Synchrotron imagery of phosphatized eggs in Waptia cf. W. fieldensis from the middle Cambrian (Miaolingian, Wuliuan) Spence Shale of Utah

Justin Moon,Jean-Bernard Caron,Robert R Gaines

doi:10.1017/jpa.2021.77

Abstract

AbstractExceptionally preserved fossil eggs and embryos provide critical information regarding paleoembryogenesis, reproductive strategies, and the early ontogeny of early arthropods, but the rarity of preservation of both eggs and egg-bearing organisms in situ limits their use in detailed evolutionary developmental (evo-devo) studies. Burgess Shale-type deposits preserve rare instances of egg-bearing arthropods as carbonaceous compressions; however, the eggs are usually poorly preserved with no compelling evidence of embryos. We describe the first record of a brooding specimen of Waptia cf. W. fieldensis from the Spence Shale, a Cambrian (Wuliuan Stage) Burgess Shale-type deposit in northeastern Utah and southeastern Idaho. This is the first record of an egg-bearing arthropod from the Spence Shale and it exhibits two distinct modes of preservation among eggs within the single clutch: carbonization and phosphatization. Unlike the egg-bearing Burgess Shale specimens, many eggs of this Utah specimen are also preserved three-dimensionally. In addition, synchrotron radiation X-ray tomographic microscopy reveals internal distributions of mineral phases, along with potential remnants of the egg membrane and attachment structures, but, as in the Burgess Shale, no explicit traces of developing embryos. The distinct modes of preservation highlight the existence of diagenetic microenvironments within some eggs, but not in others during fossilization.

Highlights

Figure 2. (1, 2) Waptia fieldensis (ROM 63354) from the Burgess Shale, middle Cambrian, Yoho National Park, British Columbia, Canada (Caron and Vannier, 2016, p. 71, fig. 2C); (1) lateral preservation showing egg cluster preserved underneath carapace under high-angle incidental lighting; (2) close up of egg cluster in (1) as a phase map showing differential preservation of the inner section and outer section of the eggs (Caron and Vannier, 2016, p. 71, fig. 2D)
W. fieldensis (KUMIP 314044) from the Langston Formation, Spence Shale Member, middle Cambrian, Cataract Canyon, Wellsville Mountains, Box Elder County, Utah, United States (e.g., Briggs et al, 2008, p. 251, fig. 12.3), right-lateral view showing potential egg cluster located on the anterior of the incomplete specimen; (4) counterpart of Waptia cf
W. fieldensis (KUMIP 314032) from the Langston Formation, Spence Shale Member, middle Cambrian, Miners Hollow, Wellsville Mountains, Utah, United States, lateral view of body fossil and egg cluster located anterior-dorsal to the body under high-angle incidental lighting; (5) same as (4), but taken underwater with high-angle cross-polarized lighting; (6) close-up of egg cluster from Figure 1.2, showing locations of three-dimensionally preserved eggs (1–11) and eggs preserved as carbonaceous compressions under low-angle incidental lighting

Summary

Introduction

Preserved eggs and embryos from the Cambrian period provide important biological information on the ontogeny, embryogenesis, and reproductive strategies of early arthropods (e.g., Zhang and Pratt, 1994; Duan et al, 2014; Caron and Vannier, 2016; Ou et al, 2020), scalidophorans (e.g., Donoghue et al, 2006a; Zhang et al, 2011), and cnidarians (e.g., Dong et al, 2013; Han et al, 2013); yet, it is only under exceptional conditions that fossilization of eggs and embryos can occur (Bengtson and Zhao, 1997; Donoghue et al, 2006b; Gostling et al, 2008; Briggs and McMahon, 2016).In Paleozoic deposits, early metazoan eggs are most commonly preserved via phosphatization, which is an important taphonomic pathway that may lead to exceptional threedimensional preservation of the ultrastructural details of labile, non-biomineralizing structures found in eggs and embryos (Allison, 1988a; Bengtson and Zhao, 1997; Donoghue et al, 2006b; Dornbos, 2011; Muscente et al, 2015). SRXTM, micro CT-scanning, and similar computed tomographic imaging have been favored over the last two decades for the study of fossil eggs and embryos in particular (e.g., Donoghue et al, 2006a; Cheng et al, 2011; Siveter et al, 2014; Ou et al, 2020) These methods provide a non-destructive alternative to imaging fossil specimens in a three-dimensional space, and have the potential to produce high resolution images of key ultrastructural details that normally cannot be detected using traditional microscopic techniques (Stampanoni et al, 2002; Donoghue et al, 2006a; Sutton, 2008; Cheng et al, 2011; Siveter et al, 2014; Eriksson et al, 2016; Yin et al, 2016)

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Discussion

Conclusion