Following consideration of 20 morphological parameters of the gastrointestinal tract of ungulates and Cetacea, as well as of the production of methane, phylogenetic trees were constructed for baleen whales (Mysticeti), toothed whales (Odontoceti) and beaked whales (Ziphiidae), as well as for four taxa of the PSHM group (Fischer and Tassy: in Szalay, F. S.; Novacek, M. J.; McKenna, M. C. (eds), Mammal Phylogeny, Placentals. New York, Berlin: Springer Verlag, pp. 217-234, 1993), the Proboscidea (elephants), Sirenia (sea cows), Hyracoidea (hyraxes) and the Equidae (horses and their kin) as representatives of the Mesaxonia. In addition five families of the Artiodactyla, namely, pigs (Suidae), peccaries (Tayassuidae), hippos (Hippopotamidae), camels and their South American kin (Camelidae), chevrotains (Tragulidae) as well as ruminants with horns or antlers (Pecora) were considered. Insectivores without a caecum (Lipotyphla) were included as an outgroup. The following interrelationship for the considered taxa was found (Fig. 8a):
(((Sirenia, (Proboscidea, (Hyracoidea, Equidae), (((((Pecora, Tragulidae), Camelidae), Hippopotamidae), Tayassuidae), Suidae)))), (Ziphiidae, (other Odontoceti, Mysticeti))), Insectivora).
Because of the improbability of the relationships within the Cetacea and because of the absence of a sister-group relationship between Tayssuidae and Suidae an enforced tree was drawn and discussed (Fig. 8b):
((Sirenia, (Proboscidea, (Hyracoidea, Equidae), ((((Pecora, Tragulidae), Camelidae), Hippopotamidae), (Suidae, Tayassuidae))))), ((Ziphiidae, other Odontoceti), Mysticeti)), Insectivora).
A stomach lined exclusively with glandular mucosa with an oesophagus that opens at the level of the mesenteric plane is plesiomorphic for ungulates. Hippopotamids are clearly grouped with the artiodactyls; there is no close relationship between whales and hippopotamids. The stomachs of hippos and whales are only superficially similar; close inspection shows major differences. All ungulates - Sirenia, Proboscidea, Hyracoidea, Equidae and Artiodactyla - are clearly separated from the Cetacea. The differentiation of a caecum in the Mysticeti could be related to their diet of planktonic crustaceans, which is rich in chitin that cannot be digested with the host's own digestive enzymes (auto-enzymatic digestion). Further investigations are needed to resolve this problem. This is also true for the degree of carbohydrate digestion in the large intestine of tayassuids. Hyracoidea and equids (Mesaxonia unambiguous data for Tapiridae and Rhinocerotidae were not available to the present author) form a monophyletic group.