Recognizing a lost nesting ground: First unambiguous Testudines eggshells from the Eocene, associated with the pleurodiran Eocenochelus (Huesca, Northern Spain)

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

“…Many of such traits would likely still be defined as discrete categories associated with a particular reptile clade – e.g. shell units with acicular aragonite crystals, long known to be a defining feature of turtle eggshells, identifiable even in diagenetic fossil specimens in which aragonite is replaced with calcite (Hirsch, 1983, 1996; Mikhailov, 1997b; Moreno‐Azanza et al, 2021; Schleich & Kästle, 1988), were recently proposed to be a synapomorphy of Testudines from direct paleontological evidence (Choi, Kim, et al, 2022). To avoid potentially confusing categories based on broad functional characteristics such as traditional eggshell types, the definition of such discrete traits should be based on explicit structural similarities, which should be argued by authors of future studies as hypotheses of primary homology to allow their subsequent testing in a phylogenetic framework.…”

“…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).…”

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.…”

“…Considering that the original diagnosis of Testudoolithus proposed by Hirsch (1996) already proposed 0.2–0.8 mm as an eggshell thickness criterion for Testudoolithus , the authority of the oospecies T. hirschi (Kohring, 1999) neglected this criterion. Parataxonomic revision is beyond the scope of the current study, but a future study may revise the concept of Testudoolithus to be more inclusive for the oofamily Testudoolithidae because current diagnoses for the oogenera of Testudoolithidae often overlap (Moreno-Azanza et al, 2021).…”

“…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).…”