Similar features were found for both species under discussion. The umbilical cord contained two arteries, two veins and the urachus. After about 20 cm, the cord divided into two branches each containing a single artery and vein. Pigmented tissue accretions of variable size and irregular distribution were found on the surface of the cord.

The amnion and allantois were translucent membranes and appeared to be avascular. The allantois was the thicker membrane and had a rugous surface. The two membranes were juxtaposed over an extensive area, forming an allanto-amnion. The surface of the allanto-amnion was covered with small accretions. In addition, accretions occurred within the allantochorion and near its surface.

The uterine surface was trabecular in appearance. In most places the placenta adhered closely to the uterine wall, but there were areas where it readily detached. We also found some areas of smooth chorionic membrane that were less well vascularised, but were unable to ascertain their extent.

The principal features of the cord are shown in Figure 1. Most of the surface was covered by simple squamous epithelium. The surface accretions were plaques of squamous metaplasia (Figure 2A). They were invested with stratified squamous epithelium (Figure 2B) and sometimes contained macrophages. The urachus was lined with transitional epithelium. The major umbilical vessels, veins as well as arteries, were endowed with vasa vasorum. The stroma of the cord was supplied with small blood vessels (Figure 2C). Smooth muscle bundles were seen within the stroma at some distance from the tunica media of the major vessels (Figure 2D) and running longitudinal to the cord.

The amnion and allanto-amnion were avascular in both species. The inner surface of the amnion was covered by simple cuboidal epithelium, apparently with a brush border, and the outer surface with simple squamous epithelium (Figure 3A). Simple squamous epithelium also covered the surface of the allanto-amnion. There were large foci of squamous metaplasia at some places within the allanto-amnion (Figure 3B). They were also found near the chorioallantoic plate. They were invested with simple cuboidal or squamous epithelium and housed macrophages (Figure 3C).

The interhemal membrane was of the epitheliochorial type. The uterine trabeculae interdigitated with folds of allantochorion that bore the fetal villi (Figure 4). Spaces occurred between the trophoblast and uterine epithelium. As discussed below, they likely were shrinkage artefacts.

The endometrium showed no sign of decidualisation. It was well vascularised (Figure 5A) and richly supplied with glands lined by actively secreting simple columnar epithelium. The cuboidal uterine epithelium was partly indented by the maternal capillaries (Figure 5B).

The folds of allantochorion were invested with two different types of trophoblastic epithelium. Usually it was of the simple low columnar type indented by the capillaries with which it was so richly supplied (Figure 5C). We were also able to observe areolar regions where the trophoblast was of the tall columnar type with an apparent brush border (Figure 5D). These regions were more sparsely supplied with fetal capillaries. Columnar trophoblast also lined the stretches of chorionic plate between the villous trees (not shown). In some parts of the placenta the fetal villi were short and during fixation had lost contact with the maternal surface (Figure 3D).

Until quite recently mammalian phylogenetics dealt with cladistic analysis of skeletal and dental characters. New insights concerning relations between the various orders have been provided by molecular tools, primarily analysis of coding sequences in mitochondrial and nuclear genes 1315161718 often supplemented by molecular characters such as short and long interspersed elements 1432 that are less subject to homoplasy. DNA sequence analysis consistently finds Cetacea nested within Artiodactyla. Therefore it is now common to consider cetaceans as part of a wider ordinal clade called Cetartiodactyla. Secondly, the whales and dolphins resolve as the sister group to hippopotamids. This contrasts with the traditional view that hippopotamids are more closely related to pigs and tayassuids. The new arrangement receives robust support from a meta-analysis for all living species of Cetartiodactyla that incorporates morphological as well as molecular data 33. Recently it was shown that the foot bones of primitive Artiodactyla and early Cetacea exhibited shared, derived characters or synapomorphies 11. Finally, renewed examination of the osteological characters of fossil and extant artiodactyls, including two early cetaceans, found that pigs and tayassuids formed a monophyletic group to the exclusion of hippopotamids, while cetaceans and hippopotamuses formed another monophyletic group 34.

These recent advances in phylogenetics have led to a re-evaluation of fetal and placental membrane characters 353637. Here we find that the placentas of Sotalia and Inia share a common pattern similar to that described for other cetaceans. It is apt to evaluate this pattern in relation to concepts such as the newly erected superorder Laurasiatheria, the combined order Cetartiodactyla and perceived relationships with the hippopotamids. Several characters in the placentation of Cetartiodactyla are shared with two other orders within Laurasiatheria, the horses and tapirs (Perissodactyla) and the scaly anteaters (Pholidota). These include the diffuse, villous and epitheliochorial placenta, and the presence of areolae 3637. Contrary to common perceptions, recent cladistic analyses agree in finding epitheliochorial placentation to be a derived state and link it to another shared character, the birth of well-developed (precocial) young following a relatively long period of gestation 353637383940. Indentation of trophoblast by fetal capillaries, and uterine epithelium by maternal ones, is common in these placentae and serves to reduce the diffusion distance 39. Uterine gland secretions are an important source of nutrients in epitheliochorial placentae 35. Columnar trophoblast, which is able to absorb these nutrients, is found in areolae and in the chorionic fossae 41. Vascularisation of the cord stroma seems to be another common feature in Cetartiodactyla 725.

There are further resemblances in cetacean placentation to that of pigs, tayassuids and hippopotami. However, some characters are shared by hippopotami 424344 and cetaceans, but not by pigs and tayassuids, in accordance with the current view that hippopotamids are the sister group to cetaceans. These features include a bilobed allantoic sac occupying both horns of the uterus, paired umbilical veins, and plaques of squamous metaplasia on the surface of the cord and amnion. It is interesting that the hippopotamus placenta, like that of Tursiops 6, has several distinct areas. One, with intimate interdigitation of chorionic villi and endometrial folds, is found mainly in the mesometrial region; the second is characterised by stunted villi and thinner endometrium; finally there is some relatively avascular chorion at the poles of the chorion and over the cervix.

In conclusion, placentation in Inia and Sotalia is similar to what has been described for other toothed whales. The limited variation in placental structures is consistent with the view that Odontoceti are monophyletic and derived from a small population of a common ancestral species 32. Secondly, the epitheliochorial placenta resembles that of even-toed hoofed mammals 45 in accordance with the current view that whales are nested within the larger clade Cetartiodactyla 12. Finally, some details of the placenta and fetal membranes support the view that whales are the sister group to hippopotamids 14.