By now we have all heard of Charles Darwin , father of the theory of Evolution by Natural Selection, of his book The Origin of Species and how controversial it was at the time. He is not so popularly known that his revolutionary ideas were conceived in a trip around the world, thanks to which he was able to observe and study nature first-hand.
The young Charles Darwin began his adventures in the natural sciences like many in his time, studying medicine. But little by little, the natural sciences were making a hole in his mind and heart. That is why he could not turn down the opportunity to join as a naturalist on board the expedition of the brigantine HMS Beagle, whose objectives were mainly geographical and hydrographic. The ship’s captain, Robert FitzRoy, worried about collecting useful information during the voyage, proposed that a scientist be found, and the chosen one was Darwin.
In 1831 they set sail for what was originally going to be a two-year voyage, but ended up being five. During this time, Darwin devoted himself to studying the geology of the places he visited and collecting specimens of animals and plants, while the Beagle team measured ocean currents and mapped the coasts along which they passed.
Darwin was compiling his observations during this trip in his logbook. A diary that we can read today published as his book A Naturalist’s Journey Around the World . Through his observations and the collection of specimens he was able to study the great diversity of current life forms… and also some extinct ones. This is the case with, for example, the Pleistocene mammalian faunas of South America . It was thanks to all these observations that he concluded that the current species of animals and plants had emerged from others in the past, and that this evolution had taken place because of the scarcity of resources in the environment and the intraspecific variability, with only the fittest surviving through a process called Natural Selection .
The South American Megafauna
But let’s get back to that fauna of extinct mammals from South America. What does it consist of? What caught the attention of young Charles Darwin that could help shape his magnum opus?
During the Cenozoic, the era that began after the Cretaceous extinction, South America was isolated, like a gigantic continent-island. Due to this, very characteristic autochthonous forms evolved in their faunas. And so it remained for millions of years. And among the most characteristic fauna is the one that lived during the Pleistocene , until about 10,000 years ago.
Among the representatives of the South American Megafauna is the megatherium , a gigantic animal related to the current sloths, and which is usually known as “giant sloth”; or the toxodon , a robust mammal belonging to the extinct group of the notoungulates, and comparable in size to that of the rhinoceros; or the macrauchenias , mammals belonging to the extinct order of the liptoternas, with an appearance similar to that of the camelids, and a large nostril that suggests that they may have had a kind of snout or small proboscis like those of the tapirs. But in this story, the mammals that Darwin found, excavated and studied were others, the glyptodonts.
Glyptodonts were gigantic, heavily armored armadillos. They possessed an enormous carapace , a vault made up of bone plates that even covered their legs. They also had armored heads and tails, which could have large tubercles or spikes, true caudal weapons such as had not been seen in terrestrial vertebrates since the extinction of armored dinosaurs such as stegosaurus or ankylosaurs. Glyptodonts could reach over 3 meters in length and a meter and a half in height, and it is estimated that they could weigh more than two tons.
Fossils of these imposing animals had been known for a long time, but Darwin was able to study first-hand some of their fossils in various locations in Argentina between 1832 and 1833. During their stay in these lands and their coasts, the crew of the Beagle frequently ate of armadillos . So for someone with eyes as adept at anatomy as he was, the connection between the two animals jumped out. Darwin compared the bones of glyptodonts with armadillos, both those collected as specimens and those on his menu, and proposed that they were related, with the gigantic glyptodonts being possible ancestors of today’s small armadillos.
Today, studies of the kinship relationships of living and fossil mammals have revealed to us that, although armadillos and glyptodonts are related, this kinship is not so strong as to propose a direct line of descent. Both groups are part of the superorder Xenarthrans , which also includes anteaters or sloths, and both would be within the order Cingulates , with the current armadillos belonging to the Dasypodidae family, and the glyptodonts belonging to the Glyptodontidae family.
The Great American Biotic Interchange
During most of the Cenozoic , the faunas of South America and North-Central America evolved independently, because they were separated by a sea barrier at the height of today’s Isthmus of Panama . When this Isthmus was formed, about 3 million years ago, a major interchange of flora and fauna began in both directions, known as the Great American Biotic Interchange .
As a result of this exchange, some South American faunas passed to North America, as is the case of megatheria, glyptodonts or notoungulates. But North American faunas also arrived in South America, as is the case of ungulates, gomphotherid mastodons or some carnivores, such as canids or saber-toothed cats.
As they say, the rest is history. This megafauna eventually disappeared. In the first place, due to the competition with the new forms that arrived after the exchange, and ultimately, due to the retreat of the glaciers and the effect of hunting by humans after arriving on this continent.
Until the revolutionary ideas of Darwin and Wallace, fossils were considered a mere curiosity. Relics of missing animals, very interesting and curious, yes, but they seemed to have no value. Charles Darwin relied on fossils to build his theory of Evolution by Natural Selection, as in the case of armadillos. And in a later edition of his Origin of Species he even mentions the finding of Archeopteryx as evidence for the evolution of birds from reptiles.
Currently, we have examples of glyptodont skeletons scattered around half the world. And one of the best collections of these animals and the Pleistocene South American faunas in general can be visited at the Museum of Natural Sciences in Valencia . There, surrounded by imposing skeletons of this fauna, you can imagine for a moment what Charles Darwin himself must have felt when he found similar fossils on the trip that would change his life, and that would change ideas in all of Natural Sciences forever.
References :
Belinchón, M. et al. 2009. Fossil Chronicles: The Paleontological Collections of the Valencia Museum of Natural Sciences . City Hall of Valencia.
Fariña, R.A. et al. 2013. Megafauna: Giant Beasts of Pleistocene South America. Indiana University Press.
Dantas, M.A.T. et al. 2013. About the occurrence of Glyptodon sp. in the Brazilian intertropical region. Quaternary International. 305: 206–208.
Gascó, F. 2021. That was not in my dinosaur history book . Guadalmazán.
