FISH AND SWIMMING REPTILES: ICHNOFAUNA OF THE BASAL MAHOGANY MEMBER OF THE TRIASSIC ANKAREH FORMATION IN CENTRAL, UTAH

The Triassic Ankareh Formation has long been considered to be nearly barren of fossils. Recently, however, a large number of fossils, mainly traces, were discovered in Spanish Fork Canyon in central Utah. There, the base of the Lower-Middle Triassic Mahogany Mbr (~ upper Moenkopi Fm) is a regressive sequence of shales, siltstones, and sandstones that interfingers with the marine Thaynes Fm. The basal Mahogany shales preserve Haplotichnus (small arthropod/worm trails), Diplichnites (many-legged arthropod traces), and a range of Undichna (fish-fin trace) morphs. The most interesting traces pertain to Rhynchosauroides, (lacertoid, lizard-like reptiles with feet ~15 mm long). Some of these tracks have detailed scale impressions, others are simply toe impressions, while others consist of fine, claw tip drag-marks. The latter ranges from short arcuate to elongate, jagged linear claw marks that are respectively interpreted as swimming strokes and ”claw anchoring” in shallow water to resist wave buffeting. Rhychosauroides trackways with tail drags are also present. The shales preserve no vertical or infaunal feeding burrows. Fish scales are the only body fossils.

The abundance of fish swim traces indicate the area was often underwater at the edge of a substantial water body while sub-cm-scale ripple marks and the small lizard-like tracks show the water was sometimes only centimeters deep. Complete lacertoid tracks and tail drags indicate brief periods of subaerial exposure followed by gentle inundation and coating by fine muds, while horizons of desiccation cracks point to longer exposure and drying. The absence of an infauna suggests the substrate either lacked food sources or was otherwise not conducive to burrowing organisms. The depositional environment is tentatively interpreted as a tidal flat that was commonly under very shallow water. These waters were frequented by fish swimming along the bottom, and lacertoids possibly feeding in the shallows.