CAUGHT IN THE ACT: ICHNOFOSSIL RECORD OF SELECTIVE PREDATION BY CAMBRIAN TRILOBITES

Predatory activity in the form of drill holes, repair scars, bite marks, and recognizable coprolitic skeletal fragments can be readily observed in the fossil record; however, evidence of the direct predatory event is far less commonly preserved. Snapshots of predatory interactions have been observed in recurring associations of trilobite traces, Rusophycus and Cruziana, with burrows of vermiform organisms—in several instances providing insights onto the behavior of the predator. This study examined a previously unreported occurrence of Rusophycus traces associated with worm burrows from the Upper Cambrian (Furongian series, Steptoean stage) Davis Formation, near Leadwood, southeastern Missouri, USA. Here, a 2 m-thick series of clay-siltstone beds yields abundant bedding-parallel burrows preserved as positive hyporelief and scattered disarticulated trilobite skeletal fragments. From 80 collected slabs of this material (with a surface area totaling 18,283.50 cm² and a median horizontal bioturbation intensity of 5.06%), a total of 1,283 vermiform burrows were measured, of which 111 (8.7%) were intersected by Rusophycus. This large number of observed interactions allowed for a more rigorous application of statistical methods to examine inferred predatory behaviors. Within these beds, Rusophycus intersect vermiform burrows more often than expected by random chance and display a positive correlation in size between paired trace-makers (Pearson correlation: r = 0.41, p < Bonferroni-corrected α of 0.0056 for Rusophycus surface area versus vermiform burrow width). In addition, the median diameter of Rusophycus-associated vermiform burrows is significantly smaller than that of the non-intersected burrows and overall population. Moreover, low angle Rusophycus-burrow interactions, though few in number, occurred more frequently than expected by random chance, supporting previous hypotheses that such low angle attacks may be an expected strategy to improve prey handling success rates. In conjunction, the interaction frequency, size-selectivity, and possible preferred angle of approach collectively suggest that the paired Rusophycus-burrow traces result from non-random encounters by behaviorally sophisticated trilobite hunters.