Late Cretaceous and early Tertiary turtles from the Big Bend Region, Brewster County, Texas
Tomlinson, Susan L.
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Fossil turtles are abundant in the Upper Cretaceous and Lower Tertiary sediments of the Aguja, Javelina, and Black Peaks Formations m Big Bend National Park, Brewster County, Texas. Nine genera of freshwater and terrestrial turtles have been identified from these deposits, including Bothremys, "Baena ", Neurmikylus. Compsemys, Adocus, Basilemys. "Aspideretes", "?Helopanoplia" and Hoplochelys. A marine turtle from the Aguja Formation represents a new genus. Turtles are most abundant in the marginal marine, brackish, and freshwater floodplain deposhs of the Aguja Formation, and within the Aguja, they are most abundant in the upper shale member. 'Aspideretes' is the dominant genus in the Upper Cretaceous sediments, followed by "Baena " Turtle fossils are rare in the fluvial floodplain deposhs of the Javehna Formation, where the dominant genus is also Aspideretes." There is a slight increase m abundance and diversity of turtles in the fluvial floodplain deposits of the overlying Black Peaks Formation, where Eoplochelys is the dominant genus. The decrease in numbers and diversity of turtles in the Javelina and Black Peaks formations, compared to the Aguja, was probably the result of a change to dry inland environments less hospitable to turtles. The diversity level of the upper shale member of the Aguja Formation is comparable to that in the correlative Fruitland and Kirtland formations of the San Juan Basin in New Mexico, and to a modem assemblage of turtles in the Brazos River drainage basin in Texas. A comparison of the shell morphologies exhibited in the Aguja fauna and in the Brazos River fauna also suggests that the diversity level is comparable. A morphometric technique was used to determine whether variability in the ornamentation patterns in trionychid shells is useful for taxonomic discrimination. Preliminary results suggest that discrete shell ornamentation patterns are discriminatory and non-random. This technique may be useful for identifying levels of fossil trionychid diversity.