Ichnofossil record of selective predation by Cambrian trilobites

- Trilobite Rusophycus traces are found intersecting vermiform burrows.
- Rusophycus trace size is positively correlated with intersected worm burrow size.
- Intersected vermiform burrows are significantly smaller than non-intersected burrows.
- Low angle attacks occur more frequently than expected due to random chance.
- Paired Davis Shale trace fossils may directly record predatory behavior.

Evidence of predatory activity can be observed in the fossil record in the form of drill holes, repair scars, bite marks, and recognizable skeletal fragments in coprolites and preserved gut tracts. It is less common, however, to find fossil snapshots of predators caught in the act of feeding on their prey. Such interactions are preserved in recurring associations of the ichnogenera Rusophycus and Cruziana, most commonly attributed to trilobites, with burrows of likely vermiform (worm-like) organisms. In this study, we examine the Cambrian (Furongian Epoch, Steptoean Stage) Davis Formation, near Leadwood, southeastern Missouri, USA. In the lower to middle Davis Fm., several silty shale beds are extensively burrowed, from which we report a new occurrence and large number of Rusophycus traces associated with burrows of vermiform organisms. Within these beds, Rusophycus traces intersect vermiform burrows more often than expected by random chance and display a positive correlation in size between paired tracemakers. The median diameter of Rusophycus-associated vermiform burrows is significantly smaller than that of the non-intersected burrows. These results suggest that the paired traces record size selective predatory behavior. Moreover, low angle predator-prey trace intersections, though few in number, occurred more frequently than expected by random chance, supporting previous hypotheses that low angle attacks are preferred as they may improve prey handling success rates.

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