A molecular phylogenetic study published today in Nature argues that South American native ungulates (SANUs) are stem perissodactyls.
As the name implies, SANUs are hoofed mammals from South America which appear to have evolved within South America. They're an entirely extinct group. Charles Darwin actually dug some up when he was the naturalist on the Beagle. During most of the past 66 million years (the Cenozoic Era), South America was an island continent, with very little faunal dispersal from other continents. The SANUs were hypothesized to be related to a few groups of animals, but with no living members, the only method of testing phylogenetic relationships was through comparative osteological studies, studies of the bones. Comparative osteological studies are good, but without some other lines of evidence (such as molecular evidence or embryonic/developmental evidence), it's tough to determine whether similarities are due to common descent or convergent evolution.
Odd-toed hoofed mammals. Living examples are horses and kin (including zebras, donkeys, and asses), rhinos, and tapirs. Extinct examples are brontotheres and chalicotheres. Some recent analyses have suggested that a somewhat odd aquatic or semi-aquatic group of extinct mammals, the desmostylians, are also part of Perissodactyla or very close to it.
The researchers got some collagen protein from very recently extinct (10,000 years old or so) bone specimens of Toxodon and Macrauchenia, two SANUs. These proteins were compared with a fairly robust sample of modern mammals, including Afrotheria (the group that contains elephants, hyraxes, and manatees), Xenarthra (the group containing anteaters, sloths, and armadillos), Artiodactyla (the group containing whales, hippos, and goats), Perissodactyla (the group containing horses, rhinos, and tapirs), and some other species to help build a credible tree, including carnivorans (cats, dogs, and kin), primates (humans, other apes, and monkeys), and even a chicken and a platypus to really create a strong base for the tree.
Analysis of the collagen proteins from modern mammals produces a tree that's mostly consistent with molecular analyses done by other researchers; there's a few places that look off to someone who studies these trees ::pushes glasses up on nose:: but it's not saying much that's in contradiction to the modern consensus of how living mammals are related to one another. It also concludes that Toxodon and Macrauchenia are the sister group to living Perissodactyla.
Such a placement means one of two conclusions. Either these two SANUs are members of Perissodactyla (in short, they are odd-toed hoofed mammals), or of all living mammals they are most closely related to Perissodactyla. The researchers are interpreting the results as saying the latter:
In future, and with more evidence, it may be appropriate to include these SANUs within an augmented definition of Perissodactyla. At present, we prefer to recognize Litopterna and Notoungulata as part of a branch-based rankless taxon Panperissodactyla, uniting all taxa more closely related to crown Perissodactyla than to any other extant taxon of placentals
So there you go. These two SANUs are stem perissodactyls. ... unless more evidence is accumulated to support a stronger, more explanatory, hypothesis.
- Not all SANUs are included in this study; two of the five main groups went extinct at least 30 million years ago, and one went extinct somewhere around 16-12 million years ago, so getting good collagen protein data from them is unlikely but not entirely impossible. So there's the possibility that SANUs are not all closely related to one another; maybe some of those extinct groups are more related to other modern groups of mammals.
- Work still needs to be done on helping to decipher which North American and South American mammals from 66-56 million years ago were (for lack of an easier term) ancestral to Perissodactyla, which were ancestral to the SANUs, and which were ancestral to neither. Since these are unlikely to have collagen protein data, that means people who are good at comparative osteological studies will have to continue looking at these fossils.
- This study's tree will have to be tested with other recent studies that have tested fossil ungulate groups, including last year's study which argued that desmostylians and anthracobunids are part of Perissodactyla.
- This study's conclusions support the divergence of Perissodactyla from other modern mammals as either very close in time to, or more likely before, the end-Cretaceous mass extinction. So some late Cretaceous mammals might be identifiable as (once again, for lack of an easier term) ancestral to Perissodactyla, some might be ancestral to SANUs, some might be ancestral to both.
To quote Álex Ramos on Twitter, Toxodon is more tapir than elephant or hippo.
Top image of a Toxodon by Rodrigo Vera.