Exceptional preservation of clam shrimp (Branchiopoda, Eucrustacea) eggs from the Early Cretaceous Jehol Biota and implications for paleoecology and taphonomy

Abstract

Fossil eggs of clam shrimps (Spinicaudata) are rare and little attention has been paid to the study of their shape and microstructures. Here, we report the discovery of exceptionally preserved three-dimensional eggs from numerous specimens of Eosestheria elliptica Chen, 1976 from the lacustrine Early Cretaceous Yixian Formation in western Liaoning, China. These three-dimensionally preserved fossil eggs display a spherical shape with smooth surface, part of the tertiary envelope, and possibly the first embryonic cuticle, which were previously unknown or ambiguous. The eggs are abundant and assumed to be attached to the exopod as in extant Spinicaudata. Moreover, the exceptional three-dimensional preservation and delicate preparation of slices of the eggs allowed us to document the microstructures and elemental composition of fossil eggs of clam shrimps from the Jehol Biota. Energy dispersive spectroscopy of the fossilized envelope revealed a calcium phosphate composition. However, the egg contents display two completely different elemental compositions. Some exhibit the same elemental composition as the envelope, whereas others has been replaced by alumino-silicate. The taphonomic process is also briefly discussed in this paper.

Confirmation of ovarian follicles in an enantiornithine (Aves) from the Jehol biota using soft tissue analyses

“…4). This same process has been reported in fossilized branchiopod (clam shrimp) eggs also from the Jehol Biota, where all the envelopes were made of calcium phosphate but some of the eggs had their internal contents replaced by alumino-silicates 41 . An explanation for the mechanism of alumino-silicate replacement was not provided for the Jehol clam shrimp eggs 41 , but we suggest that clay minerals from sediments were involved.…”

“…This same process has been reported in fossilized branchiopod (clam shrimp) eggs also from the Jehol Biota, where all the envelopes were made of calcium phosphate but some of the eggs had their internal contents replaced by alumino-silicates 41 . An explanation for the mechanism of alumino-silicate replacement was not provided for the Jehol clam shrimp eggs 41 , but we suggest that clay minerals from sediments were involved. Clay minerals have been determined to be important agents in the fossilization of soft-tissues in other settings (such as the Ordovician Soom Shale of South Africa and the Cambrian Burgess Shale of Canada) 42,43 .…”

“…However, our results demonstrate that these types of analyses can eliminate doubt and help to further understand preservation. Previous taphonomic studies conducted on Jehol invertebrates showed that alumino-silicification and pyritization help preserve soft-tissues in the Jehol 41 , 45 . In STM10–12, no pyritization was found, but the tissues apparently underwent alumino-silicification and a slight mineralization with iron, potentially iron oxides.…”

SEM Analyses of Fossilized Chondrocytes in the Extinct Birds Yanornis and Confuciusornis: Insights on Taphonomy and Modes of Preservation in the Jehol Biota

“…IVPP V13358 and IVPP V11521 represents additional examples of Jehol fossils where soft-tissues were preserved by alumino-silicification. Tissues forming the remnants of ovarian follicles preserved in the Jehol enantiornithine STM 10-12 (Bailleul et al, 2020b) and the tissues of some Jehol clam shrimp eggs were also alumino-silicified (Pan et al, 2015) showing that alumino-silicification is likely a very common mechanism of soft tissue preservation in the Jehol biota. Unfortunately, little is known about this process of aluminosilicification.…”

Late Permian conchostracans (Crustacea, Branchiopoda) from continental deposits in the Moscow Syneclise, Russia

“…Although conchostracans are currently treated as a paraphyletic group combining the monophyletic Spinicaudata, Cyclestherida, and Laevicaudata, for convenience, we use the traditional term ‘Conchostraca’ since it is in common use by other geoscientists (e.g., Gallego, 2005; Kozur and Weems, 2010; Ghosh, 2011; Weems and Lucas, 2015). However, it should be mentioned that there are also workers preferring to use the terms ‘clam shrimps’ (e.g., Li and Matsuoka, 2012; Pan et al, 2015), ‘Spinicaudata’ (e.g., Orr et al, 2008; Stigall et al, 2014), or a mixture of both ‘Spinicaudata’ and ‘Conchostraca’ (e.g., Astrop and Hegna, 2015; Morton et al, 2017).…”