Evolutionary transformations of fetal membranes and reproductive strategies.

Fetal membranes (such as the chorioallantois and yolk sac) are essential to embryonic development, and have contributed importantly to the evolutionary and ecological diversity of vertebrates. Since the mid-19th century, many scientific careers have been devoted to investigations of their structure, function, and development. However, significant gaps remain in our understanding of the diversity and evolution of fetal membranes. This symposium volume focuses on the use of cladistic principles and phylogenetic relationships to reconstruct the evolutionary morphology of fetal membranes. The main goal of the present paper is to introduce the application of such methods to the evolution of vertebrate fetal membranes, as well as to provide the reader with background information on relevant concepts and terminology. Contributions within this journal issue draw upon studies of metatherian and eutherian mammals, as well as sauropsid reptiles (notably squamates). Particular attention is given to historical transformations of fetal membranes associated with the evolution of such phenomena as placentation and matrotrophy, and reproductive strategies such as viviparity. What emerges from the contributed papers is a broad sampling of contemporary research on fetal membranes, and an overview of how these membranes have evolved to support embryonic life in diverse terrestrial and intra-uterine environments.