BORED TURTLES ALONG PALEOLAKE GOSIUTE STRANDLINES, WASATCH FORMATION, SOUTH PASS, WYOMING

Fossils of aquatic reptiles (crocodilians and emydid turtles) are common within marginal lacustrine strata of the lower Cathedral Bluffs Tongue of the Wasatch Formation (early Eocene) at South Pass, on the northeastern margin of the Green River Basin. These fossils most commonly occur in three distinct horizons interpreted as paleostrandlines.

Turtle fossils consisted of variably complete shells, many with associated postcrania. Nearly all of the shells (both plastrons and carapaces) exhibit circular borings (0.5 mm to 6.5 mm wide) on the external surface. These borings differ from the amorphous ‘shell rot' observed elsewhere by their small size, circular shape and discrete occurrence (even in highly bored shells).

Borings with depths less than 50% of the thickness of the shell are bowl-shaped whereas those with depths exceeding 70% of the shell thickness are flask-shaped (i.e. narrower externally and wider internally). All holes were initiated from the external surface of the shell or on the margins of peripheral bones, areas that would be accessible on a living turtle. Many examples occur at the sulci between epidermal scutes, a weak area in the epidermal layer.

The borings are clearly parasitic in nature, excavated whilst the host was alive. Borings that completely penetrate through the shell exhibit a raised lip or circular bone spur on the internal surface. In rare examples this lip is expanded to form a bone patch that completely reseals the hole. In these examples the boring continued into the bone patch indicating that the parasite was present for prolonged intervals and bored slowly.

The identity and purpose of the borer is unknown although a bacterial parasite is favoured. Regardless, as the host organisms were moderate-sized lake turtles the parasite must have been able to survive both periodic submergence as well as occasional, possibly prolonged, subaerial exposure. As many of the borings occur on the base of the plastron the parasite likely lived entirely within the boring to avoid being abraded off.

Analysis of emydid shells from elsewhere in the Green River Basin indicates that the borings occur primarily on fossils from a discrete stratigraphic horizon and in a limited geographic area indicating that either the parasites were host-specific or their presence in the Green River Basin was brief.