Complex sentence: Its teeth were like those of "Parahippus"; the extra crest that was variable in "Miohippus" was permanent in "Merychippus", and the other teeth were beginning to form a series of tall crests with higher crowns.
Split into simpler sentences: Its teeth were like those of "Parahippus", and the extra crest that was variable in "Miohippus" was permanent in "Merychippus". The other teeth were beginning to form a series of tall crests with higher crowns.