474
Journal of Paleontology 91(3):467–476
Table 1. Summary of measured data for Estherites? jocelynae n. sp. specimens. Length=maximum carapace length; Height=maximumdistance fromthe dorsal margin to the hinge line; Hinge = length of the hinge line; A= distance from the maximum anterior bulge to the dorsal margin; B = distance from the maximum posterior bulge to the dorsal margin; C = distance from the max- imum ventral bulge to the most anterior part of the valve; D = distance from center of the umbo to the anterior margin; units = mm; N = number of speci- mens measured.
Character Length
Height A B C D
Height/Length Hinge/Length A/Height B/Height C/Length D/Length
Hinge
mean 3.88
3.13 1.23 1.34 1.49 1.07 1.67 0.81 0.45 0.38 0.43 0.38 0.26
median 3.94
3.25 1.22 1.40 1.51 0.98 1.75 0.80 0.47 0.38 0.42 0.37 0.27
range
1.94–5.87 1.53–5.06 0.58–1.82 0.66–1.82 0.95–2.26 0.52–1.93 0.87–2.20 0.70–0.90 0.31–0.54 0.25–0.46 0.36–0.52 0.30–0.45 0.19–0.36
N
21 7 9 8 8 8
Sageman et al., 1999; Park and Downing, 2001; Martínez- Delclòs et al., 2004). The degree that phosphate is replaced by carbonate within vertebrate bioapatite has been shown to vary among depositional environments due to local geochemical conditions (Keenan et al., 2015), variations in pH (Sagemann et al., 1999), or as a byproduct of bioerosion by aquatic microorganisms (Fernández-Jalvo et al., 2016). In some instances (e.g., Fernández-Jalvo et al., 2016), bones have been replaced with a chalky version of calcium cabonate that closely resembles the preservation style of E.? jocelynae n. sp. Because spinicaudatan carapaces are primarily chitin interlayered with limited bioapatite (~5% phosphate compared to ~40% in vertebrates), it is likely that low pH lake waters or early diage- netic processes, including bioerosion, could have removed the phosphate and facilitated later carbonate precipitation within voids held open by the chitin remnants. The atypical preserva- tion exhibited by E.? jocelynae n. sp., therefore, may have been facilitated by specific geochemical or microbial conditions within the depositional environment. Additional analyses on preservation of the spinicaudatan and associated fauna of the Beaverhead Basin are in progress to investigate the relationship between preservation and geochemistry in more detail.
Evolutionary and environmental significance
Clam shrimp are common in lacustrine deposits of late Paleozoic and Mesozoic age, but rarely have been reported from Cenozoic strata (see Zhang et al., 1976; Chen and Shen, 1981; Shen et al., 2006; Gallego and Mesquita, 2010). The only pre- viously described Cenozoic clam shrimp from North American are two species from the Eocene Green River Formation (Shen et al., 2006). These species, Cyclesteriodes wyomingensis and Prolynceus laneynsis, are members of the orders Cyclestheriida and Laevicaudata, respectively. Thus, Estherites? jocelynae n. sp. is the first spinicaudatan “conchostracan” described from the Cenozoic of North America. Estherites? jocelynae n. sp. is also the second youngest clam shrimp to have been described globally—only Paraleptestheria menglaensis (Gallego and Mesquita, 2010) from the late Oligocene of Brazil is younger.
10 10 7
22 23 9 8
Figure 6. Growth plot of E.? jocelynae n. sp. specimens showing isometric growth; Height and Length in mm.
All other species of Estherites are known from older
deposits of northeast Asia, thus the presence of E.? jocelynae n. sp. in the Oligocene deposits of the Beaverhead basin suggests that the ancestors of this species dispersed to North America from Asia. Lielke et al. (2012) documented an increase in the number of plant species of East Asian affinity within the Medicine Lodge Formation, and the spinicaudatans may have utilized the same dispersal pathway. Estherites? jocelynae n. sp. is also the youngest species of this genus and demonstrates the persistence of this clade beyond the Cretaceous-Paleogene extinction. Spinicaudatans are explicit environmental indicators
because they are restricted to freshwater habitats that exhibit seasonal fluctuations in water availability (Thiéry, 1996; Vannier et al., 2003). As part of their reproductive pathway, many spinicaudatans species produce cysts that must undergo drying before hatching can occur (Thiéry, 1996; Vannier et al., 2003). Thus the basic life-history attributes of spinicaudatans match the highly seasonal, summer dry climate with a mean summer temperate of 30.5°C reconstructed by Lielke et al. (2012) for the Beaverhead Basin quite well. Additional fauna from the spinicaudatan-bearing locality
provides further insight to the depositional environment. The presence of fish remains, ostracodes (Fig. 5.5, 5.9), insects— including caddisfly cases (Fig. 5.9), bivalves, and freshwater gastropods (Fig. 5.8) indicates that portions of the lake retained water year-round, although water levels likely fluctuated, allowing marginal regions to dry enough for spinicaudatan cyst development. The presence of a limited number of Planolites burrows (Fig. 5.7, 5.11) indicates that deposit feeders were present, but uncommon on the lake bottom. A fairly limited infaunal population would have facilitated the preservation of the relatively delicate spinicaudatan carapaces.
Conclusions
With the exception of Paraleptestheria menglaensis (Gallego and Mesquita, 2010) from the late Oligocene of Brazil, Estherites? jocelynae is the youngest clam shrimp described globally. This report is also the first occurrence of a spinicau- datan from the Cenozoic of North America. The description of E.? jocelynae n. sp. extends the range of the superfamily
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