The Evolution and Diversity of Proboscideans
The order Proboscidea incorporates many families of extinct and extant animals including the recently extinct Gomphotheriidae, Mammutidae and Stegodontidae (Pleistocene 12000ya) and the older Deinotheriidae , Moeritheriidae , Amebelodontidae, Anthracobunidae, Barytheriidae, Choerolophodontidae, Gnathabelodontidae, Hemimastodontidae, Numidotheriidae.

Proboscids are known for their defining feature: a muscular but flexible trunk.

Above: Artists reconstruction of Arsinoitherium, an extinct embrithopod and member of the paenungulata.

In 1945 the eminent scientist George Simpson grouped the Proboscids, along with Sirenia, Hyracoidea, Embrithopoda and Desmostylia into a superorder, the Paenungulata meaning ‘near ungulates’. An ungulate is a generic term for animals with hoofed feet.

There are a number of different true ungulates including the odd-toed Perissodactyla e.g. horses and rhinocerous, the even-toed Artiodactyla e.g. pigs and camels, and the aardvark in the order Tubulidentata, but these taxa do not belong to the Paenungulata.

Embrithopoda went extinct approximately 23mya during the Oligocene epoch. The oldest fossil of this order was found in Oman in a lagoonal or restricted shallow subtidal; brown limestone environment, dating back to the early Eocene (40.4-37.2mya).

Above: A simplified phylogenetic tree of the probosicdeans and their relationship to other mammal orders.
Desmostylia are extinct marine mammals, fossils of which have been dated as far back as the Late Oligocene (30.8mya). They are believed to have gone extinct approximately 7.25mya in the late Miocene. Morphological similarity in skull and tusk fragments led Henry Osborn and Japanese scientists to group the Desmostylia with the Proboscids; however, in 1941 a complete skeleton was discovered on Sakhalin Island, Russia leading to the Desmostylia order being established in 1953 by Roy Reinhart.
Above: global distribution of desmostylian remains.
The Hyracoidea (hyraxes) are small, furry, rodent-like animals which live in Middle to South Africa and the Middle East. Many species of hyrax are endangered however, they are not high on the ICUN (International Union for Conservation of Nature) Red List. Well known species of Hyracoidea include the Rock Hyrax (Procavia capensis) and the Southern Tree Hyrax (Dendrohyrax arboreus).

DNA evidence supports the original hypothesis by Simpson that Hyracoidea are close relatives of elephants; they also share a number of morphological features including small tusks homologous with upper incisors, similar shaped bones and toenails. Hyraxes were first thought to be closely linked to rhinoceros because of their similar dentition. Similar morphological comparison also placed them with the odd-toed ungulates the Perissodactyla. Sequence analysis of amino acids was carried out with the results disproving the latter hypothesis and resulting in the placement of the Hyrax in their current position of basal group of the Paenungulata

Top left: A rock hyrax

Left: global distribution of hyrax remains

The Sirenia (dugongs and manatees) are found to genetically be the closest living relatives of elephants with the Hyracoidea forming an outgroup to all the Paenungulata. The Sirenia are a purely aquatic order and were first associated with elephants for their similar morphological features such as the position of teats, this was later confirmed by genetic analysis.
Above: Global distribution of sirenian remains

Literally meaning ‘of African origin’, this superorder includes proboscideans, aardvarks (Tubulidentata), golden moles (Afrosoricida), hyraxes (Hyracoidea), manatees and dougongs (Sirenia), tenrecs (Afrosoricida) and sengis (elephant shrews in the order Macroscelidea). The clade was established in 1998 by Michael Stanhope and bases its groupings on relatively new DNA sequence data and repeated molecular analysis. The other clade in this superorder is the Paenungulata. There is a large amount of data suggesting that the Afrotherian mammals all descend from one common ancestor making them more genetically dissimilar to other mammals across the world. It also suggests that once Africa split as a continent a large amount of local adaptation occurred which is highlighted by this genetic diversity. There are many morphological features to support this including similar timescales for adult tooth emergence and certain aspects of embryology after fertilization. The genetic analysis of nucleotide alignment supports the idea that Afrotheria is a monophyletic order however, more information will have to be gathered to satisfy skeptical scientists.


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