ANATOMY, PALEOECOLOGY, AND TAPHONOMY OF THE TRILOBITE BUENELLUS FROM THE SIRIUS PASSET BIOTA (CAMBRIAN), NORTH GREENLAND

Specimens of the nevadiid trilobite Buenellus higginsi from the Sirius Passet Biota (Buen Formation: Cambrian) of North Greenland show a combination of features reflecting both biomineralized and non-biomineralized anatomy, and providing direct or indirect evidence of paleoecologic associations and taphonomic history. Buenellus illustrates some key aspects of the role that trilobites played in the Early Paleozoic Marine Revolution (apparently serving as both predator and prey); its role in sediment-excavation, however, is unknown.

The Sirius Passet Biota has a low diversity and low abundance of mineralizing organisms. Buenellus specimens, which account for most of the biomineralized skeletal parts, are normally preserved as decalcified, articulated exoskeletons, parallel to bedding. Their composition is dominated by clay minerals, although quartz, chlorite, muscovite, and chloritoid, are present, indicating metamorphism of shale to greenschist grade. Specimens of B. higginsi were preferentially oriented ventral-up as collected; this suggests both that minor current reorientation of remains occurred on the sea floor, and that strata at the Sirius Passet locality are overturned.

Carnivory on B. higginsi is implied by the remains of anomalocaridids and other potential predators, and by sublethal predation scars. Slight disarticulation in the thoracic region of some fossils may have resulted from either decay or scavenging. Slender traces associated with various arthropod body fossils, including those of B. higginsi, suggests scavenging. Coprolites containing trilobite sclerites are unknown, and isolated, fragmentary sclerites are rare.

Preserved non-biomineralized parts of B. higginsi are: 1, slender antennae projecting from under the exoskeleton; and 2, axial structures (inflated cavities lined with silica and limonite). Three types of axial structures are present: 1, a large cavity in the rear-subcentral part of the cephalon (interpreted as the stomach cavity); 2, a narrow, smooth-sided, medial tube that tapers slightly posteriorly (interpreted as the intestinal tract); and 3, metamerically divided pairs of sagitally located structures (interpreted as midgut glands). A carnivorous feeding habit is suggested by the lack of sediment-filled guts in B. higginsi.