Orycteropus pilgrimi, COLBERT, 1933

Lehmann, Thomas, 2009, Phylogeny and systematics of the Orycteropodidae (Mammalia, Tubulidentata), Zoological Journal of the Linnean Society 155 (3), pp. 649-702 : 670-671

(= ORYCTEROPUS PILGRIMI COLBERT, 1933 )

Emended diagnosis: Small-sized species of Amphiorycteropus that can be distinguished from the other species of the genus by the position of its postpalatine foramina at the level of the M 2, and by the root of the zygomatic arch starting at the level of the mesial lobe of the M 2.

Holotype: AMNH 29840 , a fragment of left maxilla with M 2 and M 3 of a young individual; housed at the AMNH, New York.

Type locality: Locality 108 of Brown, 1 km south of Nathot ( GPS: 32°85′N, 73°21′E), Potwar Plateau (northern Punjab, Pakistan) .

Main occurrences: Hasnot, Mathrala, as well as Malhuwala (close to Dhok Pathan), and another imprecise locality (Kamlial or Chinji) on the Potwar Plateau (northern Punjab, Pakistan).

Age: Middle to Late Miocene.

Remarks: The holotype has been found at the base of the Nagri formation, dated to between 11.2 and 9 Mya ( Barry et al., 2002). According to Barry et al. (2002: appendix 4), the presence of Orycteropodidae is inferred from 14 to 8.1 Mya. Nonetheless, this range includes unpublished specimens and also a ‘second very small species of aardvark [...], the stratigraphic distribution of which appears to be older than that of [A.] browni .’ ( Pickford, 1978: 39), so that additional work on that material is required before assessing an accurate age range for A. browni (see below for further details about this second species).

Additional material: Fragments of cranium and mandible (YPM 13901), housed at the Yale Peabody Museum, Yale, USA; isolated left M 1 (AMNH 29997) and right maxilla with M 1 –M 3 (AMNH 29999), housed at the AMNH, New York; left metacarpal IV (GSP 11604); mandible fragment with M 2 and M 3 (GSI K13/322) housed at the Calcutta Museum (?).

Remarks: The specimen YPM 13901 could not be found in the collections; instead, I worked on a cast (AMNH 27820). Unfortunately, this cast lacks the mandible and other details. The specimens GSP 11604 and GSI K13/322 (Geological Survey Pakistan and India, respectively) are not housed at the AMNH, and are probably housed in India or Pakistan. The specimen AMNH 29999 described by Pickford (1978) is in fact a cast. This explains why the author ( Pickford, 1978: 42) also described ‘A right maxilla of very doubtful provenance [...] also preserved in Calcutta. It is so similar to [AMNH] 29999 that I thought initially that the American Museum specimen had been donated to the Calcutta Museum’. Moreover, this confusion invalidates the following argument given by Pickford (1978: 41) about a ‘strange feature anterior to the M 1 ’, namely that ‘It is unlikely that this surface arose merely as an accident of preservation or of damage, as the specimen in Calcutta is virtually identical with that in the American Museum’. The feature might be of taphonomical origin, but it is rough on the cast, and could not be accurately interpreted.

Discussion: As explained before, ‘ O. pilgrimi ’ is considered here as a synonym of A. browni . The former holotype of ‘ O. pilgrimi ’ (AMNH 29997) has been successively described as a right M 2 ( Colbert, 1933), an M 2 ( Lewis, 1938), and an M 2 or an M 1 ( Pickford, 1978). The latter author pointed out the mistakes made by Lewis. The study of this tooth shows a curvature along its height (concave on the vestibular side), a flat occlusal surface, subequal lobes, and a shallower lingual groove. These features are very similar to those of the M 1 observed on the cast of YPM 13901. Moreover, the curvature is generally an upper tooth character, and the occlusal surface confirms that the individual was adult (or at least teenager). Therefore, I suggest that the specimen AMNH 29997 is in fact a left M 1. In this hypothesis, the differences between A. gaudryi and ‘ O. pilgrimi ’ given by Colbert (1933: 6) – ‘straighter [...] anterior and posterior edges, and a slightly shallower groove on the lingual side’ – fall, and describe rather the diagnostic characters of the genus Amphiorycteropus .

Colbert (1933: figs 3, 4) performed several histological sections on the teeth (M 2) of A. browni and A. gaudryi . This author suggested that the two species diverged for the size of their tubules. However, the tubules compared are not homologous because they are taken from different parts of the tooth section. Lewis (1938: 404) already noticed this problem.

‘The histological peculiarities of the teeth of [A.] browni may be due to the location of the section. The writer found that there is histological variation related to the relative distance between the occlusal surface and the proximal growing surface in the molars of Orycteropus , and also to the transverse position between the centre and periphery of the tooth. Tangential and radial diameters of the tubules are of little significance unless the position of the tubules is designated, inasmuch as peripheral tubules have greater tangential than radial diameters, medial tubules have greater radial than tangential diameters. Moreover, there appears to be a definite zonation of tubules as to size, the peripheral, central, and medial tubules being increasingly larger in this order.’

( Lewis 1938: 404)

Furthermore, in extant juvenile individuals, the tubules tend to display a more heterogeneous size and shape than in adults ( Anthony, 1934). Likewise, the thickness of the wall of these tubules appears to be thicker in juveniles than in adults for a comparable section. In this respect, the dissimilarities observed by Colbert (1933) should be taken with reserve.

The illustration of the specimen YPM 13901 in Lewis (1938): plate 1) presents a mandible in association with the cranium. On this figure, the reconstruction of its right hemimandible suggests an angle between the mandibular rami close to 80°. If this feature is confirmed on the original specimens, it would support the membership of this species to the genus Amphiorycteropus . Among this genus, A. browni is closer to A. abundulafus and A. gaudryi than to A. depereti and A. mauritanicus , for its size.