804


Journal of Paleontology 89(5):802–820


Basin margin


Basin center


Lithostratigraphic units


Laney Member


Cathedral Bluffs Tongue


Wilkins Peak


Member


Farson S.S. Rife Bed


Scheggs Bed


Main Body


stromatolitic biolithite, and organic-rich shale are primarily included within the Green River Formation (Roehler, 1991, 1992). Brightly variegated mudstone and siltstone with laterally restricted sandstone and conglomerate beds have been interpreted as alluvial and fluvial sediments overprinted with paleosols and are included in the Wasatch Formation (Roehler, 1991, 1992; Zonneveld et al., 2003). The Wasatch Formation intertongues with the Green River Formation along most of the margin of the Green River Basin (Fig. 2) (Roehler, 1991, 1992). The fossils discussed herein were collected from the lower


beds of the Cathedral Bluffs Tongue of the Wasatch Formation (Fig. 2) in the Honeycomb Buttes area near South Pass (Fig. 1.2). Although highly variegated and brightly colored, similar to paleosol successions higher in the section, the presence of wave-rippled sandstone beds, stromatolite-covered branches and logs, as well as lacustrine vertebrate fossils such as gars, bowfins, aquatic turtles, alligators, and crocodilids supports the interpretation that the basal Cathedral Bluffs Ton- gue at Honeycomb Buttes represents lake margin to shallow lacustrine deposition with pedogenic overprinting. Supporting this interpretation as well is the presence of several specimens of the shorebird Presbyornis in association with eggshell frag- ments (suggesting a possible nesting site) and abundant fossils of emydid turtles (Echmatemys and a closely related form). The trace fossils that are the focus of this contribution occur on carapace and plastron elements of these turtles.


Terminology and acronyms Figure 2. Eocene stratigraphic units at South Pass, Wyoming. Geological setting


The study area occurs on the northeastern margin of the Green River Basin in the southwestern corner of Wyoming (Fig. 1.1). The Green River Basin formed a major depocenter during the Cretaceous and Paleogene. Subsidence in the northeastern Green River Basin, and concomitant creation of accommodation


space and sediment preservation in the study area, was a flexural response to uplift of the Wind River Mountains to the northeast (Shuster and Steidtmann, 1988; Steidtmann and Middleton, 1991). The sedimentary history of the Eocene succession in the


Green River Basin records multiple regressions and transgres- sions of Eocene Lake Gosiute, a large shallow lake that occu- pied the center of the basin (Roehler, 1991, 1992). During periods of maximum transgression, the lake lapped at the foot of the Laramide uplifts that surround the basin (Roehler, 1991, 1992; Zonneveld et al., 2003). During periods of maximum regression, the lake was limited to the center of the basin and fluvial deposition dominated in the basin (Zonneveld et al., 2003). The deposits of Paleolake Gosiute, consisting of calcareous


and dolomitic mudstone, sandstone, bioclastic packstone and grainstone, micrite, oomicrite, gypsum, trona, oil shale,


The specimens discussed herein occur on turtles that are housed in the paleontology collections at The University of Michigan Museum of Paleontology (UMMP) and the collections of The University of Alberta, Department of Earth and Atmospheric Sciences (UA-TF). Where possible, the precise shell element upon which the borings occurred is identified. Individual bones of the carapace and plastron and acronyms used to identify them are identified in Figure 3.1–3.3. The external surface refers to the surface of the shell covered by keratinized layers of the epidermal epithelium (epidermal scutes). The visceral surface of the shell refers to the surface of the plastron or carapace against which the soft tissue of the turtle would occur during life (Fig. 3.1). ‘Internal surface’ is used for the internal parts of outlying bones (such as the eipiplastrons, xiphiplastrons, nuchals, pygals, and some peripherals) which would have had distal components not covered by soft tissue but rather covered by epidermal scutes (Fig. 3.2 and 3.3). The boundaries between the scutes form seams or ‘sulci’


(Fig. 3.2 and 3.3). Although the epidermal scutes are not pre- served in any of the fossil material discussed herein, each sulcus is represented by a groove preserved on the underlying dermal bone and thus the shapes of the original scutes can be elucidated. It is worth noting that the epidermal sulci and the sutures between individual dermal bones in the plastron and carapace do not overlap (Fig. 3.2 and 3.3). Following the precedence set by previous workers


(eg. Bromley, 1981, 1993; Brett, 1985; Nielsen and Nielsen, 2001), ‘hole’ is used to identify a perforation that penetrates all the way through a shell bone, whereas ‘pits’ terminate within the bone and do not fully penetrate from the external to the internal


Wasatchian


Bridgerian


NALMA


Wasatch Formation


Green River Formation


Wasatch Formation


Green River Formation


Bridger Formation


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