PALEOECOLOGY OF THE LOWER JURASSIC NAVAJO ERG MARGIN—MICRO- TO MACROSCOPIC BIOTIC EVIDENCE FOR UNDERSTANDING SURFACES, HYDROLOGY, AND RELATIVE TIME IN AN EOLIAN LANDSCAPE

The Lower Jurassic Navajo Sandstone in SE Utah represents an erg margin of the largest sand sea in Earth history. Strata record ecological relationships of desert communities as trace fossils in a mosaic of dune, interdune, wet interdune, fluvial, palustrine-lacustrine, and playa environments. These communities reflect the interplay between (1) hydrology, (2) dune mobility and sedimentation rate, and (3) depositional energy, all of which affect landscape stability. These factors are unlike those in the marine realm, where traces are enveloped in seawater and controlled by lithofacies and media consistency of the environment. Traces occur in nearly all eolian deposits, with the greatest number and diversity in the most stable settings that produce major surfaces.

Trace-fossil occurrences represent the presence of water and vegetation. Water availability exerted control on the composition and size of community structure as well as its distribution, including fluvial and lacustrine subenvironments. Without water to support vegetation and/or microbialites as primary producers, no animals could persist in erg settings, including those that fed on windblown plant litter. Various rhizoliths, tree stumps, and steinkerns are most abundant on major stabilized surfaces.

Traces represent transient, temporary, and periodic categories for presence in the environment:

Transient: Cylindrichum was constructed for short-term use by adult arthropods on reactivation (3^rd order) and set (2^nd order) surfaces.

Temporary: Skolithos and Macanopsis were used for ambush predation and juvenile rearing. Celliforma and Fictovichnus were constructed and provisioned by adults on 1^st order surfaces, including extensive interdunes, for stationary larvae that fed on plant fluids and insects, respectively. Naktodemasis, Beaconites, and Ancorichnus represent mobile rhizophagous and detritophagous insect larvae and nymphs in interdune and major surfaces.

Periodic: Termitichnus morphotypes represent detritophagous adults that amassed decaying plant material for others and juveniles in nests, producing complex structures. Vertebrate burrows represent adult activity, from which they preyed on other animals and raised their young. These and associated traces occur on major surfaces during pauses in deposition.