Atlantogenata is a grouping of the Afrotheria (literally, “African mammal”) and Xenarthans (meaning “strange joints”). Despite their apparent physical dissimilarities, molecular analysis strongly suggests that they are descended from a single common ancestor that existed somewhere in what is now Africa when it was isolated from South America by continental drift over 100 million years ago.
Today when we think of the African continent we associate it with animals such as rhinoceroses, big cats, antelopes and primates. However the ancestors of most mammals migrated into Africa from more northerly continents following placental mammal radiation around 60-70 million years ago, at the end of the Cretaceous period.
Xenarthrans (armadillos, anteaters and sloths) are one of the most ancient groups of mammals. Paleontological records suggest that all Xenarthrans originated in South America. They are thought to have evolved from a burrowing ancestor that lived underground. Xenarthrans see the world in black and white and see best in dim light. Recent research has found that the genes that make colour-detecting cones in the retina are defective. This may have been an adaptation to low light in the burrowing ancestor where light sensitive retinal rod cells were more important than colour cones. The name “strange joint” relates to enlarged vertebral processes in the spine which could be linked to the burrowing lifestyle of their ancestor.
Afrotheria contains 5 orders of mammals grouped into the Clades Paeungulata and Afroinsectiphila. The Paenungulata are the elephants, dugongs, manatees and hyraxes which shared a common ancestor, the Tethytheria. It is easy to see that elephants and manatees are related; manatees have large nasal openings, elongated nasal bones (for muscle attachment for prehensile lips instead of a trunk), and horizontal molars which, like elephants, are continually replaced for chewing tough vegetation. Manatees have retained nails on their flippers and have mammary glands by their front flippers which are shared characteristics with their elephant relatives. Hyraxes also have shared, if less obvious, characters. They have little tusks which grow from the incisor teeth and flattened nails.
The Afroinsectiphilia (insect eating African mammals) contain two orders, the odd-looking aardvarks (Tubulidentata) and the Afrosoricida which include tenrecs, otter shrews and golden moles. Like marsupials, tenrecs are a good example of converegent evolution, with forms that strongly resemble hedgehogs, mice and shrews.