RIDING THE WAVES: FEEDING HABITS OF TRIARTHRUS BECKII AND ITS ALIGNED CRUZIANA IN A LATERALLY EXTENSIVE TRACE-FOSSIL EVENT BED IN THE UTICA SHALE, NEW YORK STATE

The olenid trilobite Triarthrus is one of the rare macrofossils commonly present in the nearly barren black shales of the Late Ordovician Utica Formation. Triarthrus paleoecology and mode of feeding are uncertain. Some previous authors have suggested that this olenid, in part because of its proclivity for deep water dysoxic facies, harbored sulfur-reducing chemosymbionts. A laterally extensive (~15 km) trace fossil event bed in the Indian Castle Member of the Utica Formation, central New York State, preserves abundant Rusophycus and Cruziana along with T. beckii body fossils (both molts and carcasses). Carcasses are rare but trace fossil densities are 10-20/100 cm². The only other fossils on this surface are orthoconic nautiloids and graptolites. The event bed surface is directly overlain by an ~10 cm-thick ash beds that evidently smothered the surface and thus provides a unique glimpse at the paleoecology of these trilobites. Orientation of over 500 individually measured traces reveals strong alignment with heads facing directly into the current, which flowed from the WSW toward the ENE (supported by aligned nautiloids). Traces grade from Rusophycus into Cruziana with the latter type strongly dominant. Rusophycus specimens resemble the size and shape of Triarthrus beckii. These data suggest that T. beckii were not farming chemosymbionts, but rather were particle feeders that in this instance employed the weak regional current to assist feeding under quiet and strongly dysoxic conditions. These results support recent findings for Triarthrus eatoni at Beecher’s Trilobite Bed and other aligned occurrences of Cruziana and Rusophycus.