PBS Eons on Toxodon, 'The Creature That Stumped Darwin,' for Darwin Day 2022

Happy Darwin Day! To celebrate, I'm continuing the tradition of sharing videos about evolution from PBS I started in 2016. Like the past four years, this one is from PBS Eons, The Creature That Stumped Darwin.

Toxodon was one of the last members of a lineage that vanished 11,000 years ago after thriving in isolation for millions of years. And its fossils would inspire a revolutionary thinker to tackle a bigger mystery than Toxodon itself: evolution.

Over at my Dreamwidth journal, I embedded this video and wrote "I show enough videos to my classes...so I won't show this. It's still good."

I'm not the only blogger I follow who has written about Toxodon and its co-star in this video, Macrauchenia. Follow over the jump for what J.K. Revell wrote about both in his blog Synapsida.

First, a passage about Toxodon from Pliocene (Pt 12): From Rabbits to Rhinos.

Despite their wider diversity in previous epochs, however, only one other family of notoungulate survived through the Pliocene. These, however, were the toxodontids, and they were the largest herbivores of their day. Represented primarily by Toxodon itself, these probably looked somewhat like smallish rhinos, and may have weighed somewhere in the region of a ton-and-a-half, standing around 150 cm (4' 11") at the shoulder. Their teeth were similar to those of the rabbit-like forms, growing perpetually throughout their life, although this seems to have been a case of convergent evolution, since their last common ancestors with the other two groups don't seem to have shared the same feature.

This would suggest that Toxodon grazed on tough vegetation, like grasses, but analysis of the chemical composition of their fossils indicates that they had a more mixed diet. They may well have been adaptable, eating more grass in the southern pampas, and more leaves further north, in the tropical forests of the Amazon. Alternatively, their choice of grass in drier climes may have been a case of necessity, rather than preference, damaging their teeth in ways that may not have helped them in the long term.

At any rate, toxodonts survived the Great American Interchan[g]e, and even made it to the northern continent, where fossils of the genus Mixotoxodon have been found from Costa Rica to Texas. This was the largest notoungulate ever to have lived, truly rhino-sized at not much less than four tons in estimated weight. Also surviving in northern South America, it was also probably the last, living through most of the Ice Ages to die out as little as 25,000 years ago.

Next time, I will turn to look at their distant relatives, the litopterns, as well as considering what it was that ate these many strange herbivores..

Now for Macrauchenia from the next installment in the series, Pliocene (Pt 13): The First Carnivorans in South America. Charles Darwin and Richard Owen play major roles in its story, too.

The second family was the more llama-like of the two. While there were a number of species, by far the best known is also the very last to survive, Macrauchenia patachonica. This made it through the Ice Ages to die out as recently as 10,000 years ago, long enough that at least some of them appear to have been butchered by humans. The first specimen was discovered by none other than Charles Darwin, during his voyage on the Beagle, although, not being an expert on fossils, he passed the specimen on to palaeontologist Richard Owen on his return, who made the first formal description and came up with the name.

Macrauchenia was one of the largest litopterns, standing around 2 metres (6 feet) at the shoulders and weighing around a ton. The name Macrauchenia literally means "long-necked", and it also had long legs that further enhanced the physical resemblance to llamas. Apart from the three toes on each foot, one of its most distinctive features was that the bony nostrils were remarkably far back, towards the top of the head, rather than at the front, where you'd expect them to be. This is usually interpreted as evidence for a short trunk, since it's similar to the way that tapir nostrils are arranged. The trunk itself would, of course, not fossilise, so absolute proof is lacking, but such an arrangement is otherwise only common in animals such as seals that like to keep their heads submerged beneath the water as much as possible. To me, at least, that seems a bit unlikely for an animal shaped like a llama.

While the original fossils were discovered in Patagonia, and most of those since have also been found in the south of the continent, at least some have also been found in mountainous Bolivia and tropical Brazil and Venezuela, suggesting that an animal that could readily adapt to different climates and terrain. Its teeth suggest a mixed diet, likely including both leaves and grass, which also might fit with an animal that was adaptable. The long legs and hooves suggest that it could run moderately fast, although an analysis of the ankle and limb bones suggest that it would also have been skilled at making sudden swerves and turns to escape predators, rather than relying on speed alone.

But what, exactly, would it have been running from? The authors of the study on Macrauchenia's agility were concerned primarily with its ability to escape from attacks by the sabretooth cat Smilodon. This makes sense, since an adult Macrauchenia was just about the largest animal that Smilodon could have taken down, and younger animals would have been an even more obvious target. But there is a problem here. While the two animals did live alongside one another for millions of years, with all that that implies, for most of Macrauchenia's existence, they didn't - sabretooth cats cannot have been what the animal originally evolved to flee.

Thereby hangs a tale, which my readers can read in the rest of the post, but also in Pliocene (Pt 14): Terror of the Marsupial Sabretooths. I learned about the existence of sparassodonts, the extinct carnivorous marsupial relatives that filled the large predatory mammal niches before the arrival of placental carnivores. I have known about the actual extinct animals for decades, but I didn't know their actual relationship to living marsupials until I read about them at Revell's blog. I told him as much in a comment on Miocene (Pt 30): Horned Armadillos and Marsupial Dogs.

Your posts are proof that even professionals like me, who worked at Rancho La Brea, can learn something new from a dedicated amateur like yourself. If I hadn't been reading this post and then followed up on Wikipedia, I might not have known that sparrasodonts (sic) are Metatheria but not part of the crown group of marsupials, that dryolestoids were therians that branched off before the eutherian-metatherian split, and that such creatures as Gondwanatheria like Patagonia even existed. Thank you, I feel much smarter as a paleontolgist and evolutionary biologist for knowing these facts.

And I'm now sharing Revell's knowledge with my readers. May they find it as interesting as I do.

I don't know the next time I'll quote Revell — probably next Darwin Day — but I'm far from through featuring videos from PBS Eons this month. I have one planned for Valentines Day, just like I did in 2019, when I posted PBS Eons on the evolution of the heart for Valentines Day. That will follow the Sunday entertainment feature, which may or may not be on the Super Bowl. I haven't decided yet, so stay tuned.