Analysis of molecular data sets has provided new insights into higher-level relationships of living Eutheria, including the recognition of Afrotheria as a novel taxon. This offers an opportunity to take a fresh look at the evolution of organ systems, including some that are little used in traditional systematics. In the present study, we attempted a reconstruction of the evolution of characters associated with placentation, the fetal membranes and the female reproductive tract. The evolutionary history of 21 characters has been traced, based on a current hypothesis of eutherian relationships, by applying the computer program MacClade. Accordingly, the analysis provides a first comprehensive interpretation of the stem species pattern of Eutheria. Of particular note, this pattern includes an endotheliochorial chorioallantoic placenta. The reconstructed pattern of Eutheria does not change in the basal nodes of the group. Thus, no character transformations occur on the stem lineages of Laurasiatheria or Euarchontoglires, and even Afrotheria has mostly plesiomorphic character conditions. However, two character transformations occur on the common stem lineage of Afrotheria and its sister taxon Xenarthra, i.e., amniogenesis by cavitation instead of folding and the precocial state of the newborn. In addition, we recognized one character transformation on the stem lineage of Afrotheria, i.e., the occurrence of a four-lobed allantoic sac. Thus, contrary to previous assertions, it is possible to identify morphological characters that could be synapomorphic for this novel taxon.

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The aim of this paper is to reconstruct the evolution of chorioallantoic placental characters in Rodentia. The analysis is based on pre-existing hypotheses of rodent relationships and the tracing of character evolution. Data on 64 rodent species of 49 genera are derived from the literature. New results refer to the hystricognath species Petromus typicus A. Smith, 1831 and Octodon degus (Molina, 1782). This comprehensive analysis confirms that the stem species pattern of Rodentia is characterised by a haemochorial placenta which is divided horizontally. Inside the placental labyrinth, fetal vessels and their trophoblastic external border build up a network through which the maternal blood flows. The trophoblastic tissue is one-layered, syncytial and possess a considerable surface extension. Within Rodentia, evolutionary transformations occurred on the macroscopic as well as the fine structural level. The results suggest that the stem species of Hystricognathi underwent transformations only on the macroscopic level, i.e., forming a ring-shaped arrangement of placental regions with centrally situated maternal arteries and the acquisition of a subplacenta. By contrast, in Muridae the chorioallantoic placenta shows derived features only in regard to the fine structure of the labyrinth, i.e. the interhaemal membrane is modified in composition, and the fetal capillary endothelium is fenestrated. Geomyoidea underwent transformations on both levels. Macroscopically, their placenta is modified into a hillock shape. Microscopically, the interhaemal membrane is formed by the cytotrophoblast. In addition to the mentioned transformations, some aspects of other fetal membrane differentiation in Rodentia are briefly discussed.

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Background: Placentas of guinea pig-related rodents are appropriate animal models for human placentation because of their striking similarities to those of humans. To optimize the pool of potential models in this context, it is essential to identify the occurrence of characters in close relatives.