The fossil record of the Placentalia (the Eutheria) extends back to the Early Cretaceous. The oldest specimen of a placental mammal is of a species called Eomaia meaning 'dawn mother'. It was discovered in the Liaoning deposits in China and has been dated at around 125 million years ago which is around 50 million years older than the second oldest specimen. Eomaia was a shrew sized mammal 16 cm long and weighed approximately 20 g. The specimen of Eomaia is exceptionally well preserved and exhibits clear fur around the skeleton. Athough it is a clear placental mammal, Eomaia still retains the epipubic bones that support the pouch in both primitive and modern marsupial species.
Placentals have the most diverse number of species of any mammal group and as such have an extesive fossil record. The Eutheria are represnted by around ten families in the Upper Cretaceous of North America, Uzbekistan and Mongolia. Zalambdalestes for example has numerous complete specimens form Mongolia. Modern zhelestids are represented by around 15 species in the Upper Cretaceous. The groups to which the Cretaceous species belonged are now largely extinct, but modern clades had some primitive representatives.
Detailed discussions of prehistoric representatives of the four placental groups can be found by following the links below.
Comprising Proboscidea, Hyracoidea, Sirenia, Tubulidentata, Macroscelidea, Afrosoricida
Comprising Cingulata, Folivora, Vermilingua
Comprising Rodentia, Lagomorpha, Primates, Scandentia, Dermoptera
Comprising Eulipotyphla, Chiroptera, Cetartiodactyla, Perissodactyla, Pholidota, Carnivora
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Author: Emma-Louise Nicholls
Last updated: 20th November 2005
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