The continent of 'trunk' animals
Molecular phylogenetic analyses have led to the radical rearrangement of mammalian systematics. Afrotheria, a clade of mammals including elephants and shrew-like animals, are nowadays regarded as the largest group of placental mammals. Long-term isolation has led these animals to develop by adaptive radiation while the Laurasiatheria developed different orders and species on the northern continents.
Early in February 2008, international newspapers carried reports about a small mammal that had recently been discovered in the Tanzanian mountains – the grey-faced elephant shrew. The photos showed a cuddly, shrew-like animal with long thin legs and a trunk-like snout. Zoologists claimed that this shrew was a relative of the much bigger elephants. It goes without saying that this became headline news.
This claim is not totally new. Elephant shrews (Rhynchocyon) have been known for quite a while, and it remains to be clarified whether the newly discovered animal is a true new species. (R. udzungwensis). Over the last few years, numerous molecular phylogenetic studies were published that alleged that the tiny animal, which weighs a maximum of 700 grammes and lives on insects, is a relative of the largest land-dwelling animal on earth, the elephant.
These findings, which were published by researchers led by Mark S. Springer, University of California, Riverside, and Stephen J. O`Brien, Laboratory of Genomic Diversity, National Cancer Institute, have considerably changed our previous ideas on the evolution of the major mammal groups as well as uncovering interesting animal-geography issues that are consistent with our knowledge of continental drift.
The trunk is not a sign of a close relationship
The elephant shrew (genus Macroscelidae) is so called due to the animal’s small trunk-like snout. It was not the newly discovered relationship with elephants (genus Proboscidea) that led to the shrew’s name. Although the idea of the trunk being a common trait between shrews and elephants seems quite exciting, the fact that the two genera have trunks must be seen as pure coincidence. The relationship between the species is not really close. Their last common ancestor most likely lived in the Upper Cretaceous period about 80 million years ago, and hence long before the dinosaurs died out.
Although elephants and the water-dwelling sea cows (Sirenia) differ in terms of phenotype, late 19th century zoologists nevertheless assumed that sea cows or manatees were the closest relatives of elephants, along with dassies (Hyracoidea). Fossil records show that all three genera, which were classified as “Paenungulata”, have their roots in Africa.
Modern molecular genetics data have confirmed that these three orders (Sirenia, Hyracoidea, Proboscidea) belonged to the paenungulates taxon and postulated a common ancestor in the oldest tertiary, the Paleocene. However, researchers could not agree on the classification of another strange creature from Africa, the antbears or aardvarks (Tubulidentata), pig-sized solitary animals who sleep in burrows by day and emerge to dig for termites after dark. The animals’ prolonged snout resembles the trunk of elephants. Initially, the aardvark was seen as the only successor of ancient ungulates, but molecular and DNA analyses have grouped the animals as paenungulates. Because of their African origin, elephant shrews, aardvarks, hyraxes, tenrecs, elephants and manatees are all classified as Afrotheria.
Afrotheria, a monophyletic superorder
The elephant shrews were previously grouped in the order of Insectivora (insect-eating animals including animals related to shrews, hedgehogs and moles) which were regarded as the basic group of all placental mammals. Mark Springer than shifted the elephant shrews and two other families from the order of Insectivora into the superorder of Afrotheria: golden moles (Chrysochloridea), mole-like animals living in arid environments in Africa, as well as tenrecs, a group with many species living in Madagascar, as well as giant otter shrews (aquatic carnivorous tenrecs). These species can be differentiated on the basis of two mitochrondrial genes as well as on the basis of a specific deletion of nine basepairs in exon 11 of the BRAC-1 gene (which plays a role in the development of breast cancer). These features are a strong argument for the monophyletic origin of Afrotheria.
It is more or less general knowledge that mammals developed after the dinosaurs had died out 65 million years ago. Some evolutionary researchers indeed assume an explosive diversification of the placental mammals above the K-T boundary (geological signature in which K is the abbreviation for the Cretaceous period, and T the abbreviation of Tertiary period). Opponents of this hypothesis assume that the placental animals separated into the major orders back in the Cretaceous period. Molecular analysis suggests an evolutionary model that lies between these two extremes: The separation into the major groups (superorders) happened in the Upper Cretaceous period, and the division into individual orders, suborders and families occurred in the Tertiary period above the K-T boundary.
Parallel developments of Afrotheria and Laurasiatheria
According to these analyses, Afrotheria represent the oldest divergence from the Placentalia lineage, which probably happened 105 million years ago. The Zenarthrae, which is a group of placental mammals extant today only in South America and includes anteaters, sloths and armadillos, was previously regarded as the earliest split. However, molecular data suggest that they separated from the main stem about 5 million years later. Africa and South America, which are fragments of the former continent Gondwana, were separated by the wide, world-spanning Tethys Ocean from the northern Laurasia (Eurasia and North America) continent before the opening of the Indian Ocean which separated Africa and South America. Their mammalian fauna, Afrotheria or Xenarthra and other related fossil groups, developed over millions of years in isolation.
The other placental animals developed on the northern continent. Another separation occurred between 90 and 95 million years ago: The Euarchontoglires, a mammalian superorder based on molecular genetic sequence analyses, combines the Glires clade, which consists of rodents (Rodentia) and lagomorphs (Lagomorpha) with the Euarchonta, a clade which consists of Scandentia, primates and Dermoptera. The Laurasiatheria evolved into several large orders that phenotypically have little in common, but are nevertheless a monophyletic group. It includes the ungulates, including the whales, odd-toed ungulates, predators, pangolins, bats and insectivora (which are not part of the Afrotheria; today called Eulipotyphla), i.e. shrews, hedgehogs and moles.
The complete isolation led to parallel adaptive radiations in Afrotheria and Laurasiatheria. Both groups developed large ungulates, insect eaters and aquatic forms, independently from each other. The separate development ended when the African plate had come close to Laurasia during the Tertiary era, and reduced the Tethys Ocean to a size that no longer prevented mammals from crossing it. 20 to 30 million years ago, the ancestors of those animal groups, which we regard as typically African (with the exception of elephants), came from the north and colonised the African continent.