13 November 2013

Fossil Vertebrate of the Month: Stegomastodon

Holyoke Stegomastodon Tusk
Denver Museum of Nature & Science
My dad grew up in Colorado.  Not in the Colorado of alpine valleys and ski resorts that most of you probably just envisioned, but on the plains of the eastern third of the state, in the town of Holyoke, to be exact.  Colorado is, of course, famous for its fossils, as the state tourism board, the architects of Denver International Airport, and at least one hotelier are always happy to remind you, but its most famous fossils are from the Morrison Formation (named for a suburb of Denver) of the mountainous western side of the state.  Despite sharing a border with Nebraska and its mother lode of Cenozoic fossils, eastern Colorado is a relatively blank spot on the paleontological map.  Or so I thought.  Passing through Holyoke on my way out to Iowa, I was surprised to learn that the Denver Museum of Nature & Science had spent the summer of 2011 excavating Pleistocene fossils from a gravel pit just outside of town (as reported by the local paper here, here, and here and by the DMNS here, here, here, and here).  The site yielded several specimens (including a possible dire wolf), but the unquestioned star of the show was a specimen of the Proboscidean Stegomastodon.  The specimen is currently being prepared at the DMNS, where I was pleasantly surprised to see it while attending last month's Geological Society of America meeting in Denver (the convention center where the meeting took place, incidentally, also housed an excellent series of murals depicting prehistoric landscapes from across Colorado, including 'Dunes,' a Pleistocene scene from Wray, just down the road from Holyoke).
Relevance to my family's history is not the only reason I'm featuring Stegomastodon this month.  It was among the last of the gomphotheres, one of the most prolific (though probably paraphyletic) groups of proboscideans (despite what the name might suggest, it was neither a close relative of the North American mastodon nor of the Asian Stegodon).  Elephants and their relatives are one of the great triumphs of mammal evolution, due in large part to their ability to disperse widely, and Stegomastodon represents an especially important milestone in this history: it was one of only two proboscidean genera to colonize South America during the American Biotic Interchange (the other being Cuvieronius, also a gomphothere).  Instead of being just an isolated specimen from eastern Colorado, then, the Holyoke Stegomastodon was part of the last great success story of a once diverse group of proboscideans, a story that unfolded not just on the Great Plains, but across Panama and into the Pampas of South America.

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