656
Journal of Paleontology 92(4):648–660
Material.—UMNH.IP 5294, valve (0.79mm L, 0.42mm H, 0.18mm W).
Remarks.—The generic diagnosis follows Holland (1934) and Kukhtinov (2004). The single specimen of Whipplella? sp. 1 is badly preserved, which allows only a tentative placement in the genus Whipplella, based on the general shape of the valves in lateral and dorsal view. Whipplella? sp. 1 and Cypridoidea indet. by Schudack (2006) could be the same species, but the specimens of the latter are badly preserved, and the attribution is at best doubtful.
Whipplella? sp. 2 Figure 5.23–5.25
Occurrence.—Hettangian, Lower Jurassic, Whitmore Point Member, Moenave Formation, Arizona and Utah, USA.
Material.—UMNH.IP 5295, valve (0.90mm L, 0.49mm H, 0.16mm W).
Remarks.—The generic diagnosis follows Holland (1934) and Kukhtinov (2004). The single specimen of Whipplella? sp. 2 is badly preserved, which allows only a tentative placement in the genus Whipplella, based on the general shape of the valves in lateral and dorsal view.
Results and discussion
In the present taxonomic review, four ostracode species were identified from sediments of the Whitmore Point Member, deposited in and along the margins of a lacustrine environment, referred throughout the region as Lake Dixie, and associated/ interbedded mudflat environments during the earliest Jurassic. The composition and stratigraphic position of this ostracode fauna makes it an interesting case to understand the demise of darwinulocopines as the main nonmarine ostracodes, both in terms of abundance and diversity, during early to middle Mesozoic times.
Evolutionary history of late Paleozoic–early Mesozoic darwi- nulocopines.—Darwinulocopines are known in the fossil record
since the late Paleozoic (Carbonel et al., 1988). The group flourished during the Carboniferous–Triassic, an interval from which the majority of darwinulocopine families, genera, and species has been so far described. During that time, the group presented greatermorphologic variety (Wang, 1980;Kashevarova and Neustrueva, 1982; Sohn, 1987; Molostovskaya, 1990) than shown by darwinulocopine flocks in earlier strata. Nonmarine faunas of the Carboniferous–Triassic also contained forms likely more closely related to the Carbonitidae Sohn, 1985 and Geisinidae Sohn in Benson et al., 1961 of the suborder Metaco- pina Sylvester-Bradley in Benson et al., 1961 (Martens et al., 1998; Tibert et al., 2013). Cytherocopines were also present in these environments, especially during the Permian, represented by orders such as the Permianoidea Sharapova in Schneider, 1948 and the Cytheroidea Baird, 1850 (Kashevarova, 1990). Late Permian darwinulocopines were highly diverse in terms of morphology and taxa, and three superfamilies are
present in the fossil record (Darwinuloidea, Suchonelloidea Mishina, 1972, and Darwinuloidoidea), each with its distinctive evolutionary history. They inhabited large, shallow lakes in which the variety of habitats was favorable to high diversity (Neustrueva, 1990). This apparently coincided with the presence of sexuality in at least some groups in this lineage, and Permian darwinulocopines commonly exhibited sexual dimorphism (Sohn, 1988; Molostovskaya, 2000). On the other hand, all modern darwinulocopines, except possibly one species (Smith et al., 2006), are exclusively parthenogenetic, and have apparently been so since late in the Mesozoic (Schön et al., 2009). The Permian-Triassic (P-Tr) extinction event greatly
reduced darwinulocopine diversity in nonmarine environments in ways similar to those observed in marine ones (Jin et al., 2000; Liu et al., 2010; Forel, 2013). It is still poorly known how these nonmarine faunas rebounded from such event during the Triassic, as they would soon be hit by another major extinction event, the ETE, which was probably caused by a disruption in the global carbon cycle due to emplacement of the Central Atlantic Magmatic Province (CAMP) (Beerling and Berner, 2002). The Upper Triassic also possibly witnessed the origin of the cytherocopine family Limnocytheridae Klie, 1938, which would become widespread in the Upper Jurassic −lower Cretaceous (Zhong, 1964; Zheng, 1976; Fang and Xu, 1978). The diversity of these faunas was maintained during the Lower Jurassic (Wakefield, 1994; Ballent and Díaz, 2012), only to be halved by the remainder of the period, before the explosive radiation of nonmarine cypridocopines, notably of the family Cyprideidae Bosquet, 1852, and the limnocytherids in the Middle–Late Jurassic (Whatley, 1988; Sames and Horne, 2012). Proposed scenarios for the widespread dispersal of non-
marine darwinulocopines in the late Paleozoic range from transportation by early tetrapods, either in wet mud adhering to their feet or in the intestines of aquatic plant-feeding animals, to humid winds in warm, subtropical environments, considering that monsoons and hurricanes can carry wet particles for long distances (Lethiers and Damotte, 1993; Horne, 2003). Evidence suggests that darwinulocopines were also tolerant of stressed environments such as shallow, warm, hypersaline lagoons and saline lakes (Gramann, 1971; McKenzie, 1981; Molostovskaya, 2000). Certainmorphophysiological traits that enabled their rapid spread through these and other habitats were present in darwinulocopine nonmarine faunas since at least the Permian (retaining eggs and juveniles in brood pouches) (Molostovskaya, 1990; Neustrueva, 1990) or the Early Jurassic (asexual reproduction by parthenogenesis). The record of parthenogenesis in darwinulocopines is a particularly remarkable one; despite being common in nonmarine ostracodes from several ages, it has not been the sole mechanism of reproduction in any other group for so long in the fossil record (Butlin et al., 1998; Martens et al., 1998; Schön et al., 2009). Exactly how long, however, is still a topic of discussion, although it is currently estimated to have persisted since 208Ma ago (Martens et al., 2003).
Lake Dixie as the ‘Last Hurrah of the Reigning Darwinuloco- pines’.—The strata of the Whitmore Point Member were deposited in Lake Dixie, soon after the ETE and the establish- ment of CAMP. The ostracode fossil record of this unit is marked by several mass mortality events, as evidenced by strata
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