Fossil Quadrupeds

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Charles Darwin wrote a book called Geological Observations on South America. Since Fitzroy needed to carry out intensive and extensive coastal mapping in South America, and Darwin was, at heart, a geologist more than anything else (at least during the Beagle’s voyage), this meant that Darwin would become the world’s expert on South American geology. Much of The Voyage is about his expeditions and observations. Part of this, of course, was figuring out the paleontology of the region.

reposted with minor revisions

Bahia Blanca is a port at the northern end of Patagonia. Chapter V of The Voyage begins:

THE Beagle arrived on the 24th of August, and a week afterwards sailed for the Plata. With Captain Fitzroy’s consent I was left behind, to travel by land to Buenos Ayres.

I tried Googling that … using “get directions.” Google maps was unable to compute a route. In fact, Google Maps has no roads whatsoever in Argentina. But, I was able to make a map showing the two locations, to give you an idea of what this must have been like. Darwin walked (well, there were horses) between these two points:


And along the way, he found some fossils. Here are brief excerpts describing some of his finds. As you read through this (it’s long, but I’ve tried to edit it down as much as possible) keep in mind the following things: Evidence for evolution, climate change, large scale global synthesis, connections between observations and theory. It is all here. This is Darwin coming to an understanding of the Big Picture of Evolution.

At Punta Alta, a low cliff, about twenty feet high, exposes a mass of [sediment] containing numerous recent shells. We may believe a similar accumulation would now take place … where tides and waves were opposed. In the gravel a considerable number of bones were embedded. … the following list may give some idea of their nature: 1st, a tolerably perfect head of a megatherium, and a fragment and teeth of two others; 2d, an animal of the order Edentata, as large as a pony, and with great scratching claws; 3d and 4th, two great Edentata related to the megatherium, and both fully as large as an ox or horse; 5th, another equally large animal, closely allied or perhaps identical with the Toxodon …, which had very flat grinding teeth, somewhat resembling those of a rodent; 6th, a large piece of the tesselated covering like that of the armadillo, but of gigantic size; 7th, a tusk which in its prismatic form, and in the disposition of the enamel, closely resembles that of the African boar; it is probable that it belonged to the same animal with the singular flat grinders. Lastly, a tooth in the same state of decay with the others: … but the part that is perfect, resembles in every respect the tooth of the common horse.* … the space in which they were collected could not have exceeded one hundred and fifty yards square. It is a remarkable circumstance that so many different species should be found together; and it proves how numerous in kind the ancient inhabitants of this country must have been.

… in another cliff of red earth, I found several fragments of bones. Among them were the teeth of a rodent, much narrower, but even larger than those of the Hydrochærus capybara; the animal which has been mentioned as exceeding in dimensions every existing member of its order. There was also part of the head of a Ctenomys; the species being different from the Tucutuco, but with a close general resemblance.

The remains … were associated … with shells of existing species. … similar to the species now living in the same bay: it is also very remarkable, that not only the species, but the proportional numbers of each kind, are nearly the same [as the modern fauna] … If I had not collected living specimens from the same bay, some of the fossils would have been thought extinct … We may feel certain that the bones have not been washed out of an older formation, and embedded in a more recent one, because the remains of one of the Edentata were lying in their proper relative position (and partly so in a second case) …

From the shells being littoral species … we may feel absolutely certain that the remains were embedded in a shallow sea, not far from the coast. From the position of the skeleton being undisturbed, and likewise from the fact that full-grown serpulæ were attached to some of the bones, we know that the mass could not have been accumulated on the beach itself. …

From the general structure of the coast of this part of South America, we are compelled to believe, that the changes of [elevation] have … of late … been in one direction, and … very gradual. If, then, we look back to the period when these quadrupeds lived, the land probably stood at a level, less elevated only by a few fathoms than at present. Therefore, its general configuration since that epoch cannot have been greatly modified; …

The surrounding country, as may have been gathered from this journal, is of a very desert character. … Here, then, is an apparent difficulty: we have the strongest evidence that there has occurred no great physical change to modify the features of the country, yet in former days, numerous large animals were supported on the plains now covered by a thin and scanty vegetation.

That large animals require a luxuriant vegetation, has been a general assumption, which has passed from one work to another. I do not hesitate, however, to say that it is completely false; and that it has vitiated the reasoning of geologists, on some points of great interest in the ancient history of the world. The prejudice has probably been derived from India, and the Indian islands, where troops of elephants, noble forests, and impenetrable jungles, are associated together in every account. If, on the other hand, we refer to any work of travels through the southern parts of Africa, we shall find allusions in almost every page either to the desert character of the country, or to the numbers of large animals inhabiting it. The same thing is rendered evident by the many sketches which have been published of various parts of the interior. When the Beagle was at Cape Town, I rode a few leagues into the country, which at least was sufficient to render that which I had read more fully intelligible.

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