Darwin and the “mega-theria” of Patagonia



Richard Owen’s restoration of Glyptodon. From Brinkman (2009).

ResearchBlogging.org Perhaps one of the primary reasons that there is so much to say about Charles Darwin is that he left us so much material to scrutinize. Outside of his famous printed works there are numerous notebooks and a staggering amount of personal correspondence which are constantly being parsed for insights into how he formulated his evolutionary ideas. Indeed, there is still scholarly debate about when Darwin embraced the idea of evolution and what observations spurred him to that intellectual turning point, and a new paper by Paul Brinkman examines the role fossils played in the young naturalist’s transformation.

According to many recent biographies Charles Darwin did not fully give up the idea that species were fixed entities until about July of 1837, a little less than a year after he returned to England from his voyage aboard the HMS Beagle. This change is often credited, in part, to the conclusions professional London naturalists had made about specimens that Darwin had collected. Though he was an enthusiastic naturalist Darwin did not have the background in comparative anatomy, paleontology, or other sciences necessary to fully understand the fossils he recovered in South America. Instead he relied on the analysis of experts, like the anatomist Richard Owen, to make the connections between the extinct forms represented by the fossils and the modern Patagonian fauna.

Much of this case rests on the mistakes Darwin made in his attempts to identify the fossils. Darwin grouped nearly all the large fossils he found into two genera, Megatherium and “Mastodon“, even though many of the bones turned out to represent entirely new creatures. Clearly Darwin was no expert paleontologist, but as Brinkman argues we should be careful in our criticism of Darwin’s science. During Darwin’s time Megatherium and “Mastodon” were the only large extinct mammals known from South America. Given that Darwin was working with the fragmentary remains of previously unknown kinds of mammals, like Toxodon and Macrauchenia, it is not surprising that he did not immediately recognize them as something new. Darwin did make good use of the Beagle library during his studies, but he lacked adequate resources to compare these strange mammals to.

Darwin also drew comparisons between some fossils and critters that scurried around the pampas. In his forays into the field Darwin often found polygonal plates that were often attributed to the giant sloth Megatherium. As Darwin knew, however, this hypothesis was controversial and the little bits of armor were very similar to that of the living armadillos of South America. This was confirmed when Owen attributed them to a new genus, Glyptodon, which he restored as a giant, extinct armadillo. This fed into what Darwin called the “law of succession of types”, for the extinct Glyptodon was clearly related to the living armadillo from the same region. Some sort of species death (and possibly birth) had happened in the past.

But did these fossils influence Darwin’s views on the transmutation of species? While it is impossible to definitively identify what caused the change in Darwin’s thinking in the summer of 1837, the fossils from South America likely contributed to his early considerations of evolution. If the giant fossils he found spoke to the “death” of species, for example, what could explain the “birth” of new species we are familiar with today? Furthermore, why were the extinct forms he discovered closely allied to modern species? There was clearly a succession of animal types that took place, but what was the process that explained the pattern? Naturalists of various stripes hinted that there was some secondary cause that could simultaneously account for both these questions, but during the 1830’s the nature of that mechanism was mysterious.

Such questions were actively considered by Darwin, and Brinkman cites evidence from Darwin’s notebooks and correspondence that the young scientist was already beginning to contemplate them during the Beagle excursion. Even more importantly, despite some of his errors in identification, Darwin did make some attempt to understand the fossils he found and was not entirely reliant on the opinions of London anatomists. The conclusions of the urban academics served more to confirm and solidify questions that Darwin was already thinking about; the young Mr. Darwin was not entirely naive about fossils and the succession of forms through time.

Brinkman, P. (2009). Charles Darwin’s Beagle Voyage, Fossil Vertebrate Succession, and “The Gradual Birth & Death of Species” Journal of the History of Biology DOI: 10.1007/s10739-009-9189-9

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