Chain, Chain, Chain

Hou, X.-G., D. J. Siveter, R. J. Aldridge & D. J. Siveter. 2008. Collective behavior in an early Cambrian arthropod. Science 322: 224.

Fossils can offer fascinating insights into the lives of long-extinct organisms. Sometimes, the lifestyles suggested are so different from anything found in living taxa that we may be at something of a loss to understand their function and significance. The new publication linked to above reports on the fascinating discovery of a collection of early arthropods from the famed Chengjiang fauna of China. (For those unfamiliar with it, the Chengjiang is similar to the Burgess Shale of North America, but even more impressive - it is only the fact that the latter was discovered earlier that gets it more press).

The fossils are of an animal very similar to Waptia, a previously-known, superficially shrimp-like animal of uncertain affinities. Waptia is not uncommon as a Cambrian fossil - according to Taylor (2002), it is the third-most common animal in the Burgess Shale, with over 1000 specimens held by the American National Museum of Natural History. Despite this abundance, Waptia has not been described in detail since Walcott's original preliminary description in 1912 (Taylor, 2002, says that he is working on a revision, but this doesn't seem to have appeared in print yet). Hou et al. (2008) refer to it as a stem-crustacean, but do not specify on what grounds. It could just as easily be a stem-chelicerate, as are the majority of known Cambrian arthropods (Cotton & Brady, 2004).

What makes this new finding so remarkable can be seen in the figure below from Hou et al. (2008). A group of 22 individuals is preserved together, each arranged head to tail in a long chain. As shown in the close-up in fig. 1C, each individual has the tail-end of the individual ahead of it cavered by its carapace. There is no evidence that the animals were lined up in a burrow, so they were most likely living above the sediment surface. The fact that the chain has not broken apart as the animals were buried indicates that they must have had an extremely firm hold on each other in life. Hou et al. interpret the chain as having been pelagic, but that seems unlikely to me - the sheer abundance of Waptia in the Burgess shale seems more consistent with a life close to the sediment surface, which would offer more opportunities for burial. Cambrian animals more likely to be pelagic, such as Amiskwia and Nectocaris, are very rare as fossils.



What on earth were these animals doing lined up like that? Hou et al. claim that such behaviour is unique, but I'm not sure just how unique. Hou et al. claim that lines formed by modern arthropods such as crayfish (for migration) and some caterpillars (feeding) are "more trains than chains", but don't explain exactly what is the difference (the curse of the super-compressed Science format strikes again?) Certainly, I've seen fireblight caterpillars here in Perth form very closely-linked chains, and one of the most horrifying sights I've ever seen was a mass of about twenty fireblights, so closely coiled that it was hard to tell where one finished and another began, moving as one. The waptiid assemblage is unlikely to be connected to feeding, as the close proximity of the mouth of one and the anus of another makes such an arrangement rather too horrible to consider. Hou et al. favour a connection to migration, probably for defense, which is a distinct possibility. I would suggest that the chain could have also been related to mating behaviour. Some animals, such as some marine gastropods, can form chains of multiple intermating individuals. Hou et al. (2008) dismiss this possibility on the grounds that "there is no precedent of arthropods of comparable aggregation for fertilisation". However, living arthropods show an absolutely enormous diversity of mating behaviours. Is it that much of a stretch to entertain the possibility that extinct forms may have shown even more?

REFERENCES

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.

Taylor, R. S. 2002. A new bivalved arthropod from the Early Cambrian Sirius Passet Fauna, north Greenland. Palaeontology 45 (1): 97-123.