Saotherium, Boisserie, 2005

Boisserie, Jean-Renaud, 2005, The phylogeny and taxonomy of Hippopotamidae (Mammalia: Artiodactyla): a review based on morphology and cladistic analysis, Zoological Journal of the Linnean Society 143 (1), pp. 1-26 : 12-15

GENUS SAOTHERIUM GEN. NOV.

Description

Diagnosis: Hexaprotodont, with the following apomorphies: cranial roof showing an antorbital angle

10Mya in lateral view; skull very high above the molars; slender mandibular symphysis in the sagittal plane. Also exhibiting these plesiomorphic or convergent features: orbits below the cranial roof; slender zygomatic arches; cylindrical braincase; slender and low sagittal crest; laterally developed occipital plate; maxillary process of the frontal separating the nasal and the lachrymal bones; short extension of the canine processes; lingual border of the lower cheek tooth alveolar process lower than the labial border.

Type species: Saotherium mingoz , from Kollé ( Chad), Lower Pliocene ( Boisserie et al., 2003) .

Other material: Saotherium cf. mingoz , from Kossom Bougoudi ( Chad), Lower Pliocene ( Boisserie et al., 2003).

Remarks: The diagnosis is the same as for the type species, excluding those characters that differentiate the two known taxa, or that have not been seen in both forms.

Etymology: From the ‘Sao’, an enigmatic medieval civilization known in the Chad basin ( Lange, 1989).

Geographical distribution: Djurab desert, Lake Chad basin ( Chad, Central Africa).

Temporal distribution: early Pliocene, between the Mio-Pliocene boundary and 4.0 Mya (see Brunet et al., 1998; Brunet & MPFT, 2000).

Discussion

In their description ( Boisserie et al., 2003), the two Djurab Pliocene hippos are shown to possess an association of original cranial features: the antorbital angle of the cranial roof and the correlated anterior convergence of the nasal toward the palate; the important relative height of the skull above the molars; the elongated braincase with a rounded transversal section and a weak postorbital constriction (‘cylindrical’ aspect). In this respect, these hippopotamids differ considerably from the other known hippos. For this reason, Boisserie et al. (2003) evoked an independent hippo lineage in central Africa originating at the Mio-Pliocene boundary if not before. This opinion is confirmed by the position of these forms in the parsimony analysis ( Figs 6 , 7 ), showing also many primitive traits in this morphology. The mandibular morphology, especially the symphysial sagittal section between the central incisors (see Fig. 9 ), reinforces this position; the association of a general thinness and of a very inclined main axis differs from the conditions seen in the other Hippopotamidae . Finally, it appears that the two Djurab Pliocene hippopotamids constitute a peculiar lineage. Following the above discussion, this lineage is separated here from all other Hippopotamidae at the generic level.

On the other hand, the parsimony analysis relates these Pliocene hippopotamids to the extant Liberian hippo. However, the long list of convergences accumulated by the latter taxon, its apomorphies and autapomorphies (see below) and the absence of the peculiar cranial structure of Saotherium obviously differentiate these animals. In fact, these taxa mainly share character states that are plesiomorphic or convergent with other taxa in the analysis, with the exception of the enlarged orbit size (character 8, state 1, see Fig. 3 and the above results). However, given the available data, it is difficult to define the most probable primitive state of this feature and hence its probable evolutionary trend. Therefore, this relationship must be carefully envisaged, but not completely ignored.

Evolutionary trends: The comparison of the Kossom Bougoudi material and the younger Kollé material led Boisserie et al. (2003) to propose some possible evolutionary trends: a relative shortening of the premolar row and a global size decrease.

Figures

Figure 3. Cranial character states. A, lateral view of a Hexaprotodon liberiensis skull. B, Lateral view of a Hippopotamus amphibius skull. C, lateral view of a Hex. mingoz skull. D, three dorsal views of the braincase (from bottom to top: in Hex. harvardi, in Hex. mingoz, in Hip. amphibius). E, four schematic anterior views of the left orbit (from right to left: in Hex. protamphibius, in Hex. harvardi, in Hip. gorgops, in Hex. sivalensis).

Figure 6. Strict consensus tree of the three most parsimonious trees obtained from the cladistic analysis (length = 81, CI = 0.6420, RI = 0.7752). In italic-bold, the bootstrap results are given as an indication of clade robustness.

Figure 7. Second of the three most parsimonious tree obtained from the cladistic analysis. The bold numbers indicate the nodes. The numbered boxes indicate the ACCTRAN character state changes (white boxes indicate reversions and convergences).

Figure 9. Mandibular anatomy within the Hippopotamidae. This figure shows the new taxonomic divisions of the family Hippopotamidae and, for each discussed taxon, some of the mandibular characters that provided additional support to the clades identified in the parsimony analysis (boxes in this figure). These features include: the general shape of the mandible, with expansion of the canine processes and relative length of the symphysis (seen in the dorsal outlines); the shape of the symphysis sagittal cross section; the length of the premolar row relative to the length of the molar row. The figure shows the following features for the taxa listed under each genus name: Saotherium, very inclined symphysis with thin cross-section and poorly developed canine processes; Archaeopotamus, relatively long and shallow symphysis with poorly developed canine processes and longer premolar rows than in any other clade; Hexaprotodon, wide symphysis but with poorly differentiated canine processes, very robust symphysis in cross section; Choeropsis, very short symphysis globular in cross section and poorly developed canine processes; Hippopotamus and aff. Hippopotamus, short symphysis globular in crosssection (lacking a projected incisor alveolar process) and strong extension of the canine processes – the latter feature being not salient in the Afar species (aff. Hip. coryndoni, aff. Hip. afarensis) and aff. Hip. cf. protamphibius from Kanapoi.