Unranked clade halwaxiids (stem group molluscs)
Odontogriphus is an early stem-group mollusc (Caron et al., 2006; Sigwart and Sutton, 2007), or a stem-group to the lophotrochozoans, a group which includes molluscs, annelids, and brachiopods (Conway Morris and Caron, 2007). A relationship to annelids (Butterfield, 2006) has been suggested, but appears less likely (Caron et al., 2007).
Odontogriphus – from the Greek odontos, “tooth,” and griphos, “puzzle, or riddle,” in reference to its uncertain affinities.
omalus – from the Greek homalos, “flat,” in reference to the animal’s flattened shape.
Holotype – USNM 196169 in the National Museum of Natural History, Smithsonian Institution, Washington, DC, USA.
Burgess Shale and vicinity: none.
Other deposits: none.
Middle Cambrian, Bathyuriscus-Elrathina Zone (approximately 505 million years ago).
The Walcott Quarry on Fossil Ridge and the Tulip Beds (S7) on Mount Stephen.
Brief history of research:
Walcott collected the first specimen between 1909 and 1924 but it remained unstudied for more than half a century. Conway Morris “rediscovered” the part and counterpart of this specimen in different sections of the Walcott collection and described it in 1976 as Odontogriphus omalus. The affinities of Odontogriphus remained uncertain until the Royal Ontario Museum discovered 189 new specimens between 1990 and 2000, allowing for a thorough redescription of the animal (Caron et al., 2006).
This entirely soft-bodied animal is ovoid and dorsoventrally compressed, reaching up to 125 mm in length and 43 mm in width. The front and back are semicircular in outline and of similar size. The mouth is ventral with a radula composed of two primary tooth rows. A muscular foot extends from behind the mouth to the posterior part of the animal and is surrounded by gills (or ctenidia), except at the front. The dorsal surface is smooth and does not bear any shells, spines or plates. Internally, a large stomach is preserved with a narrow and straight intestine ending in a sub-terminal anus.
Most specimens come from the Walcott Quarry, where Odontogriphus represents 0.42% of the community (Caron and Jackson, 2008). A single specimen comes from Mount Stephen (S7 locality).
The presence of a radula suggests that Odontogriphus was a grazer, using its teeth to rasp and ingest food. Locomotory waves within the large foot would have enabled the animal to crawl along the surface of the mud. Odontogriphus might have fed on benthic, sheet-like masses of the cyanobacterium Morania, since fossils of both are often found associated in the same layers.