472


Journal of Paleontology 91(3):467–476


Figure 4. SEM images and energy dispersive x-ray spectral data for holotype counterpart (YPM 531187) and paratype counterpart (YPM 531188) of Estherites? jocelynae n. sp. (1) Image of the carapace margins of YPM 531188, left, and YPM 531187, right, as indicated by white box in Figure 2.2, scale bar = 500 μm; (2–4) EDAX map with same dimensions as Figure 4.1, scale bars = 500 μm; elemental maps of illustrate the relative concentrations of Ca, Si, and Al, respectively. Areas with preserved growth bands have high calcium content but lack silica and aluminum and vice versa; (5) view of distal growth bands of paratype YPM 531188 outlined in the upper left corner white box in Figure 4.1, radial structures indicated by arrow, scale bar = 200 μm; (6) close up of distal margin of a growth band of the holotype (YPM 531187) outlined by the lower right white box in Figure 4.1, scale bar = 25 μm, notice the thickened distal edge, which corresponds with the stronger depressions marking the distal edge of the growth bands in moldic specimens; (7) spectral data for the growth band illustrated in Figure 4.6, location of scan marked as + in 4.6, composition is dominantly calcium carbonate.


replaced rather than recrystallized carapace material. In regions where a detailed mold of the carapace exterior is preserved (e.g., Figs. 2.13, 4.4–4.6), silicon is the primary ion (Fig. 3.3). Similar high-fidelity preservation via silica molds has been documented in other spinicaudatans, such as Carapacestheria disgregaris from the Jurassic of Antarctica (Stigall et al., 2008). Compared to other fossil spinicaudatan carapaces, the


preservation of the Beaverhead Basin specimens is unusual. Notably, although hundreds of fossil spinicaudatan species have been described, the elemental composition of carapaces has been explicitly tested for fewer than ten species. Thus sets of the comparative data are extremely limited. Most fossil species for which carapace compositions have been analyzed viaEDAXare within the ‘Estheriteoidea-Eosestheriodidea’ group, and their carapaces have reported to be preserved primarily as recrys- tallized calcium phosphate or silica molds. This group includes Triassic specimens from Poland (Olempska, 2004), Jurassic specimens from Antarctica (Stigall et al., 2008), Namibia (Stigall et al., 2014), and China (Hethke et al., 2013), and


Cretaceous specimens of Madagascar (Stigall and Hartman, 2008). Another identified mode of spinicaudatan preservation, specifically related to exceptional preservation of soft tissues and appendages, is as organic-carbon residues associated with phyllosilicates (Orr and Briggs, 1999). Carbonate preservation of spinicaudatans has only previously been reported from the Jurassic specimens of western North America (Lucas and Milner, 2006). Notably, both carbonate and organic-carbon preservation result in lower resolution of carapace details than preservation as recrystallized calcium phosphate or silicic molds. Although studies of spinicaudatan carapace taphonomy are


limited, carbonate replacement or preservation has been studied within both arthropods and vertebrates preserved in freshwater environments. Calcium carbonate precursors have been observed to form from the breakdown of organic matter in aqueous solution (Berner, 1968) and have been implicated in the development of concretions as well as exoskeleton and soft- tissue replacement in insects and shrimp (McCobb et al., 1998;


Figure 5. Images of additional spinicaudatan and associated fossils. (1–7) Additional specimens of E.? jocelynae n. sp.: (1) complete right carapace valve (YPM 531191) and partial right? valve preserved with growth bands marked by calcium carbonate replacement (YPM 531192), scale bar = 1mm; (2) posterior portion of a left carapace valve (YPM 531198) underneath the distal margin of another valve (YPM 531199), scale bar = 1mm; (3) dense assemblage of iron- stained carapace molds (YPM 531205–531214) on the underside of slab containing the holotype specimen illustrated in Figure 2.1, scale bar = 5mm; (4) anterior portion of a carapace (YPM 531203), scale bar = 1mm; (5) two carapaces replaced with carbonate (YPM 531200, upper; 531201, lower), scale bar = 1mm, margins of these specimens appear warped, probably from desiccation before burial; (6) laterally compressed specimen (YPM 531202), scale bar = 1mm; (7) E.? jocelynae n. sp. specimen YPM 531218 intersected by an iron-rich Planolites burrow, scale bar = 5mm; (8) gastropod, YPM 531204, scale bar = 1mm; (9) bedding-plane assemblage of associated fauna including ostracodes and macerated insect exoskeleton (YPM 531216), scale bar = 1mm; (10) SEM image of iron-rich burrow, field of view indicated by white box in Figure 2.1, scale bar = 250mm; (11), assemblage of Planolites burrows (YPM 531215) on the holotype slab in Figure 2.1, scale bar = 5 μm.


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