Morphological research on amniote eggs and embryos: An introduction and historical retrospective

Abstract

Evolution of the terrestrial egg of amniotes (reptiles, birds, and mammals) is often considered to be one of the most significant events in vertebrate history. Presence of an eggshell, fetal membranes, and a sizeable yolk allowed this egg to develop on land and hatch out well-developed, terrestrial offspring. For centuries, morphologicallybased studies have provided valuable information about the eggs of amniotes and the embryos that develop from them. This review explores the history of such investigations, as a contribution to this special issue of Journal of Morphology, titled Developmental Morphology and Evolution of Amniote Eggs and Embryos. Anatomically-based investigations are surveyed from the ancient Greeks through the Scientific Revolution, followed by the 19th and early 20th centuries, with a focus on major findings of historical figures who have contributed significantly to our knowledge. Recent research on various aspects of amniote eggs is summarized, including gastrulation, egg shape and eggshell morphology, eggs of Mesozoic dinosaurs, sauropsid yolk sacs, squamate placentation, embryogenesis, and the phylotypic phase of embryonic development. As documented in this review, studies on amniote eggs and embryos have relied heavily on morphological approaches in order to answer functional and evolutionary questions.

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

“…The amniotic egg, a major innovation in the evolutionary history of vertebrates, varies in size and structure associated with specific life history and incubation strategies, which have been extensively studied in reptiles (Blackburn, 2000a, 2000b; Blackburn & Stewart, 2021; Deeming & Ferguson, 1991a; Deeming & Reynolds, 2015; Deeming & Ruta, 2014; Laurin & Reisz, 1997; Reisz, 1997; Sander, 2012; Shine, 2005; Stewart, 1997; Stoddard et al, 2017). Many of these studies have focused on describing the internal egg organization and embryological development of extant egg‐laying amniote clades – i.e.…”

“…Many of these studies have focused on describing the internal egg organization and embryological development of extant egg‐laying amniote clades – i.e. Monotremata, Lepidosauria, Testudines, Crocodylia, and Aves (reviews in Blackburn & Stewart, 2021; Starck et al, 2021). The reproductive strategies of these groups have been studied in the context of their respective radiations, with some traits identified as major factors of their evolutionary success – e.g.…”

Functional connections between bird eggshell stiffness and nest characteristics through risk of egg collision in nests

“…Novel traits may allow organisms to occupy various new ecological niches, leading to adaptive radiations (Stroud & Losos, 2016). For example, cleidoic eggs allowed early amniotes to leave the water and colonise diverse terrestrial habitats (Blackburn & Stewart, 2021; Sander, 2012). Birds can breed in a wide range of habitats, which is likely associated with the diverse characteristics of both their eggs and nests.…”

Phylogeny and evolutionary history of the amniote egg

“…The structure of the egg and the extraembryonic membranes characterize the taxon Amniota. Amniota were recognized early (e.g., Haeckel, 1866; Gegenbaur, 1870; review of history in: Blackburn & Stewart, 2021), long before a strict phylogenetic toolkit (e.g., Hennig, 1966) was available. Today, the conjoint occurrence of amnion, chorion, allantois, and cellular yolk sac is considered a complex and highly integrated autapomorphic character (Ax, 2003; Mickoleit, 2004).…”