Stigall et al.—A new Oligocene spinicaudatan species from Montana
The specimens described herein meet each of these criteria for inclusion within the “Conchostraca.”
Suborder Spinicaudata Linder, 1945
Superfamily Estheriteoidea Zhang and Chen in Zhang et al., 1976
Family Estheriteidae Zhang and Chen in Zhang et al.,1976
Genus Estherites? Kobayashi and Huzita, 1943
Type species.—Estheria mitsuishii (Kobayashi andHuzita, 1942) Kobayashi and Huzita, 1943 from the Upper Cretaceous (Conianian),Nenjiang Formation, Jilin Province, northeastChina.
Diagnosis.—From Li and Batten (2005). Carapace of moderate size, gently convex, elliptical or elongate-subquadrate in out- line. Umbo projecting somewhat beyond dorsal margin; growth bands ornamented by fine lirae that are connected by bars in various directions on ventral part of carapace; those on dorsal side sculptured only with fine radial lirae.
Occurrence.—Triassic? or Cretaceous through Oligocene of China, Japan, South Korea and western North America.
Remarks.—Estherites is a genus of ~15 species of relatively small (typically 4–10mmin length) spinicaudatans characterized by an oval to elliptical carapace with a subcentally located umbo (Zhang et al., 1976). Growth bands are typicallywide throughout much of the carapace and may narrow in the distal third. The umbonal region is ornamented with punctae, which align into weak radial lirae near the dorsal margin. In some species, such as E. corrugatus, ornamentation isweakly impressed throughout the carapace. Growth band count varies from relatively few (~12) to many (40+). A comprehensive species list with associated refer- ences is provided in Li and Batten (2005). The Medicine Lodge spinicaudatans exhibit most of the
key diagnostic features of Estherites including size, elliptical shape, carapace ratios, number and width of growth bands, and weakly impressed ornamentation in the umbonal region with lirae expressed at the distal margin. The lirae and radial ornamentation are very weakly developed in this new species, which may be a taphonomic artifact or may indicate that the Medicine Lodge specimens belong to a new, yet undescribed genus. At this point, we prefer to assign the new species to Estherites while noting this uncertainty because the carapace microstructure of specimens is not preserved well enough to definitively either include or exclude the new species from Estherites. In either case, theMedicine Lodge spinicaudatans are clearly referable to the family Estheriteidae. The documentation of this newspecies extends the geologic range of the Estheriteidae and places it among the very few clamp shrimp taxa known from Cenozoic strata. Among “conchostracans,” only members of the spinicaudatan families Fushunograptidae, Loxomegaglyptidae, and Perilimnadiidae, the laevicaudatan family Lynceidae, and the cyclestherid family Cylclestheriidae have previously been documented from Cenozoic deposits (Shen et al., 2006).
Estherites? jocelynae Stigall new species Figures 2.1–2.14, 3.1–3.3, 4.1–4.7, 5.1–5.8
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Holotype.—YPM 531187, complete right carapace valve (holotype); YPM 531188, left valve; 531189, left valve, 531190, complete left valve (paratypes). All type material was collected from the Anton Creek Locality of the early Oligocene (Rupelian Stage, Orellan NALMA), Medicine Lodge Forma- tion, Beaverhead Basin, MT.
Diagnosis.—Thin carapace valves of medium size, elliptical to ovate in outline, umbo centrally located, extending slightly above hinge line; height to length ratio from 0.7 to 0.9, valve convexity moderate; growth bands of two orders: flat growth bands ~0.4mm wide in umbonal and central carapace, and rounded growth bands ~0.04mm (40 μm) wide at distal margin. Weak radial lirae and wrinkles present on dorsal growth bands.
Occurrence.—Early Oligocene (Rupelian Stage, Orellan NALMA), Medicine Lodge Formation, Beaverhead Basin, MT. Orr’s Anton Creek Locality, Sec. 27, 28, T11S, R12; 44.8527 N latitude, -113.0239Wlongitude, uncertainty 210 m.
Description.—Carapace valves of moderate size and elliptical to oval in outline. Mean carapace length and height are 3.89mm (range: 1.94–5.87mm) and 3.13mm (range: 1.53–5.06mm), respectively (Table 1). The L/H ratio averages 0.81 (range: 0.70–0.90). Carapace growth is isometric (Fig. 6). Dorsal mar- gin is weakly curved and relatively short (typically less than half of the valve length) with umbo centrally located. Anterior margin shorter and more tightly rounded than posterior. Maxi- mum anterior bulge is approximately 0.38x distance from hinge line to ventral margin, and maximum posterior bulge located approximately 0.43x distance from hinge line to ventral margin. Maximum ventral bulge is located slightly anterior of center, approximately 0.38x distance from anterior to posterior of carapace. Moldic specimens and those with preserved carapace
valves exteriors record an average of eight (range: 7–14) flat growth bands ~0.4mm wide (e.g., Figs. 2.3, 3.1, 5.1, 5.5). The distal margins of these growth bands are demarcated by a topographic impression within the mold and/or a thickened region of preserved carapace. Many specimens preserve an internal mold of the carapace valve (e.g., Figs. 2.4, 2.7, 5.2, 5.3, 5.4, 5.6) and, consequently, do not preserve any external ornament. One specimen (YPM 531189, Fig. 2.5, 2.8, 2.14) represents a carapace exterior and preserves a series of narrow concentric bands throughout the carapace, which suggests development of narrower growth bands mediodistally. Limited regions around the extreme distal margins of some specimens, notably specimens of the type series, preserve sets of narrow growth bands, ~ 40 μm wide, as siliceous molds (Figs. 2.13, 2.14, 3.5). Weakly preserved ornamentation patterns of radial lirae and concentric wrinkles are present on some of these narrow, extreme-distal growth bands (Fig. 4.5). There is no evidence for radial ornamentation in the umbo or central region of specimens where that region has been preserved as a siliceous mold. The preservation of a consistent number of wide growth bands among internal molds coupled with the occasionally preserved narrow bands suggests that two stages of concentric band development occurred during ontogeny.
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