Embryo retention in sarcopterygians, and the origin of the extra-embryonic membranes of the amniotic egg

We reevaluate a recent suggestion that the extra-embryonic membranes of the amniotic egg do not constitute an adaptation to reproduction on land, but rather appeared in an embryo-retaining ancestor, in which they facilitated interactions between the embryo and the mother. On the basis of a revised data set on the distribution of extended embryo retention in sarcopterygians, we suggest that stem-amniotes probably did not retain their eggs in utero for a great proportion of the embryonic development. Therefore, we tentatively reject the hypothesis that the initial function of the extra-embryonic membranes was to facilitate embryo-mother interactions. However, current controversies on amniote phylogeny render our conclusions tentative.