eutheria

Southern Tree Hyrax (Dendrohyrax arboreus)

Also known as the eastern tree hyrax, the southern tree hyrax is a species of hyrax (a group of rodent-like mammals more closely related to elephants than rodents) which occurs in temperate, sub/tropical dry, and lowland forests in parts of southeastern and south Africa. As evidenced by their common name, southern tree hyraxes exhibit arboreal behavior and are primarily nocturnal. Like other hyraxes, D. arboreus is herbivrorus feeding on a range of plant matter. 

Classification

Animalia-Chordata-Mammalia-Eutheria-Afrotheria-Hyracoidea-Procaviidae-Dendrohyrax-D. arboreus

Image: Charles J Sharp

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Toxodon

Mounted specimen on display at Harvard Museum of Natural History

Reconstruction by Roman Uchytel

When: Pleistocene (~2.6 million to 16,000 years ago) 

Where: South America

What: Toxodon is another one of the large herbivorous animals that roamed over South America. Charles Darwin purchased the skull of the first Toxodon known to the Old World during his journey on the Beagle. This skull was sent back to England were Sir Richard Own described it and named the animal Toxodon - ‘bow teeth' based on the curving nature of its gigantic molars. Soon complete skeletons of this amazing animal were known. The first interpratations reconstructed Toxodon as a semi-aquatic animal, much like the modern hippo, but later studies of the limbs and teeth of speciemens show this was incorrect. Toxodon  was more the analogue of today’s rhinos than a hippo, a fully terrestrial animal with teeth well adapted for grinding tough plants in somewhat arid environments. Some Toxodon specimens have been found associated with arrowheads, showing that the first people to emigrate into South America had contact with these animals, and appear to have hunted them. 

Where does Toxodon fit into the tree of life? Like its contemporary Macrauchenia (which you can see in the background of the reconstruction), its relationship to living mammals is uncertain. It falls into the larger clade of Notoungulata, literally Southern Ungulates, but the placment of this group within placental mammals is highly uncertain. They maybe have a close relationship with animals in the group Afrotheria but research in mammalian systematics is only beginning to be able to evaluate that, and other hypotheses.  So what is Toxodon? We just don’t know. 

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Eomaia scansoria

…largely considered to be the first placental mammal Eomaia was a small Mesozoic mammal that lived in early Cretaceous China. Eomaia probably acted in a similar fashion to modern insectivores as it most likely was terrestrial and fed on insects. Unlike most Mesozoic mammal fossils which are only known by their teeth Eomaia is known from a complete fossil and is noted for its complex teeth and two layers of fur. Based on the remains of this complete fossil biologists are lead to believe that Eomaia was ancestral to all placental mammals, with females sporting a primitive placenta that allowed them to birth live young.

Phylogeny 

Animalia-Chordata-Mammalia-Eutheria-Eomaia

Images: Nobu Tamura and Ghedoghedo

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Megalonyx- Jefferson’s ground sloth 

When: Late Miocene to end Pleistocene (~10 million years to 10,000 years ago)

Where: Throughout North America 

What: Megalonyx is a giant ground sloth, that grew to roughly 8-10 feet (~2.5 to 3.0 meters) long.  They are the genus of giant ground sloth most closely related to the living two-toed sloth Choloepus. Sloths originated, and most of them diversified in South America, moving northward during the great American interchange, but Megalonyx is a major exception. Its ancestors reached North America millions of years prior to the massive migrations of other South America taxa; via island hopping. This relatively early arrival allowed it to spread throughout the northern continent. Megalonyx is the only species of sloth to have reached as far north as Alaska and the Yukon. It was common in many of the lower 48 states. Like many ground sloths, Megalonyx went extinct at the end of the last glacial period. 

A more recent historical note about Megalonyx; this genus was the first fossil from the Americas to be described, and the person who did so was none other than Thomas Jefferson. He proposed the name Megalonyx for the genus, based on first material recovered - the gigantic claws. Later this genus name was formalized and a species named in his honor:  Megalonyx jeffersoni (this species is the state fossil of West Virginia). Jefferson was very hopeful that living Megalonyx would be found in the uncharted west, he told Lewis and Clark to be sure to be on the look out for this beast and report back when it was discovered.