TRILOBITE HUNTING(?) GROUNDS OF THE CAMBRIAN DAVIS FORMATION, SOUTHEASTERN MISSOURI

One of the most discussed paleoecological interactions is that of predators and their prey, commonly deliberated as a possible driving force of evolutionary change. Indeed, often cited as one of the contributing factors to the Cambrian Explosion of animal biodiversity, predation behaviors have been recognized in fossils dating to the latest Ediacaran in the form of drill holes in the mineralized tubes of Cloudina, one of the earliest known biomineralizing animals. In addition to the aforementioned drill holes, our evidence for predator-prey interactions in the fossil record comes largely from such traces, also including bite marks and repair scars in preserved prey, as well as from preserved skeletal fragments in coprolites (although this could just as easily represent scavenging behaviors). While we can infer predatory activity from each of these traces, rarely do we find snapshots of predators caught in the act of feeding on their prey. Such views are known, however, notably in the form of recurring associations of the arthropod ichnogenera Rusophycus and Cruziana (most commonly attributed to trilobites) with burrows of vermiform animals. These interactions have been interpreted to represent direct feeding behavior of the Rusophycus or Cruziana tracemaker on the burrowing worm. Interactions of this sort, although limited and sometimes debated, have been reported from the lower Paleozoic, specifically from Cambrian deposits in Sweden (Bergström 1973; Jensen 1990) and New Brunswick, Canada (Pickerill and Blissett 1999), Ordovician deposits in southwestern Ohio (Brandt et al. 1995), and Silurian deposits of New York (Osgood and Drennen 1975 after Hall 1852; Tarhan et al. 2012). Here, we examine the Upper Cambrian (Furongian, likely Franconian) Davis Formation, near Leadwood, southeastern Missouri. Shaley fossiliferous limestone and dolomite of the middle Davis Fm. are part of an intrashelf basin sequence with lateral and vertical transitions to shallow-ramp and platform carbonate facies. The rocks consist of ooid wackestones and grainstones, thrombolite boundstones, hardgrounds, and shale partings. Some silty shale beds are burrowed extensively, from which we report a new occurrence of Rusophycus traces associated with worm burrows and investigate the possible predatory nature of their interactions.