Odontogriphus
Encyclopedia
Odontogriphus is a genus
of soft-bodied animals known from middle Cambrian
Lagerstätte
. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to fossil
ization.
Originally it was known from only one specimen, but 189 new finds in the years immediately preceding 2006 made a detailed description possible. As a result Odontogriphus has become prominent in the debate that has gone on since 1990 about the evolutionary origins of molluscs, annelid
worms and brachiopod
s. A group of scientists think that Odontogriphus’s feeding apparatus, which is "nearly identical" to Wiwaxia
’s, is an early version of the molluscan radula
, a chitin
ous "tongue" that bears multiple rows of rasp
ing teeth. Hence they classify Odontogriphus and Wiwaxia as close to the ancestors of the first true molluscs. One scientist has presented a different analysis, arguing since 1990 that Wiwaxia is not closely related to molluscs but is much more like a polychaete
worm. He argues that the supposed "radula" is nothing of the sort; he classifies Odontogriphus as a basal lophotrochozoa
n, in other words close to the last common ancestor of molluscs, annelid worms and brachiopods.
Charles Doolittle Walcott
found one specimen during one of his field trips to the Burgess Shale
between 1910 and 1917. In the 1970s Simon Conway Morris
re-examined the specimen and tentatively concluded that it was a swimming lophophorate, in other words related to the ancestors of molluscs, annelid
worms and brachiopod
s. In 2006 Caron, Scheltema et al. published a new analysis based on 189 recently collected specimens, all from the Burgess Shale.
" of the animal's upper surface and the lower giving one of its underside.
Odontogriphus was a flat-bodied animal ranging from 3.3 millimetre (0.12992125984252 in) to 125 millimetres (4.9 in) in length, with parallel sides and semi-circular ends. The specimens examined by Caron, Scheltema et al. (2006) had the same ratio of length to width irrespective of size. The body outlines are bilaterally symmetrical
in all fairly complete specimens, even those in which internal features were
preserved asymmetrically. Caron, Scheltema et al. (2006) interpreted this as evidence that the animals had on their backs "shells" that were rigid enough to resist whatever stresses distorted the internal features, but were not tough enough to be preserved by fossil
ization – similar, for example, to finger nails
. Relatively broad wrinkles, parallel to each other and usually straight, run across the central region of the body in some specimens.
Caron, Scheltema et al. (2006) found evidence of a circular mouth on the underside, with two and occasionally three tooth-bearing structures that they interpreted as a feeding apparatus and very similar to that of Wiwaxia
. Odontogriphus’s feeding apparatus was located on the midline, about 15% of the total body length from the front edge of the fossils. Where a third tooth-row was found, it was less distinct, which they thought indicated that it was still growing at the time of the animal's death. Since they occasionally found pairs of tooth-rows that were not associated with body fossils but were spaced like the tooth-rows in "complete" fossils, they concluded that the tooth-rows were mounted on a material that was tough enough to keep them in the same relative positions but not suitable for fossilization.
On either side of the feeding apparatus there is a circular structure that Caron, Scheltema et al. (2006) interpreted as salivary gland
s. They also found evidence of other parts of the digestive tract: a gullet connecting to the rear of the mouth; a relatively short stomach
; and a straight much longer intestine
, ending at an anus near the rear of the underside. A pair of structures on either side of the intestine and a little behind the stomach may have been gonad
s or digestive organs.
The fossils showed signs of a thickened central structure that Caron, Scheltema et al. (2006) thought was on the underside and probably represents a muscular sole that was a little over half as wide as the whole animal. It was U-shaped, with the "open" end behind the mouth and the rounded end a little forward of the animal's rear edge. The anus apparently was slightly ahead of the rounded end. All the edges of the "foot" except the front were surrounded by darker patches, which are sometimes separated from the rest of the body by a thin layer of sediment.
worms and brachiopod
s. Caron, Scheltema et al. (2006) interpreted Odontogriphus’s feeding apparatus as a forerunner of the molluscan radula
, on the grounds that: the occasional and less distinct third tooth-row looked like evidence that the animals grew replacement tooth-rows at the rear of their mouths and shed worn-out ones from the front, as happens with molluscan radulae; the isolated pairs of tooth-rows they found, not associated with body fossils but in the same relative positions as in the more complete fossils, suggested they were mounted on a fairly tough surface, like the chitin
ous "belt" of a modern radula; they even found signs that discarded tooth-rows were sometimes eaten by the animals. Hence they classified both Odontogriphus, and Wiwaxia
with its "nearly identical" feeding apparatus, as primitive relatives of molluscs.
In line with this classification they interpreted the dark patches round the foot as gill-like ctenidia, another feature of some molluscs; and the sediment that sometimes appeared in the fossils between the foot and supposed ctenidia suggested the presence of a mantle
cavity. They also concluded that Odontogriphus was closely related to the Ediacaran
animal Kimberella
, whose fossils also show signs of a fairly rigid upper "shell" made of a material that did not fossil
ize, and which has been interpreted as a very mollusc-like organism.
They went on to classify the halkieriids as nearly modern molluscs, since in their opinion halkieriids' "chain mail" coats of mineralized sclerite
s were an advance on the unmineralized sclerites of Wiwaxia
and also resembled the armor of some living shell-less aplacophora
n molluscs, the Neomeniomorpha
. As a result they concluded that the whole Kimberella-Odontogriphus-Wiwaxia-mollusc lineage must have diverged from that of the annelid
worms some time before the appearance of Kimberella in the Ediacaran period.
siphogonotuchids) }}}}
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Butterfield's alternative: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...
of soft-bodied animals known from 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...
Lagerstätte
Lagerstätte
A Lagerstätte is a sedimentary deposit that exhibits extraordinary fossil richness or completeness.Palaeontologists distinguish two kinds....
. Reaching as much as 12.5 centimetres (4.9 in) in length, Odontogriphus is a flat, oval bilaterian which apparently had a single muscular foot, and a "shell" on its back that was moderately rigid but of a material unsuited to fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ization.
Originally it was known from only one specimen, but 189 new finds in the years immediately preceding 2006 made a detailed description possible. As a result Odontogriphus has become prominent in the debate that has gone on since 1990 about the evolutionary origins of molluscs, 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 and brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s. A group of scientists think that Odontogriphus’s feeding apparatus, which is "nearly identical" to 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...
’s, is an early version of the molluscan radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
, a chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous "tongue" that bears multiple rows of rasp
Rasp
A rasp is a tool used for shaping wood or other material. It consists of a point or the tip, then a long steel bar or the belly, then the heel or bottom, then the tang. The tang is joined to a handle, usually made of plastic or wood. The bar has sharp teeth...
ing teeth. Hence they classify Odontogriphus and Wiwaxia as close to the ancestors of the first true molluscs. One scientist has presented a different analysis, arguing since 1990 that Wiwaxia is not closely related to molluscs but is much more like a polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
worm. He argues that the supposed "radula" is nothing of the sort; he classifies Odontogriphus as a basal lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
n, in other words close to the last common ancestor of molluscs, annelid worms and brachiopods.
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 one specimen during one of his field trips to 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...
between 1910 and 1917. In the 1970s Simon 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...
re-examined the specimen and tentatively concluded that it was a swimming lophophorate, in other words related to the ancestors of molluscs, 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 and brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s. In 2006 Caron, Scheltema et al. published a new analysis based on 189 recently collected specimens, all from the Burgess Shale.
Description
Odontogriphus was apparently a very rare species, accounting for less than 0.5% of the individual organisms found in the same fossil beds. Most of the fossils consist of two parts of a split block of rock, the upper part giving a "castingCasting
In metalworking, casting involves pouring liquid metal into a mold, which contains a hollow cavity of the desired shape, and then allowing it to cool and solidify. The solidified part is also known as a casting, which is ejected or broken out of the mold to complete the process...
" of the animal's upper surface and the lower giving one of its underside.
Odontogriphus was a flat-bodied animal ranging from 3.3 millimetre (0.12992125984252 in) to 125 millimetres (4.9 in) in length, with parallel sides and semi-circular ends. The specimens examined by Caron, Scheltema et al. (2006) had the same ratio of length to width irrespective of size. The body outlines are bilaterally symmetrical
Symmetry
Symmetry generally conveys two primary meanings. The first is an imprecise sense of harmonious or aesthetically pleasing proportionality and balance; such that it reflects beauty or perfection...
in all fairly complete specimens, even those in which internal features were
preserved asymmetrically. Caron, Scheltema et al. (2006) interpreted this as evidence that the animals had on their backs "shells" that were rigid enough to resist whatever stresses distorted the internal features, but were not tough enough to be preserved by fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ization – similar, for example, to finger nails
Nail (anatomy)
A nail is a horn-like envelope covering the dorsal aspect of the terminal phalanges of fingers and toes in humans, most non-human primates, and a few other mammals. Nails are similar to claws, which are found on numerous other animals....
. Relatively broad wrinkles, parallel to each other and usually straight, run across the central region of the body in some specimens.
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...
. Odontogriphus’s feeding apparatus was located on the midline, about 15% of the total body length from the front edge of the fossils. Where a third tooth-row was found, it was less distinct, which they thought indicated that it was still growing at the time of the animal's death. Since they occasionally found pairs of tooth-rows that were not associated with body fossils but were spaced like the tooth-rows in "complete" fossils, they concluded that the tooth-rows were mounted on a material that was tough enough to keep them in the same relative positions but not suitable for fossilization.
On either side of the feeding apparatus there is a circular structure that Caron, Scheltema et al. (2006) interpreted as salivary gland
Salivary gland
The salivary glands in mammals are exocrine glands, glands with ducts, that produce saliva. They also secrete amylase, an enzyme that breaks down starch into maltose...
s. They also found evidence of other parts of the digestive tract: a gullet connecting to the rear of the mouth; a relatively short stomach
Stomach
The stomach is a muscular, hollow, dilated part of the alimentary canal which functions as an important organ of the digestive tract in some animals, including vertebrates, echinoderms, insects , and molluscs. It is involved in the second phase of digestion, following mastication .The stomach is...
; and a straight much longer intestine
Intestine
In human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
, ending at an anus near the rear of the underside. A pair of structures on either side of the intestine and a little behind the stomach may have been gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s or digestive organs.
The fossils showed signs of a thickened central structure that Caron, Scheltema et al. (2006) thought was on the underside and probably represents a muscular sole that was a little over half as wide as the whole animal. It was U-shaped, with the "open" end behind the mouth and the rounded end a little forward of the animal's rear edge. The anus apparently was slightly ahead of the rounded end. All the edges of the "foot" except the front were surrounded by darker patches, which are sometimes separated from the rest of the body by a thin layer of sediment.
Phylogeny
Odontogriphus has become prominent in the debate that has gone on since 1990 about the evolutionary origins of molluscs, annelidAnnelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...
worms and brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
s. Caron, Scheltema et al. (2006) interpreted Odontogriphus’s feeding apparatus as a forerunner of the molluscan radula
Radula
The radula is an anatomical structure that is used by molluscs for feeding, sometimes compared rather inaccurately to a tongue. It is a minutely toothed, chitinous ribbon, which is typically used for scraping or cutting food before the food enters the esophagus...
, on the grounds that: the occasional and less distinct third tooth-row looked like evidence that the animals grew replacement tooth-rows at the rear of their mouths and shed worn-out ones from the front, as happens with molluscan radulae; the isolated pairs of tooth-rows they found, not associated with body fossils but in the same relative positions as in the more complete fossils, suggested they were mounted on a fairly tough surface, like the chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...
ous "belt" of a modern radula; they even found signs that discarded tooth-rows were sometimes eaten by the animals. Hence they classified both Odontogriphus, and 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...
with its "nearly identical" feeding apparatus, as primitive relatives of molluscs.
In line with this classification they interpreted the dark patches round the foot as gill-like ctenidia, another feature of some molluscs; and the sediment that sometimes appeared in the fossils between the foot and supposed ctenidia suggested the presence of a mantle
Mantle (mollusc)
The mantle is a significant part of the anatomy of molluscs: it is the dorsal body wall which covers the visceral mass and usually protrudes in the form of flaps well beyond the visceral mass itself.In many, but by no means all, species of molluscs, the epidermis of the mantle secretes...
cavity. They also concluded that Odontogriphus was closely related to the Ediacaran
Ediacaran
The Ediacaran Period , named after the Ediacara Hills of South Australia, is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon...
animal Kimberella
Kimberella
Kimberella is a monospecific genus of bilaterian known only from rocks of the Ediacaran period. The slug-like organism fed by scratching the microbial surface on which it dwelt in a manner similar to the molluscs, although its affinity with this group is contentious.Specimens were first found in...
, whose fossils also show signs of a fairly rigid upper "shell" made of a material that did not fossil
Fossil
Fossils are the preserved remains or traces of animals , plants, and other organisms from the remote past...
ize, and which has been interpreted as a very mollusc-like organism.
They went on to classify the halkieriids as nearly modern molluscs, since in their opinion halkieriids' "chain mail" coats of mineralized sclerite
Sclerite
A sclerite is a hardened body part. The term is used in various branches of biology for various structures including hardened portions of sponges, but it is most commonly used for the hardened portions of arthropod exoskeletons....
s were an advance on the unmineralized sclerites of 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...
and also resembled the armor of some living shell-less aplacophora
Aplacophora
Aplacophora is a monophyletic group of small, deep-water, exclusively benthic, shell-less marine mollusks found in all oceans of the world. The group comprises the two clades Solenogastres and Caudofoveata , which between them contain 28 families and about 320 species...
n molluscs, the Neomeniomorpha
Neomeniomorpha
The Solenogastres , common name solenogasters, are a subclass of worm-like, small, shell-less molluscs , the other subclass being the Caudofoveata .-Morphology:...
. As a result they concluded that the whole Kimberella-Odontogriphus-Wiwaxia-mollusc lineage must have diverged from that of 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...
worms some time before the appearance of Kimberella in the Ediacaran period.
siphogonotuchids) }}}}
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This brought Odontogriphus into the center of a debate that had been going on since 1990, when Butterfield denied that Wiwaxia was a forerunner of molluscs and argued that it was an evolutionary "aunt" of annelids. In particular he had argued that: Wiwaxia’s sclerites were internally much more like the bristles of polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
annelids such as Canadia
Canadia (genus)
Canadia is a genus of extinct annelid worm present in Burgess Shale type Konservat-Lagerstätte. It is found in strata dating back to the Delamaran stage of the Middle Cambrian around 505 million years ago, during the time of the Cambrian explosion. It was about 3 centimeters in length...
than like any forerunner of molluscan shell plates; and in his opinion Wiwaxia’s feeding apparatus was more similar to that of some polychaetes than to a molluscan radula. Caron, Scheltema, et al. (2006) thought Wiwaxia bore little resemblance to polychaetes as it showed no signs of segmentation
Segmentation
Segmentation may mean:*Market segmentation, in economics and marketingBiology*A process of morphogenesis that divides a metazoan body into a series of semi-repetitive segments*Segmentation , a series of semi-repetitive segments...
, appendage
Appendage
In invertebrate biology, an appendage is an external body part, or natural prolongation, that protrudes from an organism's body . It is a general term that covers any of the homologous body parts that may extend from a body segment...
s in front of the mouth, or "legs" – all of which are typical polychaete features.
A few months later in 2006 Butterfield returned to the fray. As in 1990, he argued that Wiwaxia’s sclerites were internally much more like the bristles of polychaete
Polychaete
The Polychaeta or polychaetes are a class of annelid worms, generally marine. Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made of chitin. Indeed, polychaetes are sometimes referred to as bristle worms. More than 10,000...
annelids such as Canadia than like any forerunner of molluscan shell plates; since a 2005 paper had downplayed this argument with the comment that similar bristles also appear in molluscs and brachiopods, he pointed out that modified bristles appear as a covering over the back only in polychaetes and hence Wiwaxia’s sclerites should indeed be regarded as like polychaetes' bristles.
In addition he argued that Odontogriphus’ shedding and replacement of tooth-rows, the rows' staying in the same relative positions when isolated and the evidence that Odontogriphus sometimes swallowed discarded tooth-rows did not prove that Odontogriphus was an evolutionary "aunt" of molluscs, since eunicid
Eunicidae
Eunicidae is a family of polychaetes. Many eunicids reach a considerable size. Their jaws are known from Ordovician sediments. They live throughout the seas; a few species are parasitic....
polychaetes also molt and replace their feeding apparatus (which sometimes resembles a radula), and sometimes eat the discarded material. He also doubted whether the two tooth-rows of Odontogriphus and Wiwaxia could perform all the functions of the multi-row radula – rasping, capturing scraped food, sorting it and transporting it to the gullet. In his opinion the differences between the narrower first tooth-row and slightly wider second one in both Odontogriphus and Wiwaxia were unlike those of a molluscan radula, in which the much more numerous tooth-rows are identical; instead he argued that these two rows resembled the permanent lower jaw and moltable upper jaw of modern dorvilleid polychaetes.
While Butterfield agreed that the dark patches round the foot served as gill
Gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...
s, he denied that they were similar in structure and mode of development to molluscan ctenidia. In his opinion the flattened remains of Odontogriphus were formed by relatively tough extracellular
Extracellular
In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". This space is usually taken to be outside the plasma membranes, and occupied by fluid...
secretion
Secretion
Secretion is the process of elaborating, releasing, and oozing chemicals, or a secreted chemical substance from a cell or gland. In contrast to excretion, the substance may have a certain function, rather than being a waste product...
s, such as jaws, bristles and toughened skin, and do not include purely or primarily cellular tissues, such as muscles or gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...
s. He therefore thought the respiratory organs round the edge of Odontogriphus’ foot could not be molluscan ctenidia, since these are covered by purely cellular tissue. Instead he suggested that they might be brachiopod
Brachiopod
Brachiopods are a phylum of marine animals that have hard "valves" on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear end, while the front can be opened for feeding or closed for protection...
lophophore
Lophophore
The lophophore is a characteristic feeding organ possessed by four major groups of animals: the Brachiopoda, Bryozoa, Entoprocta, and Phoronida. All lophophores are found in aquatic organisms.-Characteristics:...
s, which are feeding organs that contain a lot of extracellular material, or polychaete branchiae, which are respiratory organs composed largely of non-cellular cuticle – both of these types of structure have been 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...
, in which all the known specimens of Odontogriphus have been discovered.
Caron, Scheltema, et al. (2006) had suggested that the wrinkles on the top surfaces of Odontogriphus specimens were caused by the rippling contractions of a mollusc-like muscular foot. Butterfield disputed this on the grounds that: a molluscan foot is also mainly composed of cellular material, which he thought unlikely to be fossilized in Burgess Shale conditions; the wrinkles were too straight and ran too precisely across the animals' bodies; the gaps between them were the same size as the gaps between the gill-like structures round the foot. Instead he argued that they were evidence of externally visible segmentation, which is found in polychaetes but not in molluscs. He concluded that Wiwaxia was an evolutionary "aunt" of polychaetes, while Odontogriphus could be an evolutionary "aunt" of polychaetes or of molluscs or of brachiopods – or even a "great aunt" of all three, as it could have been an early member of the lophotrochozoa
Lophotrochozoa
The Lophotrochozoa are a major grouping of protostome animals. The taxon was discovered based on molecular data. Molecular evidence such as a result of studies of the evolution of small-subunit ribosomal RNA supports the monophyly of the phyla listed in the infobox shown at right.-Terminology:The...
, a "super-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....
" that includes the polychaetes, molluscs and brachiopods.
In January 2007 Caron, Scheltema, et al. published a vigorous reply to Butterfield's arguments – near the end they wrote, "Many of Butterfield’s misconceptions might well have been avoided had he taken the opportunity to examine all the new material that formed the basis of our study. …" They said they had found in body fossils of Odontogriphus visible traces of the membrane on which its tooth-rows were mounted; in their opinion this was clear evidence of a basic belt-like radula assembly with regularly spaced tooth-rows, a feature unique to molluscs. On the other hand, they wrote, eunicid polychaetes' jaws have only the vaguest similarity to radulae, and other annelids' jaws grow continuously without replacement; and they supported this with a point-by-point comparison of Odontogriphus’ feeding apparatus with that of the dorvilleid polychaetes which Butterfield claimed it resembled. In answer to Butterfield's claim that the respiratory organs round the foot could not be molluscan ctenidia because these mainly cellular structures would not have fossilized in the Burgess Shale conditions, they wrote that: fairly soft cellular tissue belonging to the stomach is fossilized in many Odontogriphus specimens; some molluscan gills are stiffened by non-cellular material, for example in polyplacophorans. They pointed out that the wrinkles that appear across the body in views from the top occur only in the mid-section, and there is no sign that the tough "shell" plate on the animal's back was segmented; hence in their opinion Odontogriphus could not have been an annelid. On the other hand wrinkles are seen in the feet of dead chiton
Chiton
Chitons are small to large, primitive marine molluscs in the class Polyplacophora.There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura....
s. Wiwaxia, they argued, was clearly not segmented, as the numbers of sclerites in its three concentric groups did not match at all. They criticized Butterfield's main argument for "shoehorning" Wiwaxia into the polychaetes, that its sclerites were secreted by microvillae; such structures, they wrote, were also found in several groups of molluscs. Finally, in their opinion the absence of "legs" in Wiwaxia ruled out a close relationship with polychaetes.
In 2008 Butterfield described a set of micro-fossils dated to between and , found in the Mahto Formation in Alberta
Alberta
Alberta is a province of Canada. It had an estimated population of 3.7 million in 2010 making it the most populous of Canada's three prairie provinces...
's Jasper National Park
Jasper National Park
Jasper National Park is the largest national park in the Canadian Rockies, spanning 10,878 km² . It is located in the province of Alberta, north of Banff National Park and west of the City of Edmonton. The park includes the glaciers of the Columbia Icefield, hot springs, lakes, waterfalls and...
– this fossil bed is 5 – 10 M years older than 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...
in which the only known specimens of Odontogriphus and Wiwaxia were found. Some groupings of these micro-fossils showed a consistent arrangement that he interpreted as an "articulated apparatus" with tens of closely spaced tooth rows, apparently mounted on an organic base, and with noticeable signs of wear in the rows at one end. The rows were not quite identical, but he noted that some modern aplacophoran molluscs show similar variations. He concluded that the "articulated apparatus" was a genuine molluscan radula, most similar to those of modern aplacophorans or gastropods. He then commented on the contrast between this apparatus and the two or rarely three widely spaced and more heteregeneous tooth-rows found in fossils of Odontogriphus and Wiwaxia, and concluded that Odontogriphus and Wiwaxia were unlikely to be molluscs.
External links
- Royal Ontario Museum article on O. omalus
- http://www.cbc.ca/health/story/2006/07/12/mollusc-fossil.html