 Later morphological studies came to conflicting conclusions . A 1999 analysis of cranial measurements found that the quagga was as different from the plains zebra as the latter is from the mountain zebra . A 2004 study of skins and skulls instead suggested that the quagga was not a distinct species , but a subspecies of the plains zebra . In spite of these findings , many authors subsequently kept the plains zebra and the quagga as separate species . 
