On August 29th, Darwin’s attention returned to the muddy outcrops of Punta Alta:
“After dinner the Yawl started on a surveying cruize. I went in her. We slept at Punta Alta & I commenced a successful bone hunt; Leaving my servant & another man to continue their labours.” (Aug 29)
Whether he knew it at the time or not, Darwin and his servant, Syms Covington, were excavating more prehistoric Xenarthrans this week. So it is time for the continuation of:
In the outcrops of Punta Alta, Darwin uncovered the fossils remnants of another Xenarthran which he described in Voyage as “a large animal, with an osseous coat in compartments, very like that of an armadillo”. This creature (which turned out to play an important role in the ideas he had about evolution bouncing around in his head) was a Glyptodontidae (aka glyptodont) – specifically a member of the Glyptodon genera. So today, we’ll take a little journey on this branch of the Xenarthran family tree.
Glyptodonts – Doedicurus clavicaudatus and Glyptodon clavipes (by Robert Bruce Horsfall, 1913)
The glyptodonts can best be described as a family of giant, well-armored armadillos (both members of the Cingulata Order). Like modern armadillos, the glyptodonts had osteoderms (bony plates) in their skin that gave them a sort of armor. However, if you imagine an armadillo as having the equivalent of chain mail, you can think of the glyptodonts as wearing plate mail. The osteoderms on their back fused together to form a heavy, solid, helmet-like carapace (much like the shell of a tortoise).
Osteoderms in a glyptodon carapace (from Wikipedia Commons):
In addition, glyptodonts also had a bony plate protecting their head, and their tails were also encased in bone. In some cases the tail was surrounded by “rings” of fused osteoderms (as in the Glyptodon genera (below)), while in others, the tail was completely encased in a bony tube. Still other glyptodonts (such as the genus Doedicurus), had a large mace-like club at the end of their tail. Unlike a turtle, however, glyptodonts could not pull their head, limbs and tail into their “shell”, so this additional armoring was an important form of defense.
Glyptodon tail bone fossil – notice the ring structure (from Wikipedia Commons):
As mentioned, glyptodonts were big – the largest reaching about the size of a VW Beetle! Glyptodon one of the larger genera, with individuals reaching over 10 feet (3 m) long and weighing over 2 tons (~1800 kg). Another common characteristic found in all gylptodonts where the large bones on the side of their face (see below). These were thought to support strong jaw muscles made for chewing tough vegetation (since, based on their teeth, glyptodonts were grazers). Picture them being herded by gauchos…
Glyptodon fossil – note the “check” bones and plates of the carapace (from Wikipedia Commons)
Another family related to armadillos and glyptodonts where the pampatheres (Pampatheriidae). Although they had a similar overall look, pampatheres were smaller than glyptodonts (though still a lot bigger than armadillos) and had more flexible armor. They appear to be older than glyptodons (showing up in the Eocene (~45 million years ago) compared to the Miocene (~35 million years ago)). Both families, however, went extinct about 10,000 years ago.
Pampathere (Holmesina occidentalis) fossil (from the Royal Ontario Museum via Wikipedia Commons):
The glyptodon fossils he discovered were significant for Darwin (see Bones of an Idea). They allowed him to make the connection between the modern armadillos (like the ones he would cook up in their shells and eat while traveling) with the much larger prehistoric glyptodonts he found in the cliffs. Furthermore, since he speculated that the deposits here were not that old, he was forced to conclude that these related species had changed through time. In other words there was a process – evolution – that connected fossils of extinct species to modern relatives. Here is a little bit of what Darwin had to say about that upon returning to England:
“The relationship, though distant, between the Macrauchenia and the Guanaco, between the Toxodon and the Capybara,—the closer relationship between the many extinct Edentata and the living sloths, ant-eaters, and armadillos, now so eminently characteristic of South American zoology,—and the still closer relationship between the fossil and living species of Ctenomys and Hydrochærus, are most interesting facts. This relationship is shown wonderfully—as wonderfully as between the fossil and extinct Marsupial animals of Australia—by the great collection lately brought to Europe from the caves of Brazil by MM. Lund and Clausen. In this collection there are extinct species of all the thirty-two genera, excepting four, of the terrestrial quadrupeds now inhabiting the provinces in which the caves occur; and the extinct species are much more numerous than those now living: there are fossil ant-eaters, armadillos, tapirs, peccaries, guanacos, opossums, and numerous South American gnawers and monkeys, and other animals. This wonderful relationship in the same continent between the dead and the living, will, I do not doubt, hereafter throw more light on the appearance of organic beings on our earth, and their disappearance from it, than any other class of facts.” (Voyage of the Beagle – italics added for emphasis)*
“Shed more light”, indeed. Who would have thought that a prehistoric lumbering tank-like cousin-of-a-sloth (and the other fossils of the pampas) would have such a significant role to play in Darwin’s big idea. Score another point for Darwin the geologist! (RJV)
*PS – More on some of these other fossils animals as Darwin comes across them in the coming months.