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 (∼ 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.

Modern crinoids have the ability to use their arms to crawl along the sea floor and some are capable of swimming short distances. The first and only evidence of crinoid locomotion reported from the rock record was described from the Middle Jurassic of the Cabeço da Ladeira Lagerstätte (Portugal) resulting in description of the ichnotaxon Krinodromos bentou. Although the mechanics of crinoid movement are well documented the morphological ranges of crinoid motility tracks are unknown. This study uses observations of crinoid movement and their effects on sediment using modern comatulid crinoids to propose possible trace fossil morphologies. Using 20 experimental trials supported by photography, video analyses, 3D orthogrammetry and resin casting, the morphological ranges of crinoid motility tracks are included in five distinct morphologies attributed to ambling, crawling, walking, running, and landing/taking-off traces, the latter of which are emplaced before and after swimming.

Traces produced by ambling occur as epigenic hook-shaped grooves. Crawling traces comprise closely spaced hook-shaped grooves and ridges preserved in concave and convex epirelief. Walking traces consist of semi-bilaterally symmetrical collections of three or more grooves, and associated ridges, preserved in convex and concave epirelief.

Running traces consist of semi-bilaterally symmetrical collections of one to three straight to semi-sinusoidal grooves and associated ridges preserved in concave and convex epirelief. Landing/taking-off traces are mounded features preserved in convex epirelief, with grooves radiating from the center. The five trace types described in this paper provide insight into morphological features that can be associated with modern crinoid activities and used to identify crinoid trace fossils, which are rarely reported, in the rock record.