PREHISTORIC TRACKWAYS NATIONAL MONUMENT: A WINDOW ON A 280-MILLION-YEAR-OLD COASTAL ECOSYSTEM IN SOUTHERN NEW MEXICO

The Prehistoric Trackways National Monument (PTNM) is an ~ 20km² area in the Robledo Mountains of Doña Ana County, NM, that exposes strata of the Lower Permian Hueco Group that include more than 100 fossil localities that yield one of the world’s most significant records of nonmarine Early Permian trace fossils. The trace fossils, other associated fossils and the Hueco sediments provide a remarkable window on the local Early Permian coastal ecosystem. The Hueco section consists of (ascending): (1) the uppermost 45 m of the Shalem Colony Fm., mostly thick-bedded marine limestones with brachiopods and the middle Wolfcampian fusulinid Pseudoschwagerina; (2) Community Pit Fm., ~ 140 m of interbedded marine limestone (mostly micrite) and shale with numerous fossilized coniferous logs, evidently driftwood from the nearby land; (3) Robledo Mountains Fm., 125 m of interbedded marine limestone and shale and red bed siliciclastics that yield extensive trace fossil assemblages and fossil foliage; and (4) Apache Dam Fm., at least 25 m of mostly cherty algal limestone. The traces are almost exclusively locomotion traces of arthropods and vertebrates, though some resting traces and shallow subsurface grazing trails are known. Most of the trace fossil assemblages represent the Diplichnites ichnoguild of the Scoyenia ichnofacies, though recent discoveries of localized burrowing and marine-influenced ichnofossil assemblages indicate a complex mosaic of ichnoassemblages. The associated paleoflora is dominated by conifers of several morphotypes, as is typical of Abo paleofloras to the north. The trace-fossil-bearing red beds of the Robledo Mountains Fm. have been interpreted as either tidal flat or fluvial deposits. We interpret them as unchannelized sheetflood deposits on a distal coastal plain. This eliminates the apparent conundra of traces of animals who were not salinity tolerant and the absence of fluvial channels in the red beds. It also supports recognition of an ecosystem alien to the modern world, one in which vast coastal mud and sand flats hosted a coniferous forest flora and a fauna that apparently lacked vertebrate herbivores.