Friday, October 21, 2005

Anomalocaris

Pharyngula has a slightly different take on the same article.... focusing more on the tagmosis side of things (appendage homology based on which segment they emerge from). Tagmosis is a bit of a nightmare though... the farther back from the head you go, the more the appendages diverge into different specializations among different arthropod lineages, and the less sure you are about homology. Molecular work on developmental genes is going a long way towards sorting this out (thats how we know that the chelicerae in spiders is innervated from the deutocerebrum rather than the tritocerebrum), but for me its less interesting to argue over whether a pincer in one lineage is homologous to a paddle in another than to examine how both derived from an ancestral precursor.

That said, the more I think about linking the chelifores of sea spiders to the 'great appendages' of Anomalocaris, the more uneasy I get that we are leaping well ahead of the data. The big problem with the Cambrian lagerstatten is that they are snapshots in time... you don't get much in front of them and even less behind as they formed under fairly rare conditions. Anomalocaris appears then disappears, and its next to impossible to link it to any previous ancestor or descendant species. Although a few interesting lagerstatten fossils have shown internal anatomy, thats pretty rare, and usually confined to digestive systems. We don't have any idea how the neuroanatomy of Anomalocaris is arranged, and barring some miracle we find them swimming about a deep sea trench, we never will. The problem we have with Anomalocaris (and with other Cambrian arthropod-like fossils) is the same problem we have with contemporary arthropods... uncertainty about which segment is truly homologous with which, and (until recently) only the appendages to give a clue as to identity. What work like that of Maxmen et al. have done is show that appendage identity is really not that reliable a marker for segment homology. Unfortunately, studies such as hers are impossible on 500 million year old fossils.