Embryo retention, character optimization, and the origin of the extra‐embryonic membranes of the amniotic egg

Abstract

This study summarizes the data relevant to understanding the appearance of the amniotic egg and provides new analyses to determine the inferences that can be drawn from these data. A survey of the recent literature shows that a consensus exists on the probably primitive absence of extended embryo retention in caecilians, despite recent suggestions to the contrary. The two most recent studies on the evolution of embryo retention in sarcopterygians both suggest that early amniotes lacked extended embryo retention. New analyses of length distribution of random trees suggest that the data on embryo retention do not include a phylogenetic signal, and this implies that character optimization of these data does not yield reliable information on the primitive condition for amniotes. Thus, the study of ancestral features of amniotes will probably have to exploit data from the fossil record.

Keywords:

• Amniote
• development
• egg
• parsimony
• phylogenetic signal.

Acknowledgements

I thank Mr Damien Germain and Drs Jorge Cubo, Armand de Ricqlès, Marvalee H. Wake, and two anonymous referees for reading this manuscript.