Lower Cambrian Rusophycus from Ellesmere Island, Arctic Canada: ichnofossil of a predatory, non-trilobite arthropod

Cruziana and Rusophycus are commonly assumed to be trilobite trace fossils, although associated exoskeletons are almost invariably absent from host strata. In the lower Cambrian Dallas Bugt Formation of Ellesmere Island, Arctic Canada, high-energy, cross-laminated sandstones locally with dense populations of large Skolithos are overlain by thin-bedded, bioturbated, argillaceous, medium-grained glauconitic sandstones of lagoonal aspect, yielding Rusophycus along with an assortment of other ‘‘worm’’ burrows. Some specimens of Rusophycus are located over small Skolithos shafts and the bottoms of Arenicolites, suggesting a predator-prey relationship. Many preserve the impression of a strongly vaulted carapace with a broad, triangular dorsal outline between 1 cm and 7 cm across. A blunt anterior ‘‘prow’’ projects slightly forward and arches dorsally, forming a keyhole-shaped notch. The arthropod dug into the sediment up to 5 cm deep with an antero-posterior angle of up to ~ 458 from the sea floor; in some cases it dug obliquely laterally and rarely sideways. Retroverse scratches—preserved in hyporelief as sharp-crested ridges—suggest about 20 pairs of endopodites tipped by a pair of small claws typically dug in unison, the anterior ones starting with a V-angle of 1408 and decreasing posteriorly to 908, in rare cases more tightly. Limb length increased posteriorly, although none extended beyond the carapace. Leg muscles were strong enough to rake through the muddy sand and pull inwards either straight or in a slightly forward-curving arc, but were also slightly flexible to wiggle around obstacles. Some specimens show subsequent re-burrowing which produced cross-cutting and opposed scratches. No evidence for exopodite brushings is present. Successive carapace imprints and scratches organized in transverse rugae in some specimens suggest repeatedly plunging and digging as the arthropod pulled backward in the sediment. This was likely aided by a stout trunk and a fan-shaped uropod-like structure flanking the telson which increased grip and leverage. A pair of eye stalks may have projected from the anterior notch, and these could have been retracted during digging, but there is no evidence of antennae. The tracemaker was not a trilobite but an unrelated crustacean-like arthropod, although there is no known late early Cambrian body fossil to which it can be ascribed. The specimens are classified as Rusophycus jenningsi (Fenton and Fenton, 1937) and a neotype is selected for this ichnospecies from the Gog Group near where it was first collected. Its range extends from the southern Canadian Rocky Mountains north along depositional strike to Victoria Island and northwest to Ellesmere Island and northern Greenland. Two new ichnofamilies are established: Rusophycidae and Cruzianidae.