[Paleontology | Ichnotaxa • 2016]
Ichnofossil Record of Selective Predation by Cambrian Trilobites
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).
[1.5 COLUMN, GREYSCALE]. doi: 10.1016/j.palaeo.2015.11.033
|
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
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
Nenhum comentário:
Postar um comentário
Observação: somente um membro deste blog pode postar um comentário.