Stegomastodons

A Stegomastodon skeleton from the Smithsonian Museum of Natural History

One of the most compelling extinct creatures from South America is not as well-known as it should be because it suffers from an incredibly confusing name.  The amazing Stegomastodon was a mighty proboscidean which lived on the great grassy lowlands east of the Andes until modern times, which is to say until about 9,500 years ago (because  paleontologists have a very different definition of modern than, say, historians or artists).  Proboscideans of course are the astonishing order of large mammals which include elephants and their many extinct relatives like mammoths, mastodons, deinotheriums,  moeritheriums…and stegomastodons.  The stegomastodons first evolved in North America during the Miocene (about 3 million years ago) and they lumbered rapidly down through South America after the Great American Interchange when the Isthmus of Panama formed between the two continents.  In North America, the stegomastodons died out because of competition from the true mastodons, which crossed over from Asia via Berengia, however deep in South America, they found ecosystems which suited them and they lasted for a long, long time.

An illustration of a stegomastodon (from geologiadelparaguay.com)

Stegomastodons are neither stegodons nor mastodons, two famous and well known genera of proboscideans.  Confusingly stegomastodons are the last of the gomphotheres.  Gomphotheres wandered into Asia, became isolated and evolved into Stegodons, which, in turn, are the probable ancestors of today’s still-living elephants (assuming you are still reading this in an age when the Chinese and poachers have not wiped elephants from the globe).  If these relationships are confusing to you, you can use the proboscidean clade below (but remember that the stegomastodons are gomphotheres and they lasted much longer than is shown on the chart).

Stegomastodons were grazers: they lived on the immense fields of grass which flourished east of the Andes in what is now Paraguay, Uruguay, Bolivia, and Argentina. The creatures were smaller than modern elephants growing only to 2,8 meters (9 feet) in height and obtaining a mass of 6,000 kilograms (13,000 pounds).  It is not known what wiped out the last stegomastodons, but they died quite recently, just after the Younger Dryas stadial was ending…only shortly after humankind made its way to the southern parts of South America.

Skull of Stegomastodon waringii