Macrotarsomys Milne-Edwards and G. Grandidier, 1898

This relatively small rodent is known by three species in Madagascar. The two largest are Macrotarsomys ingens Petter, 1959 and M. petteri Goodman and Soarimalala, 2005 and the smallest M. bastardi Milne Edwards and G. Grandidier, 1898. By describing another endemic genus of Madagascar, Carleton and Goodman (1996) showed that Monticolomys koopmani is very close to M. bastardi in tooth morphology although size may discriminate between them.

A close examination of representatives of the three Macrotarsomys species and M. koopmani allowed us to find some distinctive characters.

Macrotarsomys bastardi has a long incisive foramen entering the palate between the molar rows and ending after the prelobe of M1, while M. ingens and M. petteri have a shorter one ending before the anterior root of M1. Monticolomys koopmani has the incisive foramen ending at the level of the anterior root of M1. There is also a size difference between the small M. koopmani and larger M. bastardi.

On the upper molars, in M. koopmani the M1 looks small and wide while in Macrotarsomys species it looks slightly more elongated and narrower. In the holotype of M. koopmani there is a trace of a mesoloph on M2 and M3, a large round M3 and an antero-labial cingulum on both M2 and M3. On the lower molars, the holotype of M. koopmani displays a narrow and elongated m3 and the antero-labial cusps of the anterior row of cusps are related to the single posterior cusp by a longitudinal median crest (mure). The valley separating the two lobes of m3 is long, transverse and stopped by the lingual wall of the molar. On M. koopmani, the valley is cut in two by a longitudinal median crest;there is no oblique crest connecting the two lobes. On M. bastardi the crests are more oblique on the lower molars, especially m3, and the cusps look more bunodont (Figure 4).

Different fossils of Macrotarsomys have been described in the literature. Sabatier and Legendre (1985) listed three potential species of the genus in Tsimanampetsotsa (now written as Tsimanampesotse) (SW Madagascar) – M. cf. bastardi, M. sp. and M. nov. sp. – but did not provide measurements or illustrations of their specimens. Goodman et al. (2006) found representatives of M. petteri in the subfossil sites of Andrahomana (SE Madagascar). Muldoon et al. (2009) described M. bastardi and M. petteri in Ankilitelo cave (SW Madagascar) but did not provide measurements. No fossils attributed to M. koopmani have yet been recovered. We describe here for the first time the presence of Macrotarsomys in the Children’s Cave.