Odontogriphus
Odontogriphus Temporal range:
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Odontogriphus from the Burgess Shale. From Smith (2014).[2] | |
Somewhat decayed fossil of Odontogriphus, showing 'radula' and gut. From Smith (2014)[2] | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Superphylum: | Lophotrochozoa |
Family: | Odontogriphidae Conway Morris, 1976[1] |
Genus: | Odontogriphus Conway Morris, 1976[1] |
Species | |
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Odontogriphus (from
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
History of discovery
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.[4]
Odontogriphus was a flat-bodied animal ranging from 3.3 millimetres (0.13 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 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.[4]
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.[4] 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.[5]
On either side of the feeding apparatus there is a circular structure that Caron, Scheltema et al. (2006) interpreted as
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.[4]
Phylogeny
Odontogriphus has become prominent in the debate that has gone on since 1990 about the evolutionary origins of
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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.[4] They also concluded that Odontogriphus was closely related to the Ediacaran animal Kimberella,[4] 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.[9][10]
They went on to classify the
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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
This article includes a list of general references, but it lacks sufficient corresponding inline citations. (October 2013) |
s sclerites were internally much more like the bristles of
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,[6] 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.[13]
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
While Butterfield agreed that the dark patches round the foot served as
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.[4] 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.[13]
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
In 2008 Butterfield described a set of micro-fossils dated to between 515 million years ago and 510 million years ago, found in the Mahto Formation in
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.[5] 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.[2]
See also
References
- ^ a b c d Conway Morris, S. (1976). "A new Cambrian lophophorate from the Burgess Shale of British Columbia" (PDF). Palaeontology. 19: 199–222. Archived from the original (PDF) on 2011-08-24. Retrieved 2010-10-12.
- ^ S2CID 84616434.
- S2CID 53646959.
- ^ S2CID 4431853. Archived from the original(PDF) on 2011-07-18. Retrieved 2008-07-04.
- ^ PMID 22915671.
- ^ .
- ^ S2CID 7838912. Archived from the originalon 2012-10-13. Retrieved 2008-08-13.
- ISBN 9780128143124.
- S2CID 4395089.
- S2CID 331187.
- ^ N. J., Butterfield (2007-12-18). "Lophotrochozoan roots and stems". In Budd, G. E.; Streng, M.; Daley, A.C.; Willman, S. (eds.). Programme with Abstracts. Palaeontological Association Annual Meeting. Uppsala, Sweden. pp. 26–7.
- S2CID 88100863.
- ^ S2CID 29130876. Archived from the originalon 2011-08-13. Retrieved 2008-08-06.
- ISSN 1178-9905.
- S2CID 86083492.
Further reading
- Bengtson, S. (2006-07-13). "A ghost with a bite". Nature. 442 (7099): 146–147. S2CID 2579864.
External links
- "Odontogriphus omalus". Burgess Shale Fossil Gallery. Virtual Museum of Canada. 2011. Archived from the original on 2020-11-12. Retrieved 2023-01-21.
- Royal Ontario Museum article on O. omalus
- http://www.cbc.ca/health/story/2006/07/12/mollusc-fossil.html