Journal of Paleontology, 91(3), 2017, p. 467–476 Copyright © 2017, The Paleontological Society 0022-3360/15/0088-0906 doi: 10.1017/jpa.2017.15
The first Cenozoic spinicaudatans from North America
Alycia L. Stigall,1 Roy E. Plotnick,2 and Lisa E. Park Boush3 1Department of Geological Sciences and OHIO Center for Ecology and Evolutionary Studies, Ohio University, Athens, Ohio 45701, USA
⟨
stigall@ohio.edu⟩ 2Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, USA ⟨
plotnick@uic.edu⟩ 3Center for Integrative Geosciences, University of Connecticut, Storrs, Connecticut 06269, USA ⟨
lisa.park_boush@
uconn.edu⟩
Abstract.—A new spinicaudatan species, Estherites? jocelynae new species, is described from more than fifty specimens collected from the Medicine Lodge Formation (early Oligocene) of the Beaverhead Basin in southwestern Montana, USA. This is the first spinicaudatan species reported from Cenozoic strata of North America and is the second-youngest fossil clam shrimp described globally. The new species extends the range of the superfamily Estheriteoidea into the Paleogene. Carapaces of E.? jocelynae n. sp. are preserved as a calcium carbonate replacement of the original chitin-calcium-phosphate structure, which is an uncommon style of preservation for spinicaudatans. The unique preservation coupled with the range extension suggests that the sparse Cenozoic fossil record of spinicau- datans may be partly attributable to preservation bias related to geochemical conditions rather than exclusively to diversity decline following the end-Cretaceous mass extinction. The presence of E.? jocelynae n. sp. in the Medicine Lodge Formation indicates that lakes in the Beaverhead Basin experienced seasonality and fluctuating lake levels with at least some drying at the lake margins. The ecological inferences support previous paleoenvironmental inter- pretations based on paleobotanical and other faunal evidence.
Introduction
Spinicaudatans have been important components of lacustrine ecosystems since the late Paleozoic (Park and Gierlowski- Kordesch, 2007). Spinicaudatan fossils are significant paleo- environmental indicators; because most species only occupy ephemeral pools of freshwater, their presence indicates a seasonal climate (Vannier et al., 2003). In addition, this clade is widely used for continental biostratigraphy in Mesozoic strata (Shen et al., 2004; Kozur and Weems, 2010). Paradoxically, even though the number of preserved lake deposits increases during the Cenozoic relative to the Mesozoic (Gierlowski- Kordesch and Kelts, 2000), the Cenozoic record of spinicau- datans is extremely sparse. To date, no spinicaudatan species have been formally described from Neogene strata and fewer than ten spinicaudatan occurrences have been described globally from Paleogene strata (listed in Shen et al., 2006). The substantial gap in the fossil record of spinicaudatans limits the ability to make inferences about the evolutionary history of the clade. Mesozoic faunas included many spinicaudatan families reflecting greater diversity and disparity than repre- sented among the three modern families. It is currently unclear whether spinicaudatan diversity declined due to the Cretaceous- Paleogene mass extinction, climatic or tectonic changes in the early Cenozoic, and/or preservational bias. The new species described herein is the first record of a fossil spinicaudatan species from the Cenozoic of North America. We discuss the preservation, evolutionary implications, and ecological inter- pretations of these new specimens.
Geologic setting
The fossil spinicaudatans were collected from the early Oligocene Medicine Lodge Formation in Beaverhead County, Montana (Fig. 1) by J.B. Orr during the 1960s. The specimens described herein were collected from a single locality along a small bluff on Anton Creek (Sec. 27, 28, T11S, R12), which the authors georeferenced to 44.8527 latitude, -113.0239 longitude, uncertainly 210 m. The Medicine Lodge Formation and associated sedimen-
tary units within the Beaverhead Basin include interbedded thin- to-thick shales and sandstones, including a basal conglomerate and a localized coal layer near the top, with noticeable vertical and lateral variations (Scholten et al., 1955; Becker, 1969; DeVore and Pigg, 2010; Lielke et al., 2012). These facies represent fluvial, lacustrine, fan-delta, and paludal facies (M’Gonigle and Darymple, 1996) that were deposited within a structural basin bounded by the hanging wall of a Paleogene low-angle fault on the west and the low-angle normal fault to the east (Scholten et al., 1955; Becker, 1969; DeVore and Pigg, 2010; Lielke et al., 2012). Based on paleobotanical data (e.g., Scholten et al., 1955; Becker, 1969; DeVore and Pigg, 2010; Lielke et al., 2012), the basin was estimated to have an elevation of ~1600m above sea level. Becker (1969) described three florules fromPaleogene basin-
lake deposits in Beaverhead County, Montana: the Christensen, Horse Prairie, and Medicine Lodge florules. The Medicine Lodge Florule is the youngest of these and crops out along several adjacent tributaries of Medicine Lodge Creek. The age of the
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