Blinding Me with Science

Today's edition of Science, it turns out, is packed so chock-full of goodies that I hardly know where to turn. The discussion of how to distinguish species of bacteria? The beetles with male trimorphism? Blue butterfly larvae mimicking the sounds made by queen ants in order to be tended by the deluded worker ants? All of them very cool, and well worth discussion. But lets look at option four.



This little beastie (just under ten centimetres long) is called Schinderhannes bartelsi*, and its fossil remains are described in a paper by Kühl et al. (2009) (from whence comes the above reconstruction). Some of you may immediately recognise the similarity to the famed larger animals Anomalocaris and Laggania of the Cambrian Burgess Shale. However, Schinderhannes bears a few significant differences from those taxa: (1) it has that bizarre pair of 'wings' attached to the back of the head; (2) certain details of its anatomy suggest that it is more closely related to living arthropods than is Anomalocaris, showing that arthropods are descended from an 'anomalocarid' grade; and (3) it doesn't come from the Burgess Shale, but the German Hunsrück Slate, which is from the Lower Devonian, and shows that 'anomalocarid'-type animals were around for some 100 million years longer than we previously knew. I hate to repeat the old cliché about it being like discovering a Tyrannosaurus alive today, and in fact it's not like that, because the amount of time separating Tyrannosaurus from the present is considerably less than 100 million years.

*The name Schinderhannes is apparently derived from that of an 18th century bandit in the area from which it was found. Neat name, but it hints frustratingly at a back story that we are sadly denied in the paper.

Schinderhannes resembles anomalocarids in its radial mouth, and the large pair of spiny pre-oral appendages. However, certain of its features are more like true arthropods - it has a dorsum divided into distinct, sclerotised tergite plates, and it has biramous (two-branched) appendages like crustaceans. The combination of the large 'wings' and 'flukes' on either side of the tail spine suggest that it was an active swimmer.



Large raptorial pre-oral appendages (dubbed 'great appendages') have also been found in a number of Cambrian arthropods such as Leanchoilia and Yohoia. The phylogenetic position of such 'great-appendage' arthropods has been hotly debated. Budd (2002) suggested that they were a stem grade to the arthropod crown clade, but Cotton & Brady (2004) placed them within the crown clade, in the stem group for chelicerates. Researchers have also debated whether the great appendages of these arthropods are homologous to those of anomalocarids, and whether the great appendages are homologous to the chelicerae of modern chelicerates. The (admittedly pretty rudimentary) phylogenetic analysis of Schinderhannes by Kühl et al. (2009), the results of which are shown above, supports a position of great-appendage arthropods as stem chelicerates (despite the great appendages of these arthropods being a priori coded as homologous to those of anomalocarid-grade animals), which supports the comparison between great appendages and chelicerae. It also suggests that trilobites are closer to crustaceans than chelicerates, contrary to the idea of a trilobite + chelicerate "Arachnomorpha" clade. In some regards, this would make sense - trilobites, like crustaceans and insects, have lost the plesiomorphic state of grasping pre-oral appendages as found in chelicerates and have filamentous antennae instead. However, the position of trilobites in the tree above seems to be primarily due to the presence of antennae, so I don't know if it can be considered well-supported.

REFERENCES

Budd, G. E. 2002. A palaeontological solution to the arthropod head problem. Nature 417: 271-275.

Cotton, T. J., & S. J. Braddy. 2004. The phylogeny of arachnomorph arthropods and the origin of the Chelicerata. Transactions of the Royal Society of Edinburgh: Earth Sciences 94: 169-193.

Kühl, G., D. E. G. Briggs & J. Rust. 2009. A great-appendage arthropod with a radial mouth from the Lower Devonian Hunsrück Slate, Germany. Science 323: 771-773.

13 comments:

  1. Andreas Johansson6 February 2009 17:11

    Neat.

    It's sobering to think what gaps our knowledge of paleozoic life still has. I suppose anomalocarids are insufficiently sclerotized to fossilize easily?

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  2. I suppose anomalocarids are insufficiently sclerotized to fossilize easily?

    'Fraid so. The hardest parts (and then still not particularly hard) are the ring of tooth-plates and the great appendages, both of which were described as separate entire or near-entire organisms before being recognised as part of one larger organism.

    An even more remarkable case would be Bowengriphus from the Permian, which was described as closely related to Odontogriphus that had been only known from the Cambrian. Not everyone accepts that Bowengriphus is an odontogriphid, though.

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  3. Schinderhannes the bandit, BTW, has an English wikipedia page: http://en.wikipedia.org/wiki/Schinderhannes

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  4. Damn, and I was thinking this would be a quiet weekend (aside from the Ocean Bowl), one I could use to catch up on honey dew lists!

    When I saw the image I first thought it was a new reconstruction or artist interpretation of Anomalocaris. This is very exciting.

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  5. Man!

    I thought ancestral arthropods were supposed to look like velvet worms (coincidentally mentioned in the "Incredible Organisms" thread). Or is that for an earlier stage?

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  6. Schinderhannes the bandit, BTW, has an English wikipedia page

    As does the fossil Schinderhannes. Already. Though I must say, Schinderhannes the person doesn't really sound like someone I'd want to honour with a patronym.

    I thought ancestral arthropods were supposed to look like velvet worms (coincidentally mentioned in the "Incredible Organisms" thread). Or is that for an earlier stage?

    Yep, go back a bit further for the lobopods. You can see one of the Cambrian lobopods - Aysheaia - at the root of the phylogenetic tree.

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  7. The front end is just TOO Anomalocaridid-like...
    Just how well DO we know what Anomalocaris's appendages were like? Enough to refute the "backwards" hypothesis?
    "Backwards" hypothesis that occurs to my suspicious mind):
    Schinderhannes (give or take the "wings") is actually a late surviving "stem anomalocaridarthropod," morphologically similar to the common ancestor of A. and A.: the line leading to Anomalocaris LOST its sclerotized tergites as it increased in size.
    ---
    Sorry. Amateur's comments, without having gone through the data matrix AT ALL. Disregard. ...
    I just find the conservation of great appendage and mouth morphology through a period in which so many euarthropd features evolved... incredible?

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  8. It's possible that I've misunderstood what you're suggesting (in which case, I apologise), but I fail to see what the problem is. Not all parts of an organism change at the same rate over evolution, so it shouldn't be surprising when some features of an organism look more plesiomorphic than others.

    That said, the data matrix for the Schinderhannes analysis is a little rudimentary, and possibly wouldn't be the worse off for a few extra characters.

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  9. Christopher Taylor--
    No, you understood me corectly, and I wasn't thinking very clearly: I was just bowled over by the "primitive" aspect of the head-end of the critter.
    ----
    A cladogram represents a hypothesis, not just about genealogical relations, but about the order in which novel traits appeared. Comparing Schinderhannes, Anomalocaris, and the "Great Appendaged Euarthropods," the current one suggests that evolutionary novelty, here, started from the tail: Schinderhannes seems to have derived traits behind the head, while the head (stalked eyes, circular mouth, "anomalocaridiform" great appendage) is stunningly plesiomorphic.
    ---
    And haven't I seen a relatively recent (within the past decade) report that Anomalocaris itself had a series of little lobopod-like limbs sticking down between the "flaps" on each side?

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  10. Schinderhannes seems to have derived traits behind the head, while the head (stalked eyes, circular mouth, "anomalocaridiform" great appendage) is stunningly plesiomorphic.

    It's slightly more complicated, as Schinderhannes is also coded as having an arthropod-like carapace over the head, which Anomalocaris lacks. But yes, the majority of the head does look more plesiomorphic than the body.

    And something I've just remembered - Chris Nedin wrote a post a little while back where he hypothesised that anomalocarids became extinct because of the evolution of enrollment in trilobites at the end of the Cambrian. Anomalocarid-grade animals in the Devonian rather puts the kybosh on that idea, doesn't it?

    And haven't I seen a relatively recent (within the past decade) report that Anomalocaris itself had a series of little lobopod-like limbs sticking down between the "flaps" on each side?

    As far as I know, there's no direct evidence for leg morphology in Anomalocaris (and the Schinderhannes paper has a bit of dodgy coding for Anomalocaris in that regard). But another anomalocarid-grade animal, Kerygmachela, does have preserved lobopod limbs. Going by the results of the Schinderhannes analysis, with Anomalocaris above Kerygmachela but below Schinderhannes, it's an open question whether or not Anomalocaris had lobopod limbs or more arthropod-like limbs.

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  11. Christopher Taylor--
    Thanks for your comments on my (ignorant) comments. ... As for legs on an Anomalocaris, I suspect I was remembering a picture of Parapeytoia, as at
    http://www.trilobites.info/species2.html
    though these legs seem to be more like standard arthropod legs than like lobopod appendages.

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  12. If you tire of the T. rex survival comparison, you can always use pterosaurs instead. By choosing a variety that disappeared at the right time, you can calibrate your comparison precisely. That alternative offers the added fillip of needling those people who insist that some really are still around, and you get to mention ropens.

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  13. Perhaps, but did you know that aquatic ropens suffer terribly from the depredations of parasitic Hallucigenia?

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