Tali, comparaative morphology and diagnostic value

Talus, System of measurements
Fossil tali are often well preserved and one would hope them to be very valuable for species determination. Unfortunately it is not quite the case.
Ratio diagrams in Fig.1 show that average tali of all extant species are very much alike when compared to E. hemionus onager (reference line). Apparently the only interesting point is that in most Caballines the medial length of the trochlea (measure 2) is relatively larger than the greatest length (measure 1) because distally the medial condyle reaches lower down, more or less overhanging the distal articular surface. This may however happen too in other Equus (Hartmann zebra ZH 57 in Fig.2).
In some tali the trochlear breadth is unusually wide (Fig.2) ; as it occurs in different species, it has no diagnostic value.