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.
(click for larger image)
All specimens are preserved dorso-ventrally, anterior to the top. a, Royal Ontario Museum (ROM)57712, complete specimen showing the sole (crescentic wrinkles) lying on the surface of the cyanobacterium Morania. b, ROM57725, nearly complete specimen showing putative paired gonads or digestive glands and the cyanobacterium Morania. c, ROM57713, backscattered image of an isolated two-rowed radula. d, ROM57716, three-rowed radula. e, ROM57717, four-rowed radula with putative traces of the radular membrane. f, ROM57714, view of the mouth area and anterior end of the stomach. g, ROM57721, complete specimen showing the intestine and gut content. h, i, ROM57720 complete specimen. h, Overall view; i, detail of ctenidia from h. j, ROM57723, complete specimen showing the sole (wrinkles). k, ROM57724, complete specimen with paired salivary glands and ctenidia. Scale bars: a, 2 cm; g, h, k, 1 cm; b, f, i, j, 5 mm; c–e, 1 mm. an, anus; cr, crack; ct, ctenidia; ctg, ctenidia groove; dg?, digestive glands?; gc, gut content; go?, gonads?; in, intestine; ma, mantle; mo, Morania; oe, oesophagus; ph, pharynx and mouth area; ra, radula; ram?, radular membrane?; r1, r2, r3, r4, tooth rows; sg, salivary glands; so, sole; st, stomach.
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.
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Evolutionary tree of the molluscs in the context of the Neoproterozoic-Cambrian substrate revolution. 1: Protostome bilaterian; serial replication; triploblastic. 2: Segmentation by coelomic metameres. 3: Large size; with iteration but not coelomic segmentation; ovoid; dorso-ventrally flattened; stiffened cuticular dorsum; flat, non-cuticularized ventral sole; radula of iterated, paired mirror-image teeth and radular membrane (certain for Odontogriphus); feeding on biomat? 4: Groove (mantle cavity) between dorsum and ventrum with serial ctenidia; paired salivary glands; straight digestive tract; nervous system ladder-like?; coelom posterior, restricted to reproductive and excretory organs? 5: Non-calcified scleritome, sclerites arranged in three mirror-image longitudinal zones. 6: Calcification of epidermally nucleated sclerites that pass through cuticle; calcified shell from serial shell fields; no periostracum from periostracal groove of mantle lobe. 7: Two shell fields. 8: Tubiform; reduced foot; sclerites in 1–3 longitudinal rows beside foot groove; progenetic loss of gills and shells; embryological evidence of vestigial shell fields. 9: Eight or more shell fields; sclerites not in longitudinal zones. 10: Loss of sclerites and serial shell fields; true periostracum secreted from mantle lobe; shells paired or single; reduction of gills; further variety of body plans. A, Atdabanian; B/T, Botomian/Toyonian; N-D, Nemakit-Daldynian; T, Tommotian.
Muton has more on Odontogriphus.
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.
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I think you are being unduly harsh on the term 'explosion'. How gradual is gradual? This cannot be based on a global transition from 'firm' to 'soft' substrates over this ~80 million year period, especially since this line of reasoning is based on extremely minor edicaran exposures, and thus the 'global' picture can never be fully attained.
Surely the fact that we can see all the major phyla appear as well as many that later went extinct at or around the early Cambrian period suggests this was as explosive as biological events get in the geological record. Clearly the metazoans had been around before the Cambrian (e.g. the Ediacaran fauna from Australia), however extreme diversification of the phyla using this new body plan is not recorded until the Cambrian (which may also be an artefact of the fossil record).
Clearly, the scales at which we are used to testing evolutionary change in experiments etc. are not particuarly relevant at this point in biological time. We often talk about 'species' evolution as being gradual or rapid (perhaps punctuated?), yet we use the same terms and ideas as we move further down the taxonomic tree. A gap in the logic?
I think your interpretation has a fundamental oversight in that you use common concepts used at the scale of species and apply it to a period of phylectic scale diversification. Thus I think 'explosion' is perfectly warrented, and 'gradual evolutionary change' is both missleading and innacurate.
Just out of interest, what is the ontogenetic depth of Odontogriphus?
I am specifically criticizing the creationist interpretation of the Cambrian, which implies a fallaciously abrupt and simultaneous origin of all forms. In that context, it is not an "explosion". Look at the diagram; it shows novel forms over a prolonged period, and Odontogripha is a survivor of an Ediacaran lineage.
Andy, I calculate it to be somewhere around eleventy two and a third, plus or minus a buck two fitty.
I'll publish my monograph detailing every step of my analysis...ummm, tomorrow. Yeah, that's it. Tomorrow. Look for it.
This is still not what is interesting about the Cambrian. The diagrams show very specific diversificaiton within the Mollusca, yet all phyla display equal, if not greater taxonomic radiation.
I think the experimentation with the body plan to produce most phyla is actually quite geologically abrupt. I don't care what creationists argue, if I've learnt anything from the debate chronicled on your website, it's that you can't argue with an idiot.
Maybe we should start referring to it as the 'Cambrian Deflagration'. :-)
Andy, PZ:
I had no idea what you guys were talking about so I googled 'ontogenetic depth' and came up with this page at The Panda's Thumb. I don't believe that guy left you hanging for so long (and presumably he never followed through anywhere?).
Hilarious and painful.
Whether you call it an explosion or not, it was a period of unusually rapid evolution, right?
Anyway, I never thought of this as a huge mystery. Whatever set off the Cambrian, err, thingy was presumably something that allowed for far more variability than previously existed (I saw it attributed to sexual reproduction, but I think multicellular organisms could also do it by allowing for a greater multiplicity of phenotypes with significant differences). This variability occurred in the absence of any competition. In later periods, even mutations that might be beneficial could get overwhelmed by "good enough" competitors already exploiting the same niche. But at the time of the Cambrian, by contrast, even neutral or initially deterimental mutations could be propagated, provided they were enough resources for them to reproduce anyway.
The cartoon model I have for all this comes from something I thought of years back while walking on the beach. What happens now to a sea creature with the temerity to walk on dry land for the first time? There is something waiting up there to eat it. So we would probably expect a lower frequency of water to land transitions now, whereas it could have occurred many times when land was an unexploited resource.
I have no idea if my intuition on this is in keeping with biological theory. I just personally never thought there was any great mystery as to why one would have an initial period of unusually rapid variability.
It's called an adaptive radiation.
The cartoon depiction looks kind of like a... Feminine Napkin. Now we know there was no intelligent designer - because a natural maxi pad would be so useful today... or is it proof that tampons are "fitter?"
There is no mystery, you are right. Indeed all scientists who are studying the Cambrian explosion should cease and desist. This unusually rapid evolution is more complex than just competition, and to suggest that there was an absence of competition is ridiculous. Most fossils from the Burgess Shale in particular display the Cambrian equivalent of the Cold War nuclear armament build-up.
Darwin too thought of this problem, though probably not walking on the beach, and since Darwin many more have thought long and hard about this 'mystery'.
The ease with which mutations can propogate within a population are probably not that important in adaptive radiations. The mystery is not why but how. How did the environment influence development in the Cambrian? Lowering acidity in the oceans meant organisms could now secrete CaCO3 and create more ridgid structures. This must have been a huge step developmentally for organisms to make. There are many more possibilities, and then all in combination with each other. The fact is all adaptive radiations since the Cambrian (like those following mass-extinctions) never involved the introduction of new phyla making the Cambrian fundamenally different.
I'm going to pilfer the 'Cambrian Deflagration', that is fantastic.
Echinoderm:
Oh, don't be a snot. Obviously there are many interesting questions to study and scientists have only scratched the surface. I apologize if I gave cause for a reasonable person to think I meant I thought the whole thing was simple, but actually I think I did not and that your reponse is actually unjustifiably snotty.
What I meant by "no mystery" is that creationists peddle this as some kind of show stopper, which we can agree it is not. I just don't find it counterintuitive that the rate of evolution should be significantly different under significantly different circumstances. OK, it's easy for me to make a post hoc claim that my intuition agrees with what we know actually happened. But I think it's a useful starting point for a layperson such as me to say "Does this seem at all plausible?" and come up with a rough model of why it should be plausible, contrary to creationists' false paradoxes. My rough model should not be confused with an actual scientific explanation, but I think I gave enough disclaimers not to suggest that I was confused in this way.
You are right. Sorry, I was obtuse... bad day I guess. I agree, if you can argue from the corner using some simple logic that these complex events in biological history are perfectly reasonable then it is another small piece of the conceptual puzzle that cannot be clouded by ID nonsense.
Small victories.
So, then, would my reconstructions of O. omalus, and its presumed relativeBowenogriphus perphlegis inaccurate?
I'm curious...anyone know what the earliest known appearance of a radula is?
I think it's with the Edicarian mollusc, Kimberella
The radula is something I wish evolution had NOT come up with, given the ravening snails (and slugs) in my garden!
Echinoderm,
You really need to read this:
Down with phyla!
You don't also happen to post at AtBC under the name Ghost of Paley, do you? That's the spitting image of his analytical style--I'll tear your argument a new one.....later. Right now I don't feel well/am really busy/cannot get my cranium extricated from my rectum.
I haven't gone to Nick's link, but I suspect the point is that the "body-plan" or "phyletic"-level evolution that we retrospectively impose upon the Cambrian scene is a chimera.
What was happening then was probably far closer to what is happening now--species-level evolution. After enough species evolve and split, rinse and repeat, their descendants eventually look different enough for US to group them into separate phyla and body-plans. But the initial "splits" between biomat-harvesting species, or mud-dwelling worm species, or whatever, likely looked no more remarkable or dramatic than those between two lineages of butterflies bearing differently-colored wing-spots.
Was there an ecologically-driven adaptive radiation? Sure, but how was that different from the rapid radiation of mammals post-dinosaur, the radiation of marsupials on the isolated southern continents, or the radiation of the early avian lineages into the clear blue skies?
If we were looking back on it in another 500 MY--without the benefit, hypothetically, of our current, much-closer-in-time discoveries, with further tectonic disturbance, erosion, loss of fossils, might not the branching of reptiles, mammals, and birds from their much more similar predecessors appear just as radical and "explosive"? Might not the resulting "body plans" appear just as distinct? With enough pieces missing, might the mosaic--which we are now in the process of so painstakingly reconstructing--not present a similarly abrupt, "jerky" appearance, like a poorly-cut movie?
Like the Cambrian "explosion"?
"The cartoon depiction looks kind of like a... Feminine Napkin"
vagina radula
...you know, as opposed to dentata
(these are the jokes)
I love these wonderful creatures. Please expand.
I love these wonderful creatures. Please expand.
"Now about that ontogenetic depth thing: how the heck do you calculate it?"
oh yeah....its easy....you just.....oh have to pee be back in a sec....
Thanks for the link Nick, I had not read the article before. There has obviously been long standing dissagreement with certain aspects of the Linnaean system, with some scientists suggesting a departure from it altogether (e.g: phylocode.com)
Obviously, with so many pieces missing it is hard to constrain phylogeny, regardless of how normal or easy phyla-scale divergence may have been in the Cambrian (part of Steviepinhead's point I guess). I have read that some scientists believe arthropods may be polyphyletic, which certainly makes for an interesting interpretation of that phyla if further evidence comes in its favour.
With regards to the 'down with phyla' article, I think it would be hasty to abandon conceptions of body plans and so forth just yet. While very good points are made, the shifting of the mouth in the deuterosomes is only one difference between protostomes and deuterostomes, there are many others including the style of cellular cleavage during gastrulation.
Molecular clock evidence also points to a common ancestor before the Cambrian explosion, suggesting that adaptive radiation was held back for some reason, or that we just have to many pieces missing to see a gradual metazoan evolution.
Either way the Cambrian must have been an exciting time for developmental experimentation, and I would still regard it as distinct from other adaptive radiations where the taxa would have had tighter developmental constraints. Surely by the time of the dinosaurs giving way to birds and mammals (the common examples), the mature genome that they would have assembled compared to the Cambrian fauna would have meant that their radiations operated by fundamentally different mechanisms, but to an outside observer the results might appear similar.