Thursday, February 2, 2012

Natural History of Elephants

Sixty million years ago (MYA), a common ancestor gave rise to three orders: Sirenia (represented today by manatees and dugongs), Hyracoidea (represented by the hyrax) and Proboscidea, which encompasses all extinct and living species of elephants and their close relatives. Moeritherium, a pig-sized mammal that lived in North Africa about 50 MYA, is the earliest known proboscidean and the Deinotherium line diverged 10 million years later; this lineage culminated in a massive elephant-like creature that spread throughout Eurasia and Africa during the Miocene and became extinct by the onset of the Pleistocene (2 MYA).

The Paleomastadon line emerged in North Africa about 34 MYA; Gomphotheriums split from this primary lineage about 25 MYA, giving rise to the Mastadon line about 20 MYA. The main group of Gomphotheriums died out by the end of the Pliocene (2 MYA) but the mastodons lived through the Pleistocene, occupying Africa, Eurasia and North America. Primelephas, the first member of the Family Elephantidae, appeared in North Africa about 7 MYA and this lineage would eventually give rise to mammoths and our modern elephants. The African and Indian elephant lines diverged very early in the history of this family; African elephants would then split into savannah and forest species (some say subspecies since they can interbreed) about 4 MYA and, during the same period, mammoths would split from Indian elephants. While the latter split occurred in North Africa, both species would leave their home Continent; Indian elephants spread across southern Asia (giving rise to Indian, Sumatran and Sri Lankan subspecies) while mammoth species colonized Eurasia and, during the Pleistocene, North America.

Here in North America, our elephant heritage is limited to mastadons and woolly mammoths that crossed the Bering land bridge during the glacial periods of the Pleistocene (2 MYA to 10,000 years ago). Like our ancestral bison, they were pursued by human hunters who likely colonized our Continent by 20,000 years ago. This predation, combined with the warming climate of the late Pleistocene and early Holocene led to the demise of our North American elephants; the last woolly mammoths are thought to have died out in the Arctic about 8000 years ago. Though we have lost the company of these large, sociable and intelligent creatures, we still have the manatees, their very distant cousins.