The southern mammoth (M. meridionalis) lived in a milder climate, and was likely the first mammoth to enter North America through the Bering Land Bridge. It was followed by the cold-adapted steppe mammoth (M. trogontherii), which likely gave rise to the grassland specialist Columbian mammoth (M. columbi) and the globally widespread wooly mammoth (M. primigenius). There was also a controversial branch of Columbian mammoth called the woodlands Jefferson’s mammoth (M. jeffersonii) and the diminutive pygmy mammoth from Channel Islands (M. exilis).
Traditionally, all the species of mammoths were differentiated by the geological age they lived in, and the morphological features such as skull and molar shapes. The enamel ridges on their molar teeth—which indicated the specialized diets each mammoth group evolved to—became one of the commonly used identification tool for mammoth researchers.
One joint study by Canadian and American researchers, however, argued that existing studies had been biased toward the permafrost-preserved specimens of woolly mammoth. By sequencing 67 complete mitochondrial genomes from various specimens found across North America, the researchers found out that their genetics were murkier than previously expected.