Trachytherus alloxus

Billet, Guillaume, Muizon, Christian De & Quispe, Bernardino Mamani, 2008, Late Oligocene mesotheriids (Mammalia, Notoungulata) from Salla and Lacayani (Bolivia): implications for basal mesotheriid phylogeny and distribution, Zoological Journal of the Linnean Society 152 (1), pp. 153-200 : 174-175

TRACHYTHERUS ALLOXUS

The measurements taken on Trachytherus alloxus cheek teeth show distinct trends relative to wear (Appendices 3, 4). The clearest trends have been observed for M1–3 and m3. It is noteworthy that there is an opposite trend in M1 and M3: M1 shows a mesio-distal shortening with wear while M3 lengthens. M2 does not present a mesio-distal trend with wear and is more stable in length; it is intermediate between M1 and M3. Changes in tooth dimensions with wear have already been observed in other notoungulates: in toxodontids ( Madden, 1990, 1997) and in archaeohyracids ( Croft et al., 2003).

The rostrum (measured from I1 to M3) of Trachytherus alloxus has been measured on nine specimens (Appendix 4, Table A3): the range found is 107– 121 mm with a mean of 115 mm. Shockey et al. (2006) gave the same measurements for Salla mesotheriid remains housed in the UF collections (N = 8): they found a range of 111–126 mm with a mean of 121 mm. These data constitute an interesting point of comparison with Trachytherus spegazzinianus . Measurements for this later species provide a range of 115–150 mm with a mean of 139 mm (N = 6, Appendix 4, Table A5). This shows firstly the great range of variation for this species, greater than that of T. alloxus . Secondly, even if there is an important overlap between these two variation ranges, T. alloxus differs from T. spegazzinianus in always having rostrum smaller than 126 mm. The differences in the wear stages appear not to affect consistently the tooth row length in the two species.

Moreover, measurements on M1 for these two species at equivalent wear stages may distinguish them (Appendix 3, Tables A1b and A 4). As mentioned in the diagnosis of T. alloxus , the M1 of T. spegazzinianus is relatively wider labio-lingually than that of T. alloxus . A clear difference in size is also observed between the mesio-distal length of the I1 of the two species. However, it is possible that with additional specimens of T. spegazzinianus , these measurements would have revealed a greater variation range for this species, less distinct from that of T. alloxus .

Aside from that, it is noteworthy that the morphological differences between Trachytherus alloxus and Trachytherus spegazzinianus mainly concern the ontogeny and structure of the upper molar lobes and sulcus (no clear differences on lower cheekteeeth have been determined, but some may exist such as the possibly more resistant lingual sulcus on the lower premolars and molars of T. spegazzinianus ). It is difficult to evaluate how strongly these morphological and ontogenic differences are linked but there is no doubt that a relationship exists.