Supposed evolutionary impasse a mollusc ancestor?
New fossil specimens of an animal species that has been extinct for 500 million years shed new light on the relationships between early life forms. The species Odontogriphus omalus, which was widespread in the Cambrian and had an uncertain pedigree until now, was apparently an ancestor of today's molluscs, believe Jean-Bernard Caron and his colleagues from the Royal Ontario Museum. They came to this conclusion by analyzing 189 previously understudied specimens of the species recovered from the Burgess Shale fossil site over the past 15 years.
Odontogriphus was first described in 1976 and was mostly thought to be a flattened, segmented tube without appendages, with a possibly toothed mouth hole and two sensory organ pits, the phylum of which died out in the course of evolution without descendants. After analyzing the extremely well-preserved new finds, which are up to twelve centimeters long, Caron and colleagues now interpret certain zigzag lines in the fossils as the remains of a radula, the typical rasping tongue of mollusks.
The scientists also recognized very similar oral apparatus and radula remains on the Cambrian-dwelling Wiwaxia corrugata, which has so far been vaguely classified as a member of the Lophotrochozoa - a progenitor group of the Lophophorates, mollusks and annelids. In addition, Odontogriphus resembles the even older Kimberella mollusc ancestors, whose remains are often associated with typical grazing marks in fossil bacterial lawns. Today's mollusks also leave very similar feeding tracks. Overall, this indicates that the origins of the mollusks predated the so-called Cambrian Species Explosion 540 million years ago by at least 18 million years.
The researchers believe that the original mollusks may have been shellless grazers of microbes, which were displaced by larger shelled ones as the widespread colonization of lake floors by bacterial mats declined in the Middle Cambrian.
British Columbia's famous Burgess Shale deposit conserves the largest amount of Cambrian soft- and hard-tissue fossils. The once marine animal species are preserved in sediments deposited near the equator around 505 million years ago.