HIPPOPOTAMIDAE

Boisserie, Jean-Renaud, Lihoreau, Fabrice, Orliac, Maeva, Fisher, Rebecca E., Weston, Eleanor M. & Ducrocq, Stéphane, 2010, Morphology and phylogenetic relationships of the earliest known hippopotamids (Cetartiodactyla, Hippopotamidae, Kenyapotaminae), Zoological Journal of the Linnean Society 158 (2), pp. 325-366 : 352

EMERGENCE OF HIPPOPOTAMIDAE : SCENARIOS FOR

HIPPOPOTAMOID EVOLUTION

Boisserie & Lihoreau (2006) described two phylogenetic scenarios of hippopotamid origins relying on different biogeographical events. The first one proposed the migration from Asia to Africa of a common ancestor exclusive to Libycosaurus and Hippopotamidae after 15 Mya. This scenario seems unlikely because: (1) our phylogenetic results do not support Libycosaurus as exclusive sister group for the Hippopotamidae ; (2) confirmation of K. ternani within Hippopotamidae implies a first appearance datum possibly as early as 16 Mya. The second scenario corresponded to the emergence of Hippopotamidae within advanced bothriodontines, i.e. with a sistergroup relationship between Hippopotamidae and crown bothriodontines. In this case, the bothriodontine forerunners of Hippopotamidae and African advanced bothriodontines would have probably migrated from Asia during the early Miocene, closely to the first appearance of advanced bothriodontines Sivameryx and Afromeryx in eastern and northern Africa (see Fig. 11 , hypothetical relation 1). This would correspond to the first main faunal interchange between Eurasia and Africa triggered by firm land bridge formation around 18 ± 1 Mya ( Thomas, 1985; Rögl, 1999).

Compared to an emergence within advanced bothriodontines, a scenario relating Hippopotamidae to archaic bothriodontines would rely on quite different spatial and temporal grounds ( Fig. 11 , hypothetical relation 2). It would potentially link the origins of Hippopotamidae to anthracotheriids found in Africa as early as the late Eocene ( Lihoreau & Ducrocq, 2007). These anthracotheriids include Bothriogenys and Quatraniodon from the Jebel Qatrani Formation, Fayum, Egypt (early Oligocene, see Ducrocq, 1997; Seiffert, 2006), Brachyodus (early to middle Miocene, Lihoreau & Ducrocq, 2007), and potentially other nondescribed material (e.g. Sanders, Kappelman & Rasmussen, 2004). Brachyodus exhibits a derived dental morphology (modified anterior dentition, extremely developed styles on upper molars, see Dineur, 1981) probably excluding it from the lineage that led to Hippopotamidae . However, the Fayum anthracotheriids could represent the stem for a putative anthracotheriid lineage, the stem group of Hippopotamidae . Unfortunately, the African anthracotheriid fossil record from the upper Oligocene and the lowermost Miocene is particularly poor. The presence of a relatively bunodont anthracotheriid in the lower Miocene of Kenya ( Coryndon, 1978a, b; Pickford, 2007a) is of interest with regard to this scenario, but relies on quite fragmentary remains and its affinities within Anthracotheriidae need to be clarified.

Figures

Figure 11. Biogeographical background and evolutionary scenarios for the emergence of Kenyapotamus and Hippopotaminae. Abbreviations: 1 and 2, see text for comments; archaic bothrio., archaic bothriodontines (excluding Brachyodus); Pleis., Pleistocene; Siva. – Afro., group of advanced bothriodontines including the African Afromeryx and the Afro-Asian Sivameryx. Maps indicate known distribution areas of taxa at approximate regional scale. Distributions for: Libycosaurus from Lihoreau et al. (2006) and Boisserie & Lihoreau (2006); Merycopotamus from Lihoreau et al. (2007); Sivameryx– Afromeryx from Lihoreau (2003); Elomeryx from Ducrocq & Lihoreau (2006); archaic bothriodontines from Ducrocq (1997).