TAPHONOMIC INFLUENCE OF CARNIVORES ON THE MACROFAUNA COMPOSITION OF THE POLETA FORMATION, NEVADA

The middle member of the Poleta Formation (Cambrian Series 2; upper Montezuman-lower Dyeran according to Laurentian terminology) in Esmeralda County, Nevada, is one of the oldest Laurentian deposits yielding exceptionally preserved fossils. The unit, composed of thin-bedded gray shales with thin calcareous siltstone interbeds and sandstones, was deposited under storm-influenced, oxic-water shelf conditions in a transgressive to highstand systems tract. Biomineralized fossils, principally trilobites, brachiopods, hyolithids, and helicoplacoids, dominate in abundance over non-biomineralized fossils. Non-biomineralizing arthropods and other organisms are relatively rare, although preserved mantle setae of brachiopods and gut traces of hyolithids are moderately common. Trace fossils, including bedding-plane disruptions and coprolites composed of helicoplacoid ossicles and trilobite sclerites, are common in some intervals.

The taphonomic influence of macroscopic, carnivorous biodegraders in the Poleta depositional environment is demonstrated by fragmented trilobite sclerites, coprolites, and trace fossils resulting from trilobite predation on infaunal worms. The frequency of trilobite sclerite breakage, both those within coprolites and those isolated from them, exceeds 70% of cephala and 80% of thoracic sclerites. Similar high levels of sclerite breakage have been observed in the Middle Devonian of New York. Together, the evidence suggests that carnivory played a significant role as a taphonomic filter. The relative scarcity of macroscopic non-biomineralized material, other than in brachiopods and hyoliths, is also inferred to be related to high levels of carnivory.