Trace Fossil Evidence for Late Ordovician Animals on Land

Fossil burrows within newly recognized buried soils in the Late Ordovician Juniata Formation, near Potters Mills in central Pennsylvania, represent the oldest reported nonmarine trace fossils. They are thought to have been an original part of the soil because their greater density toward the top of the paleosols corresponds with mineralogical, microstructural, and chemical changes attributed to ancient weathering and because about half the burrows are encrusted with nodular carbonate, interpreted as caliche. Associated fossil caliche, the size distribution of the burrows, and their W-shaped backfills are evidence that the burrows may have been excavated by bilaterally symmetrical organisms that grew in well-defined growth increments and were able to withstand desiccation. Among well-known soil organisms, millipedes are burrowing animals that satisfy these requirements, but have a fossil record not quite this old. This trace fossil evidence for animals on land, together with recent palynological evidence for land plants of a bryophytic grade of evolution during Late Ordovician time, are indications of terrestrial ecosystems of slightly greater antiquity and complexity than hitherto suspected.