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Fig. 8. Diagram of trilobite interactions with worm, progressing step-wise from (A) to (C), with underside views of perpendicular handling (D) and parallel handing (E).
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Highlights
• 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.
Abstract
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.
Keywords: Cambrian; Davis Formation; Rusophycus; vermiform burrows; predation
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Fig. 3. Slab photograph, template for calculation of horizontal bioturbation intensity, and three-dimensional surface rendering. Sample JWH-DAV-01 showing the bottom of the slab with (A) light photography, (B) illustration of different traces (light grey = unknown, medium grey = vermiform, and dark grey = Rusophycus) used for bioturbation intensity calculations, and (C) 3D surface rendering topographic view below the upper plane of the slab (as it is preserved as positive hyporelief). Scale bar = 5 cm, with 1 cm demarcation. Color topography scale = 0–16 mm.
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Fig. 4. Samples 13-DAV-20-057 (A–B), 13-DAV-071 (C–E), 13-DAV-20-003 (F), and 13-DAV-20-039 (G), showing the bottom of the slabs with (A, C, E–G) light photography, and (B, D) 3D surface rendering topographic view below the upper plane of the slab (as these traces are preserved as positive hyporelief). (A–F) Examples of Rusophycus-vermiform burrow intersections; (G) Examples of interpreted matground punctures. Scale bars = 1 cm, with 0.5 cm demarcation. Color topography scale in B = 0–12 mm, in D = 0–13 mm.
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Conclusions
As indicated by our quantitative approach, the intersections between Rusophycus and vermiform burrows in the Davis Formation are most likely representative of active predatory behavior. Based on surface area, the Rusophycus traces appear to intersect vermiform burrows far more frequently than what would be expected by random chance alone. The frequency of these Rusophycus intersecting with vermiform burrows reveal that 30.7% of the traces reported here have the potential to be predatory. Of the traces that do show a potential predatory interaction, there is evidence of prey size selectivity. In fact, the trilobites chose from among the vermiform organisms a smaller, or more precisely narrower, prey size. Indeed, those prey selected show a significant and positive correlation with the size of the Rusophycus predator. Once the reported ichnofossil intersections were established as non-random in nature, angle of attack was assessed to determine if there was a preferred orientation. When modeling for a uniform distribution of angles, while simultaneously accounting for a reduced likelihood of intersection with reduced angle of intersection, we found that, though few in number, low angle attacks occurred more frequently than expected by random chance. We interpret the non-random distribution of angles of intersection to support the hypotheses of Jensen (1990) and Tarhan et al. (2012) that actively predating trilobites attacked at lower angles to maximize appendage to vermiform organism body exposure. While decreasing the chance of intersection or visibility during approach, this method would allow for trilobites to increase their grappling/handling success and efficiency by using their appendages to aid in both locating and capturing their prey. In sum, these results highlight the importance of the availability of large sample sizes that, in turn, enable a more rigorous quantitative approach to understand the nature and behavior of trace fossils and their makers.
Tara Selly, John Warren Huntley, Kevin L. Shelton and James D. Schiffbauer. 2015. Ichnofossil Record of Selective Predation by Cambrian Trilobites.
Palaeogeography, Palaeoclimatology, Palaeoecology. In Press. doi:
10.1016/j.palaeo.2015.11.033