A new report in this week’s Nature clears up a mystery about an enigmatic fossil from the Cambrian. This small creature has been pegged as everything from a chordate to a polychaete, but a detailed analysis has determined that it has a key feature, a radula, that places it firmly in the molluscan lineage. It was a kind of small Cambrian slug that crawled over matted sheets of algae and bacteria, scraping away a meal.
Here it is, a most unprepossessing creature. It was small (less than a 5 inches long), a flattened oval with few striking features, with a small mouth containing chevron shaped rows of mineralized teeth. This structure is called a radula, a kind of toothed tongue, that we can see nowadays in slugs and snails. It’s a clue to its lifestyle, too—it’s found in conjunction with fossilized sheets of a cyanobacterium, Morania, which would have been present in dense mats. Odontogriphus would have crept over those mats, dining on the material it could scrape away.
Another interesting feature of this organism is that it represents an inhabitant of a transitional environment. The pre-Cambrian and early Cambrian were dominated by a firm sea floor surfaced with these bacterial mats; later, as animal life invaded new niches and began more thoroughly churning up the floor (a process called bioturbation), we see more changes in the world and its biota.
Body (Kimberella) and trace (Radulichnus) fossil evidence from Ediacaran shallow marine sediments demonstrates the establishment of a bilaterian microphagous mat-grazing guild by at least 555 million years ago. The subsequent transition from Neoproterozoic biomat-dominated sea floors to Phanerozoic-style seafloor conditions, characterized by increasingly fluidized substrates, was driven by a shift to more intensive and vertically oriented bioturbation across the Precambrian/Cambrian boundary. The redistribution of extensive microbial mats and biofilms (along with metazoan grazers) from the open shallow marine to stressed nearshore and shelf-edge to deep-sea settings was not abrupt, but took place over a protracted interval in the Cambrian during which relict mat-based communities persisted in at least some marine environments, including the Burgess Shale. This persistence is evident from the intimate association of Odontogriphus and Wiwaxia with dense, sheet-like aggregates of the fossil cyanobacterium Morania, which often cover extensive bedding surfaces. Morania probably provided a food source and stable substrate for an array of Middle Cambrian benthic grazers adapted to Neoproterozoic-style substrates.
Now I know the creationist caricature of the Cambrian is that it was a discrete, sudden event in which complex animal life just suddenly appeared, but the more accurate version is that this was a long period of rapid, steady change, not just in the animals but in the environment. I thought the diagram below was wonderfully useful, showing not only the phylogeny of the molluscs, but illustrating the span of time from the Ediacaran to the Ordovician, and the pattern of change that we can see. You can call it an “explosion” if you want, but it clearly was a period of evolutionary change, not sudden creation.
Caron J-B, Scheltema A, Schander C, Rudkin D (2006) A soft-bodied mollusc with radula from the Middle Cambrian Burgess Shale. Nature 442:159-163.