INVERTEBRATE ICHNOFAUNA AND LUNGFISH BURROWS FROM THE UPPER TRIASSIC REDONDA FORMATION OF EAST-CENTRAL NEW MEXICO

The upper Norian Redonda Formation (Chinle Group) at Mesa Redonda, Tucumcari Basin, New Mexico, contains two distinct ichnofaunal associations assigned to the Scoyenia ichnofacies, and an unusual occurrence of lungfish burrows. Numerous well-preserved Lockeia siliquaria are found within laterally continuous calcarenite beds ~14 meters below the top of the Redonda (disconformable contact with overlying Middle Jurassic Entrada Sandstone). Larger specimens measure between 9 mm long and 4 mm wide, smaller than most published measurements of Lockeia. These beds contain large estherian conchostracan valves, which are of similar dimensions to the Lockeia and were the probable local producers of this ichnotaxon. Arenicolites isp. occurs in low abundance. Abundant vertebrate tracks assigned to the ichnogenera Brachychirotherium, Pseudotetrasauropus, and Grallator are found in this association. The paleoenvironment was a lacustrine shoreline. A second ichnofaunal association is found ~7 meters below the top of the Redonda. Numerous Scoyenia gracilis and scarce Skolithos isp. burrows occur in sandstones and mudstones. A localized occurrence of non-branching, large diameter (4.8-7.2 cm), cylindrical casts is found within siltstones representing a lacustrine margin facies. These casts are mostly comparable to lungfish burrows known from other exposures of the Chinle Group and the Permian of Texas and Kansas. Unlike these previously described fossil burrows, which represent estivation within vertical shafts, the Mesa Redonda burrows differ in that some shafts are obliquely angled and weakly helical. In this they resemble certain modern lungfish burrows used for spawning and nesting. The Mesa Redonda burrows may be the oldest record of diverse behavior in extinct lungfish. Both low diversity invertebrate trace fossil associations at Mesa Redonda are similar to other reports of the Scoyenia ichnofacies from Triassic lake margin environments.