Parallelisms represent a vital component of structural evolution. Ignoring this factor and its causal roots (each stem-clade is characterized by its own unique system of morphogenetic constraints) leads to an incomplete understanding of any clade’s morphogenetic history and strips the terms “macrotaxon” and “taxonomic rank” of their essential biological meaning. Mineralized tissues exemplify this, as constraints on their modification are rooted in the dual nature of biocrystalline materials (BCM). One such material, well-preserved in the fossil record, is sauropsid eggshell, whose structural diversity—as in other BCMs—is directly linked to the complex 3D interaction of organic matrix and mineral phase. The most complex structure (micro-ultrastructural BCM zonality) among sauropsid eggshell types occurs only in the Pennaraptora clade (birds and near-bird dinosaurs), where a key acquisition was the emergence of a distinct BCM zone with the squamatic ultrastructure (SqU)—a true composite material based on a complex 3D organic matrix network. Explicit novelties in standard BCM zonality of ornithoid-type eggshell formula: ML + SqZ or ML + SqZ + EZ did not appear in the history of Avialae/Aves clade frequently, and all clearly associated with early, basal divergence of orders at the Cretaceous–Paleogene boundary. Such novelties likely reflect threshold changes in basic physiological regulation (e.g., calcium metabolism), with zonal modifications as mere side effects (“noise”) of important homeostasis processes. This study provides the first comprehensive analysis of ornithoid-type eggshell structural diversity. A matrix of all micro-ultrastructural characters is compiled (Part 2) and illustrated on 17 plates with SEM images and several stylized drawings. All extant bird families and selected Cretaceous Pennaraptora groups are assigned “eggshell formulas” for BCM zonality profiles derived from the character matrix (Appendix Table 1, Figs. 10−11). The detailed matrix discussion is preceded by methodology for BCM interpretation (Part 1): vertical and horizontal aspects of BCM arrangement, BCM visualizations at various scales and under different observation methods (thin sections in ULM/PLM, fractures in SEM, EBSD). This justified character matrix serves as a basis to identify the frequency of structural parallelisms in Pennaraptora and Avialae clades (Part 3). The ideal situation for detecting parallelisms is an independent genealogical framework from reliable molecular data—now available for all extant bird orders and families over the past five years—as a template for taxonomic calibration of BCM zonality changes. Applications are discussed via vivid examples of zonality innovations with the main rule: do not assign families with distinct novelties to the same order.
К. Е. Михайлов (Mon,) studied this question.