Complex sentence: It lived alongside the Jefferson's Mammoth, "M. jeffersonii", and the Columbian Mammoth, "M. columbi", which is believed to be the same species as "M. imperator", because of it's similair size and fossil range.
Split into simpler sentences: It lived alongside the Jefferson's Mammoth ("M. jeffersonii ") and the Columbian Mammoth ("M. columbi "). The latter is believed to be the same species as"M. imperator", because of its similar size and fossil range.