Fossil turtle eggs from the Upper Cretaceous Gaogou Formation, Xiaguan-Gaoqiu Basin, Neixiang County, Henan Province, China: Interpretation of the transformation from aragonite to calcite in fossil turtle eggshell

Turtle eggs from the Lower Cretaceous Hasandong Formation (South Korea) with relict aragonite under significant thermal maturity

“…Because the aim of the EBSD analysis was to detect the preservation of relict aragonite in the eggshells (e.g., S. Choi, Kim et al, 2022; Moreno-Azanza et al, 2021; Xu et al, 2022), we included aragonite and calcite in the phase candidates. In addition, quartz, plagioclase, and chlorite were included because they were detected in a preliminary analysis of specimens.…”

“…The shell units have height-to-width ratio of 1.7–2.7:1 with an average of 2.2:1. From the inner surface to the middle of the eggshell, concave-down parallel and prominent growth lines are observed, which is another character of turtle eggshells (Hirsch, 1983; Hirsch & Lopez-Jurado, 1987; Kohring, 1990; Lawver & Jackson, 2016, 2017; Poust, 2017; Xu et al, 2022). The outer surface of the eggshell is darker than the main eggshell of which the position is reminiscent of the cuticle (Fig.…”

“…However, fossil eggs from clades other than nonavialan dinosaurs are rare. Turtle eggs are no exception and there are only a few reliable reports from East Asia, including Cretaceous eggs from Mongolia (Mikhailov et al, 1994), China (Jackson et al, 2008(Jackson et al, , 2018Ke et al, 2021;Xu et al, 2022), and Japan (Fukuda & Obata, 1991;Isaji et al, 2006;Uematsu et al, 2022). There is still a large knowledge gap in the fossil record of East Asian turtle eggs and new findings could shed light on the reproductive history of turtles.…”

“…EBSD has therefore been adopted by invertebrate paleontologists to test the preservation of relict aragonite (e.g., Balthasar et al, 2011). EBSD has been subsequently adopted by vertebrate paleontologists to test the preservation of aragonite in turtle fossil eggshells (Moreno-Azanza et al, 2021), and in situ mappings of relict aragonite have been acquired from Late Cretaceous turtle eggshells from Montana (U.S.A.) and Henan (China) (S. Choi, Kim et al, 2022; Xu et al, 2022).…”

The diverse terminology of reptile eggshell microstructure and its effect on phylogenetic comparative analyses

“…The CL consists in “fan‐shaped” shell units with a spherulitic arrangement of acicular aragonite crystals (Figure 1; Hirsch, 1983; Lawver & Jackson, 2016; Silyn‐Roberts & Sharp, 1985), resulting in large spaces between their bases, distinct from those found in archosaurs (Schleich & Kästle, 1988). This shell unit morphology, unique to turtle eggs, has allowed paleontologists to assign many fossil eggshells to Testudines (Hirsch, 1983; Lawver & Jackson, 2014; Mikhailov, 1997b; Moreno‐Azanza et al, 2021; Schleich & Kästle, 1988; but see Ke et al, 2021; Xu et al, 2022, for fossil turtle eggs with an unusual shell microstructure), with an earliest record in the Late Jurassic (Kohring, 1990) and empirical evidence of aragonitic shells in the Late Cretaceous (Choi, Kim, et al, 2022; Ferguson & Tapanila, 2022; Xu et al, 2022). The relative shape and size of turtle egg shell units compared with the thickness of the underlying membrana testacea varies significantly (Cadena et al, 2019; Lawver & Jackson, 2014), from thick and columnar in most terrestrial turtles to reduced and as thin as the membrane in some freshwater turtles (Hirsch, 1983; Packard et al, 1979; Packard & Packard, 1980, 1988; Schleich & Kästle, 1988).…”