FOSSIL CHITONS AND MONOMORPHICHNUS FROM THE EDIACARAN CLEMENTE FORMATION, SONORA, MEXICO

Two fossil chiton species belonging to different genera (family Leptochitonidae) occur in the Ediacaran Clemente Formation of Sonora, Mexico. Valve and girdle morphologies characteristic for Polyplacophora occur in the two species. Considering its stratigraphic position beneath the Clemente oolite, and its Proterozoic age (c. 580-585 million years), the older species represents the earliest confidently identified megascopic animal body fossil. The ichnospecies Monomorphichnus multilineatus Alpert, 1976 has also been recovered from beneath the Clemente oolite. This trace fossil, consisting of 5 or 6 sets of parallel scratches, extends across a bedding sole surface for a distance of 25 mm. The specimen represents the earliest evidence for a trilobitoid animal in the fossil record. Its morphology confirms the biogenic character (questioned in one report; Palaeontology 50:169-175) of a specimen of Rusophycus multilineatus McMenamin, 2001 from the overlying La Ciénega Formation. The latter, a dual ichnofossil, shows a R. multilineatus tracemaker attacking a Planolites tracemaker, and thus represents the oldest direct evidence in the fossil record for a predation event as well as the first evidence for a direct ecological interaction between unrelated animal species. These new data impart considerable paleontological importance to the Ediacaran‑Cambrian strata of northwestern Sonora, Mexico, a stratigraphic sequence that has produced evidence for a diverse and very ancient ecosystem. This ecosystem hosted both animals and non‑animal (vendobiont) Ediacarans.