Posted by: Rob Viens | January 19, 2014

The Last of the Red-Hot Litopterns

On the 9th of January Darwin arrived in Puerto San Julián:

“During these days we surveyed the coast & at night either anchored or stood out to sea. There are many rocks & breakers lying some way from the land & a ship ought not to come near them. The table land of Port Desire, is continued to St Julian, but in many places interrupted by great vallies; & large patches have been entirely removed, so that the outline resembles fortifications. The Beagle anchored off the mouth of the harbor & the Captain went in to sound the bars. He landed me & I found some most interesting geological facts. — At sunset we went on board, & the Captain took the ship into the harbor.” (Jan 5-9)

Among the “interesting geological facts” was the last in a suite of “megafauna” collected by Darwin (and later described by Richard Owen in Zoology of the Voyage of the Beagle).  It is pretty amazing – Darwin actually discovered or at least described representatives of almost all of large mammals that roamed South America in the Pleistocene (toxodons, mammoths, ground sloths, glyptodonts, oh my!).  Had he found any predator fossils (namely large saber-toothed cats) he would have covered pretty much everything. On the other hand, it is interesting that he did not uncover a single dinosaur fossils – for which Argentina is particularly well known. Maybe because he was exploring younger sedimentary rocks along the coast. Or the fossils he uncovered came from relatively soft, unconsolidated sediments and sedimentary rocks.  He may well have walked on rocks with dinos, but if they were too hard to release their treasures he would never have known.

In a letter to John Henslow, Darwin later wrote of his love for these “fossil-bearing rocks” and the age-old conflict (at least among geologists) between the choice between “soft-rock” geology (sedimentary rocks and fossils) and “hard-rock” geology (igneous and metamorphic rocks):

“I am quite charmed with Geology but like the wise animal between two bundles of hay, I do not know which to like the best, the old crystalline group of rocks or the softer & fossiliferous beds.— When puzzling about stratification &c, I feel inclined to cry a fig for your big oysters & your bigger Megatheriums.— But then when digging out some fine bones, I wonder how any man can tire his arms with hammering granite.” (Correspondence to John Henslow, March 1834)

Geological in-jokes aside, this last fossil that Darwin uncovered (and was the first to “discover”) was from the Order Litopterna– a llama-like grazing mammal of the pampas. In particular, it was later identified as Macrauchenia patachonica, which would go on to become one of the best-known representatives of the litopterns.

Reconstruction of a Macrauchenia (on Wikipedia Commons by Kobrina Olga)

Macrauchenia

Macrauchenia was almost a creature out of mythology – having the body of a large camel, the head of a tapir and the 3-toed feet of a rhino. Darwin describes it as such:

“At Port St. Julian, in some red mud capping the gravel on the 90-feet plain, I found half the skeleton of the Macrauchenia Patachonica, a remarkable quadruped, full as large as a camel. It belongs to the same division of the Pachydermata with the rhinoceros, tapir, and palæotherium; but in the structure of the bones of its long neck it shows a clear relation to the camel, or rather to the guanaco and llama.” (Voyage of the Beagle)

Foot bones of a macrauchenia from Zoology of the Voyage of the Beagle

Macrauchenia foot bones

Macrauchenia grew to be about 10-feet long (~3 m) and weighed around half a ton. Herds of these litopterns would have been found grazing the ancient pampas, using their trunks to help bring food to their mouth (like an elephant).  It is also thought that the trunks were an adaptation to dry, dusty conditions – serving as a sort-of filter to the nostrils which were located on the top of the skull.  And those long legs with rhino like hoofs?  Those where thought to help protect these prehistoric ungulates by allowing them to deliver powerful kicks (like giraffes) and evade predators by running, leaping and jumping.  In fact, studies of the bones structure of the lower leg and feet suggest that they were very agile and had the ability to rapidly change direction while running.  And they had some nasty predators to avoid. Not only where the grasslands inhabited by the aforementioned saber-toothed cats (which came from North America about 3 million years ago), but they were also the long-time home to the 3-10 ft (~1-3 m) tall “terror birds” that evolved in South America and made up the top of the food chain.

Macrauchenia skeleton (from the Royal Ontario Museum)

macrauchenia skeleton

Litopterns appear to be are closely related to the toxodons (see Toxodon Dentistry), and both orders may have evolved from the same primitive ungulate (the condylarth) that led to modern hoofed animals – such as the artiodactyls (cows, deer, hippos, pigs, whales, camels, etc.), perissodactyls (horses, tapirs, rhinos, etc.) and proboscids (elephants). Others think that the litopterns may have branched off from the other ungulates much earlier in the Cenozoic Era.  But either way, even though they look like a cross between a camel and a tapir, litopterns are an entirely different beast.

Ironically, the litopterns that survived the “Great American Interchange” (see Bridging the Continents) ended up in direct competition with a very similar group of animals that evolved in North America – the camels and llamas.  As Darwin could tell you by the abundance of guanacos (a type of camel) – it was the later that where still around in 1834. The macrauchenia and their kin didn’t make it into the Holocene – having gone completely extinct (along with the ground sloths, toxodons and glyptodons) by about 10,000 years ago. As I’ve probably said before, the world is a poorer place for their loss. (RJV)

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