Uses and limitations of mammalian fetal membranes and placenta for phylogenetic reconstruction

Comparative studies on the development of the mammalian fetal membranes and placenta, and discussions of the possible evolutionary significance of these data, have been carried out for more than a century. Throughout much of this period, however, such data were considered in isolation from cranioskeletal and other biological findings, so that only rarely were placental features incorporated into comparative assessments of mammalian evolution. Evolutionary interpretations of the fetal membrane evidence were commonly based on the concept of overall similarity, and most placentologists were strongly influenced by the widely accepted hypothesis that living "insectivores" were closely related to the ancestral eutherian mammals. Mossman's emphasis on the use of all available developmental data on the fetal membranes and placenta, rather than simply the nature of the definitive placenta, marked a turning point in the history of using such findings for assessing eutherian mammalian evolution. Recent advances in the methodology of phylogenetic inference, with emphasis on evaluating convergence versus homology, and on distinguishing between relatively primitive versus derived homologous similarities shared by two or more taxa, have provided a sound framework for the phylogenetic analysis of fetal membrane data. Character analysis of mammalian fetal membrane traits provides a mechanism for testing phylogenetic hypotheses formulated by cranioskeletal features. It is emphasized in this study that fetal membrane data have their greatest input into assessing mammalian phylogeny when they are evaluated in conjunction with the available cranioskeletal and molecular evidence.