Synapsid Burrows in the Lower Jurassic Navajo Sandstone, Utah

Abstract

Enigmatic structures below interdune deposits of the Lower Jurassic Navajo Sandstone near Moab, Utah are interpreted as synapsid burrows based on similar morphologies to fossil and extant synapsid burrows. Two types of burrows are distinguished by their size. Type I burrows are large diameter and comprise complex, high density sinuous tunnels, Y- and T- branched tunnels, sinuous ramps, and chambers. Type I burrows at one locality weather into mounds averaging 33 m x 22 m and extend ~ 1 m 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. These burrows mostly have smooth walls, though some have scalloped walls. Type I burrows represent a new ichnogenera and ichnospecies Labyrinthopolis odieri. These burrows are best explained by multiple individuals living together in social groups similar to modern vole (social) and mole rats (eusocial), and likely represent permanent dwelling structures for foraging, nesting, hiding, and food storage. Type II burrows are mega diameter with 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. Type II burrows represents a new ichnogenera and ichnospecies Schemalitus psalihyponomes. Type II burrows were likely constructed by therapsids based on similarities to therapsid burrows found in South Africa and Antarctica, and likely represents a permanent shelter used for dwelling and brooding. Alternate excavators for Type I and II burrows are rejected by reviewing and comparing burrow morphologies of fossil and extant vertebrate groups because morphologies are consistent within the major groups in both fossil and extant vertebrates. Burrow morphologies reflect the tracemaker's anatomy, social structure, media consistency, and food availability.