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First North American Caballines
I consider as certain ‘Caballines’ fossils in which cranium, and upper cheek teeth, and lower cheek teeth have caballine characters.
1. According to Azzarroli (1998) the first caballine in North America may be represented by the right mandible UNSM 93078, a fragmental jaw UNSM 93082, and two MT III: UNSM 93080 and 93081. They were collected in the early Irvingtonian Red Cloud Formation, in the Ferry gravel pit, Nebraska.
The mandible is very large (maximal length 543mm; 127.4 for the muzzle length; 67 for the muzzle breadth). According to correlations in extant caballines, the basilar length of the skull would have been around 590mm, and the muzzle length - 139mm.
The shape of the mandible is indeed horse-like; the teeth, however are rather small and not altogether caballine.
2. Another fossil referred by Azzaroli to E. ferus, UCMP 32879, (1998, Plate 1, figs 1) from the Irvingtonian locality V 3605 of California lacks the muzzle and the occiput (no Franck’ Index) but the palate is short (125mm) relative to the ST-HO distance (about 119mm): the Palatal index is not caballine.
3. E. scotti section852E. scotti, Introduction
The best represented old caballine is E. scotti from Rock Creek, Texas, and the best associated skull and mandible are those preserved in Ottawa, NMC 2381. The NMC 2381 mandible is slightly larger than in UNSM 93078, and the symphysis and cheek teeth series are relatively longer. The lower molars are more caballine.