Atlantogenata

The Atlantogenata includes the eutherian mammals that evolved in South America and Africa, which split off from the northern continent more than 100 Ma, prior splitting off from each other. The Xenarthrans evolved in South America and the Afrotheria evolved in Africa. Molecular DNA analysis indicates that the Xenarthra are more closely related to the Afrotheria (Figure 12‑47) than to the northern placental mammals in Laurasia. Thus, scientists group them within the Atlantogenata (see chapter 11).

The extinct South American ungulates (placental grazing animals) also evolved in South America during the Cenozoic Era, but they are more closely aligned with the Laurasiatheria than the Atlantogenata. The fact that the Xenarthra, South American ungulates, and Afrotheria became placental mammals after the End Cretaceous extinction indicates that all eutherians had at least the potential for the placental condition prior to the separation of Gondwana from Laurasia, which took place tens of millions of years prior to the End Cretaceous extinction.

The Xenarthra have some significant differences from other placental mammals. The name Xenarthra, “reverse joints,” refers to their unique vertebral joints. They also have a different hip structure from other mammals. Physiologically, they have single-color vision with sensitivity to long-wavelength light, a much lower metabolism rate than other placentals, and the males have internal testicles. Their brains are similar to other placentals, with the major difference being the lack of a pineal gland.

Figure 12‑43. Nine-banded armadillo (Dasypus novemcinctus) in Kansas. Credit: Hans Steiglitz. Used here per CC BY-SA 3.0

The South American superorder Xenarthra includes armadillos (Figure 12‑44), anteaters (Figure 12‑45), and sloths (Figure 12‑46). The fossil record of the Xenarthra begins 60 Ma with Utaetus, an early armadillo. Armadillos were confined to South America until the land bridge was established between North and South America at the beginning of the Pleistocene. The nine-banded armadillo (Figure 12‑43) is now in Kansas. It protects itself from predators by hiding inside its shell. Other armadillos do not have the capability to protect themselves in this way.

Figure 12‑44. Pink fairy armadillo (Chlamyphorus truncatus). Credit: Cliff. Used here per CC BY 2.0.

The Xenarthra bone structure is optimized for digging. They probably evolved from a eutherian that looked more like an armadillo or anteater than a sloth. An example of the digging capability is seen in feet of the pink fairy armadillo, which burrows and dig for ants in the dry soils of the deserts of Argentina (Figure 12‑44). Armadillos average 75 cm (30 inches) in length, but the pink fairy armadillo is only 15 cm (6 inches) long. At the other end of the scale, the Pleistocene glyptodon was an enormous armadillo that weighed 2 tons. As with the pink fairy armadillo, anteaters (Figure 12‑45) dig for ants or other insects in the ground.

Figure 12‑45. Giant anteater. Credit: Dave Pape. Used here per CC BY-SA 3.0

The sloths (Figure 12‑46) have an interesting evolutionary story. Their digestive system has four chambers, which allowed them to digest plant fibers that other animals could not digest. Their teeth have low enamel and continually grow so their teeth do not wear away as they chew tough foods. Thus, they could eat leaves and other plant materials and did not need to compete with other animals for these food resources. Sloths evolved into many specialized niches to consume various food resources. For example, giant sloths grew extremely large as they consumed leaves in tall trees.

One of the more interesting sloths was the Thalassocnus, which consumed underwater vegetation in the sea. As with the pakicetids, it developed dense bones to help it stay below the water surface. Rather than swim, it pulled itself along the sea floor with its large claws. Giant ground sloths dug caves enormous caves. Some caves were tall enough for humans to walk in and up to 80 m long. Scientists think that Lestodon, which was as big as an elephant, might have dug these burrows. Diabolotherium was able to climb cliffs and live in caves on the cliffs. All of the ancient sloths went extinct before or during the ice ages.

Both of the modern groups of sloths are suspensorial (Figure 12‑46), which means that they hang from tree limbs. Hanging in trees protected them from predators. Scientists do not know where they came from because there are no similar sloths in the fossil record.

Figure 12‑46. A two-toed sloth (Choloepus hoffmanni) at La Selva Biological Station, Sarapiqui, Costa Rica. Credit: Geoff Gallice.

The Afrotheria include a wide range of animal types, including elephants aardvarks, dugongs, elephant shrews, manatees, golden moles, rock hyraxes, and tailless tenrecs. Scientists link the elephants, as well as the dugongs and manatees, with the rock hyrax , which might be surprising but really not when considering the stories of the evolution of horses, whales, and marsupials from tiny animals. Scientists can see links between the rock hyrax bone structure and elephants.

As with other placental taxa, the Afrotheria probably evolved from a small insectivore. here are few fossils from the Paleocene in Africa. Ocepeia (60 Ma) appears to be transitional between two groups of Afrotheria, the Afroinsectiphilia (aardvarks, golden moles, and tenrecs) and the Paenungulata (elephants and hyraxes). It weighed approximately 3.5 kg. It had a mix of primitive eutherian and placental mammal traits.

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African forest elephants. Credit: US Fish and Wildlife Service.

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