The amniote ancestor of both the mammalian and avian lineages was poikilothermic and had ribs from the head to the sacrum.  It had a parchment-like egg with a large yolk and extraembryonic membranes the assisted in nutrient mobilization and respiratory gas exchange that resulted in relatively large, self-feeding hatchlings emerging into terrestrial habitats (   ).  For the mammalian lineage, failure to find fossilized eggs from the period before viviparity suggests the lineage had always been characterized by soft, permeable eggs.

     Lauren (    ) estimated the stage of embryo development at the time of egg laying or birth for the base members of 12 extant amniote groups and assigned numbers from 0 (external fertilization) to 4 (viviparous).  For example, birds were assigned 1 (blastula), monotremes 3 (post-neurula), and Theria 4.

     To arrive at the most parsimonious scenario (fewest number of changes in egg retention on the phylogenetic map), he assigned a value of 1(gastrula) to base amniote.  Based on this analysis, at the time endothermy arose in the therapsid lineage, eggs would have been retained at least until axial patterning had begun.

     Lauren's analysis suggests that in the avian lineage egg retention decreased.  Among extant birds, the longest egg retention is the early gastrula, which plays a role in parasitizing nests of other birds.  Several factors may have selected against longer egg retention in birds.

      Although living crocodilians retain eggs until the neurula stage, the thicker shells developed in the avian lineage may have selected for shorter retention.  Egg shape and next arrangements suggested that dinosaurs related to birds laid a single egg per oviduct prior to the next ovulation (   ).  It has been suggested that this ovulation behavior and development of a larger egg resulted in the limitation to a single functioning oviduct that has been adaptive in weight reduction necessary for flight (   ).  Under these conditions, long egg retention would likely be selected against since it would limit clutch size.  Development of bird eggs temporarily stops (diapause) after laying until brooding begins (  ).  Diapause might not be possible if eggs were retained to a later stage.  It has also been suggested that the very high body temperatures (41-43 degrees C) are incompatible with internal embryonic development (    ).