Reptilian Eggshell Anatomy

Traditionally, reptilian anatomy has been classified based on differentiating between “hard” and “soft” eggshells. These definitions of hard and soft influence inferred deep-time evolutionary patterns. Unfortunately, there is no consensus among even experts. There are studies placing eggs in both categories. Researchers at The University of Texas at Austin recently published a paper in Journal of Anatomy proposing abandoning these guidelines in favor of focusing on functionally relevant characteristics. Where were the eggs laid? Was the nest buried with mud or sand, up in a tree? How much moisture could pass through the eggshell? While eggshell formation in birds has been well studied, that’s not the case with reptiles.
Looking at the inner structures of shells, the membranes is tricky at best and often relies on visual estimations, which can be subjective. It also omits to study fossils, making it difficult to place in a phylogenetic species context. That’s another way of saying you’re not using fossils to help figure out where species belong in the tree of life. Measuring other traits such as thickness, porosity, and calcium content through such tools as spectrometry or spectroscopy to enable chemical analysis would greatly enhance the precise comparison of samples.
Dr. Lucas J. Legendre and colleagues propose guidelines covering a range of topics. The intent is not meant to be exhaustive, but to help researchers clarify, essentially to improve placement of branches on Darwin’s “Tree of Life.” Topics include things like details on sampling (from different layers, parts of the eggshell), combining methods (visual, chemical), sample sources (fossils, egg shells), different types of species, body sizes, and defining descriptive terms like “soft” and “hard [1].


[1] Legendre, Lucas J., et al. “The Diverse Terminology of Reptile Eggshell Microstructure and Its Effect on Phylogenetic Comparative Analyses.” Journal of Anatomy, vol. 241, no. 3, June 2022, pp. 641–66,
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