by Matthew Cobb
Jerry’s recent post about Steve Marley’s fantastic photo of a trilobite eye reminded me of one of the odder paleontological findings. Many trilobites have chunks taken out of them, presumably by predators. This fantastic CGI video – an extract of a display at the Chicago Field Museum – gives you some idea of what might have happened: at the end, a hapless Cambrian trilobite is snarfed by the top predator of the time, Anomalocaris. (This idea has recently been disputed by Whitey Hagadorn of Amherst College, who has made computer models which suggests Anomalocaris simply couldn’t have crunched the hard exoskeleton of a trilobite. Other people aren’t convinced and point out we’ll have to find some Anomalocaris poop to settle the issue…)
What’s odd about the munched trilobites is that, when you look at the distribution of injuries, you find a 2:1 ratio of injuries on the right hand side of the animals to injuries on the left-hand side of the animal. Like these:
This figure is taken from an article in Nature in 1989 by Loren Babcock and Richard Robison of the University of Kansas. The table showing the data (rather poorly reproduced below) is pretty convincing, but WHY?
The authors suggested that “those scars we attribute to sublethal predation… are significantly more frequently found on the right side of the trilobites, suggesting that predators preferred to attack that side”, and they pointed the finger at Anomalocaris.
There is a precedent for this kind of predator preference. In 1993, Michio Hori of Wakayama Medial College, Japan, reported in Science that scale-eating chichlid fish in Lake Tanganyika showed handedness and that “attacking from behind, right-handed individuals snatches scales from the prey’s left flank and left-handed ones from the right flank”. Maybe Anomalocaris (or some other Cambrian predator) showed similar preferences?
A few weeks after Babcock and Robison published their piece, Nature published two replies, one from David Maitland, the other from Stephen Stigler, contesting their interpretation. Maitland suggested the apparent preference might be a consequence of the trilobites curling up to defend themselves (I don’t really follow his argument, to be honest), while Stigler, more interestingly, suggests this might be a problem of sample biais. Maybe the underlying assymetry is not in the predators, suggested Stigler, but in the internal organs of the trilobites: “it may be that trilobites were attacked equally often on both sides, but more of those attacked on the right side survived to join Babcock and Robison’s sample.”
That seems to have been the last word on the question, for the moment. But I am reminded of a study I heard about a while back, which was based on thousands of beachcombing records from the English Channel, where it was found (I think in Holland), that significantly more left shoes were washed up than right (or it may have been the other way round). I’ve been unable to track down this article, or any sensible explanation. All suggestions gratefully received!