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

Abstract

Reptile eggshell ensures water and gas exchange during incubation and plays a key role in reproductive success. The diversity of reptilian incubation and life history strategies has led to many clade-specific structural adaptations of their eggshell, which have been studied in extant taxa (i.e. birds, crocodilians, turtles, and lepidosaurs). Most studies on non-avian eggshells were performed over 30 years ago and categorized reptile eggshells into two main types: "hard" and "soft" -sometimes with a third intermediate category, "semi-rigid." In recent years, however, debate over the evolution of eggshell structure of major reptile clades has revealed how definitions of hard and soft eggshells influence inferred deep-time evolutionary patterns. Here, we

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

“…This Upper Cretaceous egg was found in a marine deposit and was associated with an ammonite ( Baculites ) fossil (Fukuda & Obata, 1991). Because all known sea turtles lay ‘soft’ eggs (but see Legendre et al, 2022 for a critical appraisal of the ‘soft’ and ‘hard’ dichotomy), which are rarely fossilized (Hirsch, 1996; Legendre et al, 2020), Fukuda and Obata (1991) inferred that the ‘hard’ egg does not belong to sea turtles and was allochthonously transported to the marine deposit. However, Cadena et al (2019) confirmed that protostegid marine turtles laid ‘hard’ eggs unlike ‘soft’ eggs of modern sea turtles.…”

A Review: Comparative Genomics and Physiology of Parity Mode Evolution in Amniotes

“…Other than two lineages of geckos with hard shells, oviparous squamates have parchment-shelled eggs with a thin layer of calcium deposits on the outer surface of the shell membrane (Blackburn & Stewart, 2021;Choi et al, 2018). Monotremes have an eggshell but far less has been documented about its structure compared to other amniotes (Legendre et al, 2022). The structure and physiological mechanisms involved with eggshell calcification are most well resolved in birds (Choi et al, 2018;Francesch et al, 1997;Jonchere et al, 2010Jonchere et al, , 2012Mikšík et al, 2010;Rose-Martel, Du, & Hincke, 2012).…”

“…In birds, the organic components of uterine fluid promote the formation of calcite (Hernández-Hernández, Gomez-Morales et al, 2008;Hernández-Hernández, Rodriguez, et al, 2008;. Most amniotes use this polymorph (Hernández-Hernández, Gomez- Hernández-Hernández, Rodriguez, et al, 2008;Legendre et al, 2022;.…”

“…There are differing accounts on the microstructure of monotreme eggshells and further studies are needed to determine secondary homology (Legendre et al, 2022). Nonetheless, they are described as proteinaceous, permeable, and flexible (Hughes, 1984).…”

“…Mammillary knobs are a distinct layer between the outer eggshell membrane and the calcified shell matrix layer (Hamilton, 1986). These are characteristic of Archelosaur eggshells (Legendre et al, 2022;Zelenitsky et al, 2002;Zelenitsky & Modesto, 2003). Part of the mammillary knobs, called basal caps, are embedded into the outer eggshell membrane fibers (Tyler, 1965).…”