Our understanding of the history of the Proboscideans, which include all the modern elephants, is a bit of a mess. We're not even entirely sure how many distinct species of elephants there are today, and the situation is even worse when we look to the past. The closest relatives of elephants that are still alive last shared an ancestor with them back when dinosaurs were around, and all of the extinct species except mammoths died out too long ago for most DNA sequencing to work.
Fortunately, techniques for working with ancient DNA are improving, putting the past in reach. An Open Access publication has now described the entire mitochondrial genome of the mastodon. The sample came out of a tooth that had been stuck in permafrost since the Pleistocene, at least 50,000 years ago. Since mastodons split from modern elephants less than 30 million years ago, they provide a good handle on the Proboscidean lineage.
This isn't the first mastodon sequence announced, but the data from this study suggests that the previous sequence was the result of contamination, as it looks suspiciously similar to either modern elephants or primates. The new sequence differs substantially from the earlier work.
Doing comparisons with mitochondrial genomes from existing elephants and frozen mammoths, the relationships among these group members becomes far more clear. The mammoth is most closely related to Asian elephants. The data also argues that African elephants are actually two species, forest and savannah, which have been diverging for over four million years. Meanwhile, the sequence comparison sets the date of diversification of these three species at around 6-9 million years ago, which the authors note is suspiciously similar to the diversification of African primates into gorillas, chimps, and hominids. The authors suggest that this was a time of major climate changes, which may have triggered many other speciation events.
Elsewhere, the BBC is reporting on a fossil mastodon find with some pretty spectacular dimensions. Despite being about 3.5 m tall at the shoulders, the tusks of the beast extended straight out from its body for five meters. The tusks were found intact, so this will be an impressive sight once it's reconstructed and put on display.