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Journal of Paleontology 91(3):548–553
been generally observed that larger fish tend to live in deeper water systems (Werner et al., 1977; Schlosser, 1988; Harvey and Stewart, 1991). The presence of moderately sized fish in the Navajo deposits supports previous suggestions of relatively deep and possibly long-lived ponds in the Navajo dune system. Loope and Rowe (2003) conservatively estimated the wet, plu- vial episodes within the Navajo Sandstone may have lasted between 4,000 and 5,000 years, during which time yearly monsoons were capable of depositing up to 170mm of rain per storm (Loope et al., 2001). These authors note that an analogous environment existed in the mid-Holocene Selima Oasis of Sudan, where wet periods reached a precipitation rate of up to 200mm/yr causing high stands of pluvial Lake Selima to reach depths up to 17m (Haynes, 1987; Haynes et al., 1989; Loope and Rowe, 2003). Although it is conceivable such water depths in the Navajo could have occurred, the fish material presented here can only in the broadest sense tell us that water depth was
deep enough to support a small population of moderate-sized fishes for more than a single year. Some interdune deposits in the Navajo have been interpreted as fluvial in origin (Loope and Rowe, 2003). Because these fish must have dispersed to this particular interdune lake from some larger body of water, further work on Navajo interdune deposits may reveal more about how these localized habitats were connected during wetter periods in the Early Jurassic erg.
Implications for the Early Jurassic fossil fish record.—Unlike the relatively well-documented fish assemblages of the eastern coast, Early Jurassic fishes of the western USA are incompletely known (Milner et al., 2006, and references therein). Currently, only two formations from the Jurassic west have yielded recognizable fossil fish remains. The earliest is the Hettanginian “Lake Dixie fauna” from the Whitmore Point Member of the Moenave Formation of southwestern Utah and northwestern Arizona (Kirkland et al., 2014). This fauna has yielded a sur- prising diversity of material, including a hybodont shark, a palaeonisciform, a possible perleidiform, several semionoti- forms, and at least two sarcopterygians (Milner and Kirkland, 2006; Milner et al., 2006). Semionotiforms are the numerically dominant group, with at least three known species historically recognized from the fauna (Eastman, 1917; Hesse, 1935; Schaeffer and Dunkle, 1950; see Milner and Kirkland, 2006 and Milner et al., 2006 for discussion of the validity of these taxa). The next oldest ichthyofauna is from the middle Sinemurian to early Pliensbachian Kayenta Formation, which contains two sharks and two dipnoans, as well as an undescribed coelacanth, semionotid, and palaeoniscoid (Curtis and Padian, 1999; Milner et al., 2006, 2012; Frederickson and Cifelli, 2017; personal communication, A.R.C. Milner, 2017). After these faunas, a gap in the Western Interior fossil fish record exists until the rela- tively diverse Middle Jurassic Sundance and Wanakah faunas (Schaeffer and Patterson, 1984; Wilson and Bruner, 2004). Schaeffer and Patterson (1984) described, but did not name, a species of Lepidotes among six other fish genera from the marine Sundance Formation. The Sundance Lepidotes is only known from a few fragmentary specimens, and is much rarer than the well known, but phylogenetically obscure, Hulletia and the basal teleosts Occithrissops and Todiltia. The fish occur- rence described here from the Navajo Sandstone would fall into
this gap, slightly after the semionotid-dominated Early Jurassic ichthyofaunas and well before the more diverse assemblages of Middle Jurassic formations.
Acknowledgments
We would like to thank A. Titus (BLM: Grand Staircase- Escalante National Monument) for his invaluable assistance with field reconnaissance. An anonymous reviewer and A. Milner (St. George Dinosaur Discovery Site, UT) improved the quality of the manuscript. Partial support for this project was provided by a National Geographic Society Waitt Grant to BD (#W266-13).
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