Orycteropus djourabensis, LEHMANN ET AL., 2004

Lehmann, Thomas, 2009, Phylogeny and systematics of the Orycteropodidae (Mammalia, Tubulidentata), Zoological Journal of the Linnean Society 155 (3), pp. 649-702 : 686-687

SPECIES ORYCTEROPUS DJOURABENSIS LEHMANN ET AL., 2004

Emended diagnosis: Species of Orycteropus showing small- to medium-sized post-cranial elements, but a large skull. Orycteropus djourabensis differs from the other species of the genus by the long premolars and lower molars, the continuous border between the tibial tuberosity and the fibula on the proximal epiphysis of the tibio-fibula, as in Amphiorycteropus , as well as the proportionally short and slender hands. This species can also be distinguished from the other Orycteropus by the following features: the absence of incisura mandibulae; the shallow lingual groove on the upper molars; the oblique orientation of the articulation axis of the semilunar notch, with respect to the ulnar diaphysis; and the short and slender tibial tuberosity.

Remarks: The longer length of the lower molars in O. djourabensis is best seen when comparing the cumulative length (arithmetical addition) of the three lower molars. The talus of O. djourabensis is quadrate, as in the other Orycteropus species , but the shape of the condyle for the navicular is different. Indeed, it is compressed ventrodorsally, and thus limits the movement of the ankle in the Chadian aardvark. This character has only been observed on the holotype, so far, and additional specimens are thus required before considering it a diagnostic feature.

Holotype: KL09-98-001, a subcomplete skeleton found in anatomical connection, housed at the CNAR, N’Djaména.

Type locality: Kollé ( GPS: 16°20′N, 19°00′E) ( Chad) .

Main occurrences: Type locality, as well as possibly Ileret and Koobi Fora (Area 103) from the Koobi Fora region, east of the lake Turkana ( Kenya) (see Lehmann, 2008b) .

Age: Early Pliocene to Early Pleistocene, from between 4 and 1.52 Mya ( Lehmann, 2008b).

Additional material: A subcomplete ulna (KNM ER 875); elements of the skeleton of an adult individual (KNM ER 876); elements of the skeleton of a juvenile individual found in association with the preceding adult (KNM ER 877).

Remarks: Lehmann (2008b) demonstrated that these specimens belong to O. djourabensis .

Discussion: Orycteropus djourabensis is the oldest known species of the genus Orycteropus so far. It possesses characters that are less derived than in the other members of the genus, as well as unique features. Moreover, this taxon confirms but also questions some of the hypotheses on the evolutionary trends of the Orycteropodidae suggested by numerous authors (e.g. Colbert, 1941; Patterson, 1975; Pickford, 1975; van der Made, 2003). For instance, the length of the forelimb of O. djourabensis , intermediate between that of the Miocene Amphiorycteropus and the Plio– Pleistocene Orycteropus , compared with the large size of its skull, shows that the evolution of the Tubulidentata is more complex than has been assumed. The trend towards a general increase in size over time for the Orycteropodidae thus varies according to the element of the skeleton used to assess the size of the animal. On the other hand, the hand of O. djourabensis is significantly shorter than in the other Orycteropus species , whereas the length of the foot is comparable among these taxa. This would confirm the observation made by Colbert (1941) that there is a relative increase in size for the manus between the Early Pliocene and recent times. Accordingly, allom- etric evolution over time should be considered for Tubulidentata . Finally, O. djourabensis shows the difficulties in identifiying Plio–Pleistocene species on the basis of post-cranial elements or cranial remains only. These difficulties are best illustrated in the identification of the aardvark specimens from South Africa (see Lehmann, 2004, 2006b) and from East Africa ( Lehmann, 2008b).

SPECIES ORYCTEROPUS AFF. DJOURABENSIS LOTHAGAM LOWER NAWATA

Material: Elements of the hindlimb (KNM LT 28641) described by Milledge (2003) as Orycteropus sp. and housed at the NMK, Nairobi.

Locality and age: Lothagam, Lower Nawata Formation ( Kenya). This formation is dated to between 7.4 and 6.5 Mya (Late Miocene) ( Leakey & Harris, 2003).

Discussion: These specimens are the oldest-known representatives of the genus Orycteropus to date. The tibia of the Lothagam aardvark lacks a cnemial tuberosity typical of Myorycteropus , and its dimensions are larger than those in Leptorycteropus . The talus is quadrate, as in Orycteropus , but is dissimilar to Amphiorycteropus . The cotyloid facet for the medial malleolus of the tibia is concave and developed, unlike in Myorycteropus . The dimensions of the talus are significantly smaller than those in O. afer , whereas they are close, but still smaller, to those of O. djourabensis . Noticeably, the articular condyle for the navicular is compressed dorsoventrally, as in O. djourabensis . One can be assured that the Lothagam Lower Nawata aardvarks belong to the genus Orycteropus . However, the absence of cranial and dental remains makes it difficult to accurately identify the species. If the diagnostic status of the dorsoventrally compressed articular condyle for the navicular on the talus were confirmed, these specimens would be brought closer to O. djourabensis . Therefore, I recommend referring this material to Orycteropus aff. djourabensis until new material is discovered.

SPECIES ORYCTEROPUS AFF. DJOURABENSIS LOTHAGAM UPPER NAWATA

Material: A partial right calcaneum (KNM LT 25136) and the proximal part of a right ulna (KNM LT 32873) described by Milledge (2003) as Orycteropus sp. , housed at the NMK, Nairobi.

Locality and age: Lothagam, Upper Nawata Formation ( Kenya). This formation is dated to between 6.5 and 5.5 Mya (Late Miocene) ( Leakey & Harris, 2003). Discussion: The ulna presents an oblique articulation axis of the semilunar notch, as in O. djourabensis and Amphiorycteropus . However, the dimensions of this fragment are larger than those in any of the other Miocene species, and are closer to O. djourabensis . Conversely, the calcaneum is distinctively small, with a size close to that of A. mauritanicus and the largest A. gaudryi . One could consider that the two specimens belong to two different taxa, or that the calcaneum belongs to a juvenile individual. There is no direct comparison possible between the aardvarks from the Lower and Upper Nawata Formation in Lothagam. Nonetheless, I also tentatively refer the latter specimens to O. aff. djourabensis . These fossil aardvarks, along with the specimens from Asa Koma, Lothagam Lower Nawata, and Lukeino, are then the oldest specimens of Orycteropus .

SPECIES ORYCTEROPUS AFF. DJOURABENSIS LUKEINO

Material: A left talus ( KNM LU 668 ) and the proximal epiphysis of a left proximal phalanx IV of the hand ( KNM LU 750 ), described by Pickford (1975) as ‘ Orycteropus sp. indet.’, housed at the NMK, Nairobi .

Locality and age: Lukeino ( Kenya). The Lukeino Formation is dated to between 6.2 and 5.6 Mya (Hill, 2002).

Discussion: In his description, Pickford (1975) reported two isolated specimens from the Lukeino Formation that he characterized as belonging to a tiny and a large orycteropodid, respectively. The phalanx is very small, and is only comparable with the smallest Orycteropodidae , such as M. africanus and aff. M. chemeldoi . No homologous phalanx is known for aff. M. minutus at this time. The possibility that this specimen belongs to a very juvenile individual cannot be excluded. Conversely, the talus is very large, similar in size to the talus of the type of O. djourabensis , but distinctly smaller than those of O. afer and a fortiori of O. crassidens . Although damaged, the Lukeino talus seems quadrate, as in Myorycteropus and Orycteropus taxa, and is thus distinct from the genus Amphiorycteropus . Moreover, the cotyloid facet for the medial malleolus of the tibia appears concave and developed, unlike in Myorycteropus . This talus, considered alone, would thus be very close to O. djourabensis . It is not yet clear if the two specimens were found in the same stratigraphic level, and if KNM LU 750 belongs to a juvenile individual of the same form as the talus. Therefore, until new material is found, I suggest placing the aardvarks from Lukeino in affinis with O. djourabensis .