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.