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 Simpson’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 – close to Wild Asses ; between 4 and 2-5, 28 and 9 – 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.