Cranial Growth

Well preserved skulls are rare. It is very frustrating when they belong to juvenile animals, and thus cannot be directly compared to adults. It may, however, be possible to "extrapolate" the juvenile dimensions to adult ones. I have tried to do so by applying what I know about the skull growth of extant Equus species.

Four species were considered: Grevy’s zebra, Plains zebras, Hemiones, and Przewalsi’s horses. Comparison was made between average adult skulls and juvenile skulls of different ages by calculating the % rate of "growth" of the later.

For example, the average basilar length of 19 adult E. burchelli being 465.2 cm, the average basilar length of 5 E. burchelli 2 years old being 439cm,
the difference is 465.2-439=26.2, and the rate of growth is 100*26.2/439=5.98. Thus, to get the adult dimensions from the juvenile, one has to multiply 5.98 by 439 and add the result to 439.

Samples of adult skulls were chosen to correspond as much as possible to the available (always poor) juvenile samples. For instance, only adult E. burchelli from Etosha Pan were used because the best sample of juvenile skulls comes from Etosha Pan. For adult skulls, means were calculated on ca. 60 E. grevyi (but Cheek length and depth of the Naso-Incisival notch are known only in 5 specimens), and ca. 60 Hemiones (excluding Hemippes, Kiangs, and Mongolian Hemiones), 22 to 31 E. przewalskii, and 15 E. burchelli. Juvenile samples vary from one age group to another.

Measurements are my usual ones; some are sums of cranial segments.

About one year
Juvenile samples consist of 1 E. grevyi, 6 E. burchelli, 2-5 E. hemionus, and 5 E. przewalskii (Table 1, Fig.1).

One-two years
Juvenile samples consist of 9 E. grevyi, 9-10 E. burchelli, 6-11 E. hemionus, and 7-13 E. przewalskii (Table 2, Fig. 2).

About three years
Juvenile samples consist of 1-5 E. grevyi, 4-7 E. burchelli, 5-10 E. hemionus, and 1-5 E. przewalskii (Table 3, Fig. 3).

Naturally, the highest percentages concern the "growth" of the cheek teeth, since there are only three decidual teeth instead of six adult. The smallest percentages concern the growth in length of the muzzle. Obviously, skulls of different species grow in different ways. In E. grevyi the muzzle width grows less than in other species while the length of the Naso-Incisival notch seems to increase more. On the whole, however, the concordance seems good enough to justify tentative reconstructions of juvenile skulls.