with each other and their environments...
Charles Walcott. Walcott thought he saw three buccal spines in the fossils, and therefore categorized Amiskwia as a chaetognath
Chaetognatha
Chaetognatha, meaning hair-jaws, and commonly known as arrow worms, are a phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic, that is belonging to the lowest zone of the ocean, or benthic zone, and can attach to algae and...
worm (arrow worm). However, Amiskwia appears to lack the characteristic grasping spines and teeth of other Burgess fossil arrow worms. Later scientists suggested an affinity with the nemertea
Nemertea
Nemertea is a phylum of invertebrate animals also known as "ribbon worms" or "proboscis worms". Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela. Although most are less than long, one specimen has been estimated at , which would make it the longest animal ever...
ns (ribbon worms), but the evidence for this was somewhat inadequate. Conway Morris
Simon Conway Morris
Simon Conway Morris FRS is an English paleontologist made known by his detailed and careful study of the Burgess Shale fossils, an exploit celebrated in Wonderful Life by Stephen Jay Gould...
, on re-examining of the Burgess Shale fauna in the 1970s, described it as being the single known species in an otherwise unknown phylum, given that it has two tentacles near its mouth, rather than the characteristic single tentacle of true nemerteans. (Nemerteans do not have a single tentacle. However, a pair of antero-lateral tentacles is present in 2 of the many genera of pelagic nemerteans. Nemerteans do have a single eversible - normally internal - proboscis, which when everted could resemble an anterior median tentacle if fossilized. Whether retracted or everted, the proboscis is the only structure in pelagic nemerteans likely to fossilize, as it is the only structure with substantial connective tissue and muscle. The body wall has almost no muscle or connective tissue and is exceedingly unlikely to fossilize; hence, a pelagic nemertean fossil would be only the proboscis). Butterfield implies from the appearance of the fossils that the organisms may have lacked a cuticle: whilst this is also true of the nemerteans, these organisms lack a coelom
Coelom
The coelom is a fluid-filled cavity formed within the mesoderm. Coeloms developed in triploblasts but were subsequently lost in several lineages. Loss of coelom is correlated with reduction in body size...
and are thus unlikely to fossilise. He goes on to argue that the absence of cuticle is characteristic of the Chaetognaths; whilst teeth would be expected, a similar fossil, Wiwaxia
Wiwaxia
Wiwaxia is a genus of soft-bodied, scale-covered animals known from Burgess shale type Lagerstätte dating from the upper Lower Cambrian to Middle Cambrian. The organisms are mainly known from dispersed sclerites; articulated specimens, where found, range from to a little over 50.8 millimeters in...
, only shows such structures in 10% of the expected instances, and Anamolocarid
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...
s are often found detached from their mouthparts, so the absence may be taphonomic rather than genuine. The absence of spines could simply mean that the fossils represent young organisms — or that later chaetognath evolution involved paedomorphosis.
While more work on undescribed specimens is required before firm consensus is reached, the discovery of P. spinosa, which strongly resembles modern Chaetognaths, in sediments of a similar age in Chengjiang biota
Maotianshan shales
The Maotianshan Shales are a series of lower Cambrian deposits in the Chiungchussu formation, famous for their Konservat Lagerstätten, or high number of fossils preserved in place...
places doubts on its position in the Chaetognath crown group. It has also been likened to the molluscs, and bears limited resemblance to unusual forms such as Pterotrachea
Pterotrachea
Pterotrachea is a taxonomic genus of medium-sized to large floating sea slugs, marine pelagic gastropod molluscs in the family Pterotracheidae, which is in the infraorder Littorinimorpha...
. Of course, many similarly enigmatic Cambrian fossils probably represent stem groups to living taxa, and there is no reason to assume that this organism must fall into an extant crown group at all.
The name Amiskwia sagittiformis derives from the nearby Amiskwia river, and its sagittiform shape.