Antonietto et al.—Ostracoda from the Lower Jurassic Moenave Formation


657


Figure 6. Ostracode extinction levels observed in the present work (1, 2) in comparison with the inorganic and organic carbon isotope curves (δ13C) of the Potter Canyon section (Suarez et al., 2017), Mohave County, Arizona, USA. Note the up-and-down trend of the inorganic (CO3) curve (VPDB = Vienna Pee Dee Belemnite standard).


in the Olsen Canyon containing numerous, piled-up ostracode carapaces and valves that were deposited during what might be an up-and-down trend in local carbon isotope records (Suarez et al., 2017) (Fig. 6). These values are like those observed during the P-Tr extinction event, which was also associated with vol- canic activity on a large igneous province in Siberia, Russia (Korte and Kozur, 2010). Several horizons preserving mud cracks, triclinic sulfate


crystal casts and algal mats indicate that Lake Dixie, although large, was relatively shallow, with sporadically fluctuating base levels (Kirkland and Milner, 2006; Kirkland et al., 2014). Sandstones with dinosaur tracks at the SGDS are interpreted as the shoreline area of the lake, and salt crystals were formed in the sediment of these shores during times of drought (Kirkland et al., 2002; Milner et al., 2006). Lake Dixie was bounded southeastward by coastal and fluvial deposits of the upper Dinosaur Canyon Member, and further east by the eolian, sand- dune desert deposits known as the Wingate Sandstone (Blakey, 1994). Westward it was limited by the first stages of what would become the Nevadan orogeny by the end of the Jurassic (Renne and Turrin, 1987). The diversity and composition of the Whitmore Point


Member ostracode fauna agree with previous interpretations of the paleoenvironments of Lake Dixie. After the ETE, a very diversity-depleted fauna populated the member, comprising few species, one of which (Suchonellina globosa) was dominant in all of the currently analyzed samples containing sizable


ostracode faunas. The majority of specimens recorded in the present work are females, and the very few that could be males are also interpretable as A-1 juveniles, according to the morphological parameters of Martens et al. (2003). Similar faunas, both in terms of diversity and population


composition, were also found in several formations that are coeval to the Moenave in Arizona, New Mexico, and Texas (Kietzke, 1987, 1989a, b; Kietzke and Lucas, 1991, 1995; Lucas and Kietzke, 1993). Lake Dixie was a shallow, occasionally saline lake, located at the center of a network of similar paleoenvironments in the southwestern USA. Along these regions, depauperated darwinulocopine faunas mostly had to overcome the effects of CAMP by deploying metabolic, but more probably reproductive (parthenogenesis), strategies that allowed them to occupy previously stressed environments after their recovery.


Conclusions


Four ostracode species of darwinulocopines were recovered from sediments of the Hettangian, Lower Jurassic, Whitmore Point Member of the Moenave Formation. These sediments were deposited in and along the margins of Lake Dixie and associated/interbedded mudflat environments in the counties of Washington and Mohave, Utah and Arizona, USA. The diver- sity and composition of the Whitmore Point Member ostracode fauna agree with previous interpretations of Lake Dixie as a


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