Fossil turtle carapace and plastron bones from the earliest Bridgerian (Ypresian, early Eocene) lowermost Bridger Formation in the Bridger Basin of southwestern Wyoming are characterized by numerous biogenic bone modification features. These include the ichnotaxa Karethraichnus and Thatchtelithichnus (interpreted as invertebrate parasitism traces) and the ichnotaxa Nihilichnus and Linichnus (interpreted as predation/scavenging traces). Carapace and plastron bones with ovoid to amorphous depressions of etched or corroded bone occur at several horizons and are particularly abundant (similar to 80%) at one horizon. In some examples the external cortical layer is corroded away and the underlying upper trabecular layer is exposed. However, in more severe cases, the deterioration of the shell bone penetrates halfway or more through the trabecular layer. These depressions occur solely on the external surface of bone elements and are consistent with the ichnotaxon Gunnellichnus. Comparison of the Bridger turtles with modern emydid turtle taxa affected by various shell disease suggest that these corrosion depressions record the occurrence of ulcerative shell disease (USD) on Eocene turtles. Ulcerative shell disease occurs rarely on Baena (Baenidae) and Baptemys (Dermatemydidae) but is particularly common on geoemydid turtles of the genus Echmatemys (E. septaria, E. wyomingensis, and Echmatemys sp.). It is notably absent on trionychid and carettocheylid turtles. The prevalence of USD on Echmatemys spp. is interpreted to reflect differing environmental preferences of Bridger turtles. The abundance of turtles with USD within one narrow stratigraphic horizon which may reflect a short interval of increased rainfall and cloud cover resulting in decreased basking efficacy, which is a primary factor in the occurrence of USD in modern turtles.
