Synapsid Burrows and Associated Trace Fossils in the Lower Jurassic Navajo Sandstone, Southeastern Utah, U.S.A., Indicates a Diverse Community Living in a Wet Desert Ecosystem

Abstract

Abstract Enigmatic structures in the Lower Jurassic Navajo Sandstone near Moab, Utah, occur at three localities below carbonate beds that record interdune lake deposits. These structures are interpreted as mammal and therapsid burrows based on their architectural and surficial burrow morphologies: Type I (large diameter) and Type II (mega diameter) burrows. Type I burrows include sinuous tunnels, Y- and T-branched tunnels, sinuous ramps, and chambers, and weather into mounds averaging 33 m × 22 m and extend ~ 1m above the surface. Type I burrows are dorsoventrally flattened, in cross section averaging 9.3 cm wide and 4.2 cm high, and are sand filled and structureless. Type I burrows mostly have smooth walls, though some have scalloped walls. Type II burrows are simple, inclined tunnels ~ 35 cm wide and ~ 20 cm high, and exhibit well-preserved bilobate morphology along the underside of the tunnel. The walls preserve a series of 3 or 4 thin (~ 4–8 mm), inclined scratch marks from the upper part of the wall and along the floor. The great complexity and high density of Type I burrow systems is best explained by multiple individuals living together in social groups. These burrows are more complex than Early Triassic therapsid burrows from South Africa and are most similar to burrows of extant social (e.g., voles) and eusocial mammals (e.g., naked mole rats). Type II burrows were likely constructed by therapsids, based on burrow size and comparison to Permian and Triassic therapsid burrows from South Africa and Antarctica.