Phylogeny and evolutionary history of the amniote egg

We review morphological features of the amniote egg and embryos in a comparative phylogenetic framework, including all major clades extant vertebrates. discuss 40 characters that are relevant for an analysis evolutionary history vertebrate egg. Special attention is given to morphology cellular yolk sac, eggshell, extraembryonic membranes. Many typically assigned amniotes, such as large delayed deposition, terrestrial reproduction have evolved independently convergently numerous use character mapping ancestral state reconstruction tools recognize sequence, order, patterns evolution deduce hypothesis Besides amnion chorioallantois, amniotes ancestrally possess copulatory organs (secondarily reduced most birds), internal fertilization, deposition eggs contain embryo primitive streak or early somite stage. Except amnion, type these almost aquatic vertebrates possibly response selective factors predation, hostile environmental conditions development, adjust hatching young favorable season. A functionally important feature membrane its myogenic contractility moves (early) prevents adhering growing materials. This function liquid-filled cavity may under requirements developing embryos. The chorioallantois temporary embryonic exchange organ supports development. possible scenario presents exaptation paved pathway on land. As shown by examples from anamniotes, land has occurred multiple times among vertebrates—the presenting one “solution” enabled conquest reproduction.