Isohyaenodon SAVAGE , 1965

Morales, Jorge & Pickford, Martin, 2017, New Hyaenodonts (Ferae, Mammalia) From The Early Miocene Of Napak (Uganda), Koru (Kenya) And Grillental (Namibia), Fossil Imprint 73 (3 - 4), pp. 332-359 : 344

Genus Isohyaenodon SAVAGE, 1965

1985 Leakitherium Savage ; Dashzeveg, p. 234.

1999 Metapterodon Stromer ; Holroyd, p. 11.

T y p e s p e c i e s. Isohyaenodon andrewsi SAVAGE, 1965 (holotype: right mandible with m1 – m3 (M-15048); type locality: Ombo , Kenya) .

D i a g n o s i s. See Morales et al. (1998a).

O t h e r s p e c i e s. Isohyaenodon zadoki SAVAGE, 1965 (= Isohyaenodon matthewi SAVAGE, 1965 ) and Isohyaenodon sp. (Morales et al. 2008).

N.B. Part of Savage’s (1965) hypodigm of Isohyaenodon matthewi (CMF 4060 NHMUK M 2947), left m 2 in a mandible fragment, fits perfectly onto a specimen attributed by the same author to Isohyaenodon andrewsi (CMF 4023 NHMUK M 2948) a left m 3 in a mandible fragment. Both fragments came from site R3, Rusinga Island and represent a single individual.

D i s c u s s i o n. Soon after its creation, this genus, as was mentioned in the introduction, was the subject of discussion by van Valen (1967) and subsequently by Morales et al. (1998a, 2007) and Lewis and Morlo (2010). The holotype mandible is similar in dimensions (at least the m3) to Metapterodon stromeri from the locality of Langental ( Morales et al. 1998a). However, there are morphological reasons for separating the two species, I. andrewsi possesses a slightly more sectorial m3, with a moderate tendency to enlarge the protoconid. The two species share the absence of the metaconid and the greatly reduced talonid in the m3. But in M. stromeri the m2 retains a more developed talonid. Reasonably, the mandible of I. andrewsi , which shows a greater sectorial tendency, could correspond to the upper dentitions more sectorial than the Metapterodon species. Borths et al. (2016: supplementary table 2) support the speculation that Isohyaenodon andrewsi and Metapterodon represent the lower and upper dentition of the same taxon based on the size of the occluding carnassial portion of the upper and lower dentitions. However, very sectorial lower teeth are known in Africa from the Late Eocene in the Fayum succession, the case with the species previously identified as Metapterodon schlosseri by Holroyd (1999). This was realised by Savage (1965), when he included in Isohyaenodon one of the Fayum mandibles described by Andrews (1906). The upper teeth of this species are known from a single maxilla ( Holroyd 1999: fig. 8B) which differs from species of Metapterodon by the reduction of the protocone and the extension of the metastyle in the M1 – M2 ( Text-fig. 8k ). This morphological pattern can be seen in the form described by Savage (1965: text-fig. 29, pl. 4, fig. 2) as Metapterodon zadoki from the site of Rusinga, Kenya, which could correspond to the lower dentition of the species Isohyaenodon matthewi from the locality of Songhor, Kenya ( Savage 1965: text-figs 41–43). Apparently, M. zadoki seems to have progressed further in the reduction of the protocone of the molars, as was noted by Savage (1965), which are also more robust and without a parastyle (Pl. 3, Fig. 1). To a certain extent, these characters approach Isohyaenodon to the new genus Sectisodon gen. nov., but the difference in robustness of the M1 is more than noticeable. It is evident that the solution is not easy, as there exists a whole range of morphologies in the upper teeth which do not correspond directly to the sectorial lower teeth. Thus, we propose to restrict the species content of Isohyaenodon to I. andrewsi and Isohyaenodon zadoki (= I. matthewi ) from Kenya, as well as Isohyaenodon sp. (Pl. 3, Fig. 2) from Elisabethfeld, Namibia (Morales et al. 2008) in the hope that new fossil finds may clarify the characteristics of this genus.

Figures

Text-fig. 8. Drawings of upper dentition in occlusal view of African Miocene Hyainailourinae. Comparison with other selected Hyaenodontida taxa. a: Brychotherium ephalmos, b: Anasinopa leakeyi, c: Teratodon spekei, d: Mlanyama sugu, e: Leakitherium hiwegi, f: Buhakia sp. from Karungu, g: Dissopsalis carnifex, h: Metapterodon stromeri, i: Pterodon dasyuroides, j: Hyainailouros napakensis, k: Falcatodon schlosseri, l: Sectisodon markgrafi, m: Isohyaenodon zadoki, n: Isohyaenodon sp. from Elisabethfeld, o: Sectisodon occultus, p–q: Exiguodon pilgrimi, p: from Napak, q: from Koru.