The material coming from Ocucaje, Samaca (Pujos et al., 2004) clearly belongs to another species. MUSM 492 is a juvenile cranium about 1.5 to 2 years old (Fig 3 and 3bis). The Palate to Vomer and Vomer to Basion dimensions (measurements 3 and 4) being debatable, I shall further use their means.
There is a way to extrapolate juvenile cranial dimensions into tentative adult ones Cranial Growth Tables, Cranial Growth Figures.
Fig. 4 compares Simpsonâ€™s diagrams of MUSM 492 to extant Equus about the same age. MUSM 492 appears to have measurements 16, 23, 3, 4, 13, 10, 28, and 31 close to E. grevyi ; proportions between 5 17, and 17bis close to Hemiones ; between 10 and 25 and between 31 and 32 â€“ close to Wild Asses ; between 4 and 2-5, 28 and 9 â€“ close to E. burchelli.
I took them as base to determine the probable adult dimensions and proportions of MUSM 492 (Table 1, Fig. 5).
Unfortunately I have no data on the cranium of E. curvidens MLP 6-1 from the Lujan Formation in Argentina other than the profile in Text-fig. 3 of Azzaroli, 1992 (Fig. 6). It seems, however, that its naso-incisival notch (measure 31) is much shorter.
As much as I know, comparisons show (Fig. 7) that the less dissimilar cranium to MUSM 492 is EPN V.68 of Equus santae-elenae from La Carolina, Ecuador (Fig. 8 after Azzaroli, 1992, Text-fig. 2). The upper cheek teeth are similar too (Fig. 9).
There is also a lower molar MUSM 494, (Fig. 10) resembling specimens from Tarija.