Origin, architecture, and paleoecology of the early Eocene continental ichnofossil Scaphichnium hamatum— integration of ichnology and paleopedology

Abstract

Ichnofossils and alluvial paleosols from the lower Eocene Willwood Formation of the Bighorn Basin in northwest Wyoming documents the origin and paleoecology of the ichnofossil Scaphichnium hamatum as well as provide insight on changes in paleosol hydrology and development through time. In the Willwood Formation, 5. hamatum occurs as a discrete hamate (hook‐shaped) to lunate meniscate burrow with a lower bulbous termination in assemblages that range from a few to hundreds of individuals. The burrows are concentrated within the Bg horizon (gleyed B horizon) of hydromorphic paleosols representing distal floodplain settings. Until recently, the architect responsible for its construction was unknown. Comparisons with modern examples of insect nests and burrows suggests that the architect of S. hamatum was a beetle of the family Scarabaeidae and that nest construction occurred during periods characterized by relatively dry soil conditions and lowered water tables.

Interpretations of the trace fossil succession, which includes S. hamatum, in typical hydromorphic paleosols in the study area corroborate sedimentologic and geochemical evidence for cumulative profile development. The trace fossil succession documents changes in the paleosol hydrology (water table fluctuations) through time from alternating periods of wetting and drying to relatively drier conditions and finally to complete soil saturation and gleyed conditions.

The combination of understanding the burrow architect and burrow function with respect to the sedimentologic, paleopedologic, and infaunal succession history of the units allows us to make a first approximation of the depths and fluctuations of the paleo‐water table from the paleobiologic qualifications (attributes) of infaunal assemblages from unit to unit. The integration of ichnology and paleopedology in the studies of continental deposits provide an important means for improving paleoecological and paleoenvironmental interpretations.

Key words:

• Scaphichnium hamatum
• Scarabaeidae beetles
• brood cells
• nests
• Eocene
• paleopedology
• paleohydrology
• continental ichnofossils