Leakitherium hiwegi Savage, 1965

Borths, Matthew R. & Stevens, Nancy J., 2017, Deciduous dentition and dental eruption of Hyainailouroidea (Hyaenodonta, “ Creodonta, ” Placentalia, Mammalia), Palaeontologia Electronica 20 (3), pp. 1-34 : 4-9

Leakitherium hiwegi Savage, 1965

Figure 3 , Tables 1-2

Referred specimens. KNM-RU 2949, left maxilla fragment with dP 3 and dP 4 ( CMF 4025 in Savage, 1965; Rusinga Island, Kenya); KNM-RU 15182, maxilla fragment with dP 3, dP 4, P 4 in crypt (Rusinga Island, Kenya).

Description

KNM-RU 2949, a maxilla fragment of Leakitherium hiwegi ( Figure 3 ), preserves the crowns of both dP 3 and dP 4. This specimen was originally numbered CMF 4025 and was briefly described and illustrated by Savage (1965) as P 4 and M 1 and designated as the paratype of Leakitherium . Though the infraorbital foramen is not preserved intact, the floor of the infraorbital canal traces to a point superior to the mesial root of dP 3. The parastyle of dP 3 is relatively short compared to the mesiodistally elongate metastyle. The paracone and metacone are fused at the base and for most of the height of the metacone. The apex of the metacone is distinct from the paracone, but it is a smaller, mesiodistally shorter cusp. The postmetacrista slopes to the deep carnassial notch formed with the metastyle. No buccal cingulum is observed along the buccal margin of the tooth, though it should be noted the specimens are heavily abraded. A thin, lingual cingulum traces the low and mesiodistally wide protocone. The primary cusp of the protocone rises only to the height of the parastyle.

The dP 3 contacts dP 4 lingual to the relatively small dP 4 parastyle. The parastyle exhibits a small cusp that connects to the preparacrista. The paracone and metacone are fused for most of their height. The paracone is slightly taller than the lower, more buccolingually compressed metacone. The postmetacrista forms a distinct carnassial notch with the metastyle. The metastylar blade exhibits a slight undulation. The protocone of dP 4 is mesiodistally narrower and projects more mesially than does the protocone of dP 3. The distinct protocone cusp frames a shallow and clearly delineated trigon basin. The protocone on dP 4 is low and does not rise to the level of the parastyle on dP 4. The palate between the protocones of dP 3 and dP 4 and distal to the protocone of dP 4 is deeply embayed, presumably to accommodate tall trigonids on dP 4 and M 1.

A second maxilla fragment of Leakitheirum, KNM-RU 15182 ( Figure 3 ), also preserves dP 3 and dP 4. Both crowns are fragmented or worn, though fragmentary material. See methods for definitions of these measurements.

the morphology of the bases of each is consistent with the morphology observed on KNM-RU 2949, including the emergence of the infraorbital canal superior to the mesial root of dP 3. KNM-RU 15182 also preserves the enamel bell of P 4 in the crypt superior to the roots of dP 4. The developing tooth bears the distinctive crenulated enamel often associated with Leakitherium ( Savage, 1965; Lewis and Morlo, 2010). There is no developing tooth apparent in the crypt superior to dP 3. This may be evidence that P 4 developed and erupted before P 3 in Leakitherium . It is also possible that the germ of P 3 was more developed than P 4 and fell out of the specimen when it was fractured along the P 3 crypt.

Remarks

The holotype of Leakitherium (BMNH M19083) is a pair of molars, M 1 and M 2, which both exhibit large protocones and expansive trigon basins ( Savage, 1965). The lower dentition of Leakitherium has not been described. Some authors ( Van Valen, 1967; Morales et al., 2007) have suggested Leakitherium hiwegi is the upper dentition of Isohyaenodon andrewsi . Holroyd (1999) made the case that the Metapterodon kaiseri (known from upper dentition) and Isohyaenodon andrewsi (known from lower dentition) are in fact synonymous, and Leakitherium is a distinct taxon. Neither Isohyaenodon andrewsi , nor Metapterodon kaiseri exhibit the distinct, crenulated enamel of Leakitherium . Further, Isohyaenodon does not possess distinct talonids nor the buccolingually broad trigonids expected to occlude with the M 1 and M 2 of Leakitherium . The specimens referred to Leakitherium in this study share the crenulated enamel and buccolingual breadth noted in the M 1 and M 2 of the holotype ( Savage, 1965). The rugosity of the enamel is even more pronounced on the unerupted P 4 (KNM-RU 15182, Figure 3 ), than on the more worn and abraded dP 4. Based on these observations, we support the hypothesis of Holroyd (1999) that Leakitherium is a distinct taxon.

The Borths et al. (2016) analysis places Leakitherium within Hyainailourinae , a result congruent with earlier phylogenetic hypotheses (Lewis and Morlo, 2010). One feature uniting hyainailourines is the metacone morphology on M 1 and M 2, reduced into a buccolingually compressed cusp that is partially or almost entirely fused to the paracone (Borths et al., 2016). Interestingly, the dP 4 of Leakitherium ( Figure 3 ) exhibits a distinct metacone that is lower than and partially fused to the paracone, anticipating the morphology of M 1 in the holotype of Leakitherium . Indeed, the morphology the dP 4 metacone in Leakitherium is similar to the dP 4 metacone of Paroxyaena pavlovi , a smaller hyainailourine from the late Eocene of France (Lavrov, 2007). This reflects the observation that in many mammals, a deciduous tooth resembles the adult tooth in the immediately distal position ( Zack, 2012).

Deciduous teeth also provide additional features useful in differentiating among hyainailourine taxa. For example, dP 3 s are known for at least three hyainailourines: Leakitherium , Paroxyaena , and Pterodon dasyuroides . Among these, Leakitherium exhibits the most individuated metacone on dP 3 with a distinct furrow on the buccal surface of the tooth defining the bases of the paracone and metacone. In Pterodon and Paroxyaena the metacone is a low cusp with a distally pointing apex, lacking any deep furrow separating it from the paracone.

Leakitherium offers additional data on deciduous dental replacement order in Hyainailouroidea . The KNM-RU 15182 ( Figure 3 ) specimen suggests that in Leakitherium , dP 3 was retained longer than dP 4. This deciduous replacement order is consistent with the deciduous replacement order that Bastl and Nagel (2013) observed in Hyaenodon , a hyaenodont taxon that likely diverged from the clade that includes Hyainailouroidea during the late Cretaceous or early Paleocene (Borths et al., 2016).

Figures

FIGURE 3. Leakitherium hiwegi maxilla fragments from Rusinga Island (early Miocene), Kenya. KNM-RU 2949 (CMF 4025 in Savage, 1965) is a left maxilla fragment dP3 and dP4 shown in 1) occlusal view; 2) buccal view; and 3) lingual view. KNM-RU 15182 is a right maxilla fragment with dP3 and dP4 and the germ of P4 shown in 4) occlusal view; 5) buccal view; 6) lingual view; 7) dorsal view; 8) and distal view. The well-developed germ of P4 is best viewed in dorsal and distal view. Specimens are to scale and scale bar is 10 mm.