Molecular evidence indicates that Paenungulata (or at least its extant members) is part of the cohort
Afrotheria, an ancient assemblage of mainly
African mammals of great diversity. The other members of this cohort are the orders
Afrosoricida (
tenrecs and
golden moles), Macroscelidea (
elephant shrews) and Tubulidentata (
aardvarks).[2]
Of the five orders, hyraxes are the most basal, followed by embrithopods; the remaining orders (sirenians and elephants) are more closely related. These latter three are grouped as the Tethytheria, because it is believed that their common ancestors lived on the shores of the prehistoric
Tethys Sea; however, recent
myoglobin studies indicate that even
Hyracoidea had an aquatic ancestor.[3]
History
In 1945,
George Gaylord Simpson used traditional
taxonomic techniques to group these spectacularly diverse mammals in the superorder he named Paenungulata ("almost
ungulates"), but there were many loose threads in unravelling their genealogy.[4] For example, hyraxes in his Paenungulata had some characteristics suggesting they might be connected to the
Perissodactyla (odd-toed ungulates, such as horses and rhinos). Indeed, early taxonomists placed the Hyracoidea closest to the rhinoceroses because of their dentition.
When genetic techniques were developed for inspecting
amino acid differences among
haemoglobin sequences the most
parsimoniouscladograms depicted Simpson's Paenungulata as an authentic
clade and as one of the first groups to diversify from the basal placental mammals (
Eutheria). The amino acid sequences reject a connection between extant paenungulates and
perissodactyls (odd-toed ungulates).[4]
Each of the
extinct orders, the
Embrithopoda and
Desmostylia,[a] was as unique in its members' ways of making a living as the three orders that survive. Embrithopods were
rhinoceros-like herbivorous mammals with
plantigrade feet, and desmostylians were
hippopotamus-like amphibious animals. Their walking posture and diet have been the subject of speculation, but tooth wear indicates that desmostylians browsed on terrestrial plants and had a posture similar to other large hoofed mammals.[2]
^
abDesmostylians, however, have been placed in
Perissodactyla by a 2014
cladistic analysis,[5] and the taxonomic placement of embrithopods has also been questioned[7] though recently supported.[8]
Gheerbrant, E. (2005).
"Paenungulata (Sirenia, Proboscidea, Hyracoidea, and Relatives)". In Rose, Kenneth D.; Archibald, J. David (eds.). The Rise of Placental Mammals: Origins and relationships of the major extant clades. Baltimore, MD: Johns Hopkins University Press. pp. 84–105.
ISBN080188022X – via Google Books.