Odontogriphus

''Odontogriphus'' (from el, ὀδούς , 'tooth' and el, γρῖφος , 'riddle') is a genus of soft-bodied animals known from middle Cambrian Lagerstätte. Reaching as much as 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 fossilization.

Originally it was known from only one specimen, but 189 new finds in the years immediately preceding 2006 made a detailed description possible. (221 specimens of ''Odontogriphus'' are known from the Greater Phyllopod bed, where they comprise 0.42% of the community.) 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 brachiopods. It is thought that ''Odontogriphus''’s feeding apparatus, which is "nearly identical" to ''Wiwaxia''’s, is an early version of the molluscan radula, a chitinous "tongue" that bears multiple rows of rasping teeth. Hence ''Odontogriphus'' and ''Wiwaxia'' are often classified as closely related to true molluscs.

History of discovery

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 brachiopods. In 2006 Caron, Scheltema ''et al.'' published a new analysis based on 189 recently collected specimens, all from the Burgess Shale. A full pre-publication draft, free but without pictures, may be available at

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 "casting" of the animal's upper surface and the lower giving one of its underside.

''Odontogriphus'' was a flat-bodied animal ranging from to 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 fossilization – 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.
The mouthparts comprised two to three rows, each comprising about two dozen carbonaceous teeth arranged symmetrically about a medial tooth, with one or two lateral teeth substantially smaller than the central teeth. The teeth operated by passing around an underlying "tongue", with the tooth rows deforming as they did so. The teeth probably scooped through the sea-floor mud, feeding on any detritus within it.

On either side of the feeding apparatus there is a circular structure that Caron, Scheltema ''et al.'' (2006) interpreted as salivary glands. 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 large gland apparently overlies the posterior portion of the gut. A pair of structures on either side of the intestine and a little behind the stomach may have been gonads or digestive organs. Muscular tissue is apparently preserved in the head and the foot, with a distribution that strikingly resembles that observed in modern chitons.

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, annelid worms and brachiopods. 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 fairly tough surfaces, like the chitinous "belts" of modern radulae; 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-lik
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 fossilize, 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 sclerites were an advance on the unmineralized sclerites of ''Wiwaxia'' and also resembled the armor of some living shell-less aplacophoran 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.

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 annelids such as '' Canadia'' 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, appendages 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 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 polychaetes also molt and replace their feeding apparatus (which sometimes resembles a radulaButtefield points to Fig 10F in and Fig 4B in ; a range of annelid jaws can be observed at ), 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 gills, 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 secretions, such as jaws, bristles and toughened skin, and do not include purely or primarily cellular tissues, such as muscles or gonads. 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 lophophore – both of these types of structure have been found in the Burgess Shale, 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, a "super-phylum" 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 chitons. ''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's Jasper National Park – this fossil bed is 5 – 10 M years older than the Burgess Shale 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 heterogeneous tooth-rows found in fossils of ''Odontogriphus'' and ''Wiwaxia'', and concluded that ''Odontogriphus'' and ''Wiwaxia'' were unlikely to be molluscs.

However, a 2012 study re-examined the mouthparts of these two genera, and identified problems with previous interpretations. Most importantly, the mouthparts contain a central rachidian tooth—a key radular characteristic. In light of this new reconstruction, a molluscan affinity seems well supported. Further support for a position in the molluscan total group is provided by details of its musculature, and from further details obtained from its close relative ''Wiwaxia''.

References

Further reading

*

External links

*

Royal Ontario Museum article on ''O. omalus''
* http://www.cbc.ca/health/story/2006/07/12/mollusc-fossil.html

{{Taxonbar, from=Q2307052

Lophotrochozoa
Prehistoric protostome genera
Cambrian invertebrates
Burgess Shale fossils
Protostome enigmatic taxa
Cambrian genus extinctions