Box The Phylogeny Of Placental Mammals

The phylogeny of the placental mammals has been studied in more detail than that of any other group of organisms, and yet it has taken some 30 years of intensive effort by cladists and molecular phylogenists to resolve the main patterns. Novacek et al. (1988) found that Xenarthra were the basalmost placental order, and they identified the Paenungulata/Tethytheria (hyracoids, sirenians, proboscideans), Glires (rodents, lagomorphs) and Archonta (scandentians, primates, dermopterans, chiropter-ans), conclusions that have been generally confirmed subsequently (see cladogram).

Early molecular phylogenies, in the 1970s and 1980s, produced rather variable results. In the 1990s, some of the clades noted by morphologists were confirmed, and in a flurry of important papers the remainder of the basal relationships of pla-centals were worked out: Springer et al. (1997) identified the Afrotheria as a clade, Xenarthra was the second clade to split and the remaining mammals, the Boreoeutheria, were seen to fall into two clades, the Euarchontoglires (Glires + Archonta) and the Laurasiatheria (insectivores, bats, carnivores, ungulates). This pattern has been confirmed in its essentials by Madsen et al. (2001), Murphy etal. (2001), Arnason etal. (2002), Huchon et al. (2002) and Springer etal. (2003), among many other papers.

Placentalia Phylogeny

Cladograms showing the postulated relationships of the placental mammals. The outlines of the tree are founded on recent molecular analyses, and morphological synapomorphies are taken from Novacek etal. (1988) and other sources. Synapomorphies for all nodes have yet to be discovered: A PLACENTALIA, chorioallantoic placenta, prolonged gestation in uterus, median vagina, epipubic bones and pouch absent, shell membrane absent, narrow stylar shelves on upper molars, optic foramen widely separated from sphenorbital fissure; B AFROTHERIA, no synapomorphies identified (except perhaps the 'trunk'); C, no synapomorphies identified; D, no synapomorphies identified; E, PAENUNGULATA, amastoidy (mastoid process concealed by expansion and overlap of squamosal), jugal extends posteriorly as a prominent ventral crest to anterolateral border of the glenoid fossa, carpals dorsoventrally compressed and serially arranged; F TETHYTHERIA, bilophodont cheek teeth with tendency to form additional lobe on posterior part of cingulum, forward displacement of orbits, infraorbital canal very short, zygomatic process of squamosal robust and extends dorsally and laterally, premaxilla with strong posterior process extending around reduced nasals and nearly contacting frontals; G, no synapomorphies identified; H BOREOEUTHERIA, no synapomorphies identified; I LAURASIATHERIA, no synapomorphies identified; J, no synapomorphies identified; K FERUNGULATA, no synapomorphies identified; L CETARTIODACTYLA, trochlea (groove) on navicular bone in ankle, narrow calcaneum and elongate heel process; M, no synapomorphies identified; N, no synapomorphies identified; O EUARCHONTOGLIRES, no synapomorphies identified; P ARCHONTA, sustentacular facet of astragalus in distinct medial contact with distal astragalar facets, pendulous penis suspended by reduced sheath between genital pouch and abdomen; Q, no synapomorphies identified; R GLIRES, posterior process of premaxilla long and contacts frontal, maxilla does not contact frontal, premaxilla and maxilla equally exposed in palate, glenoid fossa (jaw joint) set well dorsally of basicranium, upper and lower first incisor teeth absent, ever-growing incisors.

clear at present whether more ancient fossils will be found, or whether the molecular dates for ordinal originations will be revised upwards.

10.6.2 Aardvarks, tenrecs and golden moles

The first division of the Afrotheria is an unnamed clade consisting of the aardvark, the tenrecs, the golden mole and the elephant shrews. The aardvark is the sole living representative of the Tubulidentata. It is a bulky animal with a tubular snout and reduced teeth that lives in burrows and digs for termites. Fossil aardvarks date back to the Miocene (Figure 10.22(a)).

Tenrecs and the golden mole pair off as a specifically African group of insectivores, the Afrosoricida (Douady et al., 2004). There are 24 species of living tenrecs, insect-eating mammals that are found mainly in Madagascar, and some in west Africa. Many are small, but some range up to cat-sized and some are semi-aquatic. Many have spines and they look generally hedgehog-like (Figure 10.22(b)), so it is no wonder they were previously classified in the Lipotyphla, with the other insectivorous mammals. The oldest fossil tenrecs are Miocene.

Golden moles (Chrysochloridae), a group of 18 species from southern Africa, are small insect-eaters that burrow using their paws and a leathery pad on the nose. They retain primitive characters such as the tabular bone and a single cloaca. The oldest fossils are Miocene.

Repeated molecular assessments have shown that the sister group ofAfrosoricida is the Macroscelidea,the elephant shrews (Figure 10.22(c)). The rare elephant shrews, some 19 living species, date back to the middle Eocene (Tabuce et al., 2001). The skull is superficially shrew-like and it shares long incisors with rodents and rabbits.

10.6.3 Paenungulata:elephants and their relatives

The two living species of elephant, the Indian and the African, are a sorry remnant of the former diversity of the group (Proboscidea). The closest relatives of proboscideans are the sirenians, or sea cows, which

Proboscidean Palate

10 mm

10 mm


Fig. 10.22 Diverse afrotheres: (a) the Miocene aardvark Orycteropus gaudryi; (b) the tenrec Tenrec; (c) skull of the living elephant shrew Elephantulus. [Figure (a) after Andrews, 1896; (b,c) redrawn from Young, 1981.]

Fig. 10.22 Diverse afrotheres: (a) the Miocene aardvark Orycteropus gaudryi; (b) the tenrec Tenrec; (c) skull of the living elephant shrew Elephantulus. [Figure (a) after Andrews, 1896; (b,c) redrawn from Young, 1981.]

might seem faintly plausible. The next outgroup, the hyraxes, however, look more like rabbits than elephants. Nonetheless, morphological evidence indicates that all three groups together form a clade, the Paenungulata, and that the Hyracoidea is the outgroup of the Tethytheria, which consists of Sirenia and Proboscidea.

Paenungulates are characterized by posterior extension of the jugal to the front margin of the jaw joint and by serial arrangement of the wrist bones (Novacek et al., 1988). Tethytheres share a forward position of the orbit over the anterior premolars, and bunolophodont molars (cusps arranged opposite each other transversely). The alternative suggestion, that paenungulates and perissodactyls share morphological characters (e.g. Fischer and Tassy, 1993), is ruled out by the combined morphological and molecular evidence for Afrotheria.

Fossil Hyrax
Fig. 10.23 Proboscidean relatives: (a) the modern hyrax Heterohyrax; (b) the Miocene dugong Dusisiren. [Figure (a) after Young, 1981; (b) after Domning, 1978.]

10.6.4 Hyracoidea and Sirenia: hyraxes and sea cows

The hyraxes (sometimes called dassies or conies) are rabbit-sized animals (Figure 10.23(a)) that live in Africa and the Middle East, feeding on a mixed vegetable diet. They have short limbs, four-fingered hands and three-toed feet. The fossil record of hyraxes dates back to the Eocene and the group radiated in the Oligocene and Miocene, before declining to its present diversity of six species.

The sea cows are large, fat animals that live in coastal seas or freshwaters of tropical regions and feed on water plants. They arose in the early Eocene and radiated during the Eocene to Miocene (Domning, 1978). The first sirenian, Pezosiren (Domning, 2001), has short legs for walking on land. Later forms, such as the Miocene dugong Dusisiren (Figure 10.23(b)), show the strange down-turned snout and the reduced dentition (only four cheek teeth on each side), as well as aquatic adaptations in the broad thickened ribs (for extra weight during diving), front paddles, reduced hindlimbs and a whale-like tail.

Two extinct groups may be closely related to proboscideans and sirenians. The desmostylians were large ungainly semi-aquatic animals found in marine beds of the Oligocene and Miocene, and restricted to the north Pacific Ocean. The embrithopods, represented best by the large horned herbivore Arsinoitherium from the Oligocene of Egypt, also show tethytherian characters.

10.6.5 Proboscidea: elephants and relatives

Proboscideans are characterized by a number of features (Tassy, 1990;Shoshani and Tassy, 1996): a reduced jugal and orbit that opens in the maxilla, enlarged second upper incisors (these become the tusks in most later forms), lower canines and first premolars absent, broad molar teeth with thickened cusps and ridges and adaptations of the limbs for weight-supporting.

The early evolution of the group took place mainly in Africa. The oldest proboscidean is Phosphatherium from the lower Eocene of Morocco (Gheerbrant et al., 1996), which is rather like Moeritherium from the upper Eocene and Oligocene of North Africa (Figure 10.24(a, b)), which has a deep skull with the upper and lower second incisors enlarged as short projecting tusks. The skeleton indicates a long-bodied animal that was about 1m tall and probably lived in freshwaters, rather like a small hippo.

There were several further Eocene proboscideans in North Africa, before a split into the deinotheres and the elephantiforms (Tassy, 1990). The deinotheres lived until the mid-Pleistocene in the Old World. They have a pair of lower tusks curling under the chin from the lower jaw (Figure 10.24(c)), which may have been used in scraping the bark from trees. The upper tusks have been lost.

The elephantiforms consist of the palaeomastodon-tids, a group known from the upper Eocene and lower

Palaeomastodon Tusk

Fig. 10.24 Proboscidean evolution: (a,b) early Eocene Moeritherium; (c) Miocene Deinotherium; (d) Miocene Gomphotherium; (e) evolution of elephant molars from the low mounded teeth of the Miocene Gomphotherium (bottom), through the more incized teeth of the Pliocene Stegodon (middle), to the deeply ridged teeth of the living Elephas; teeth are shown in lateral, occlusal and section views; enamel is black, cementum heavy stipple and dentine light stipple. [Figures (a, b, d) after Andrews 1906; (c) after Flower and Lydekker, 1891; (e) after Savage and Long, 1986.]

Fig. 10.24 Proboscidean evolution: (a,b) early Eocene Moeritherium; (c) Miocene Deinotherium; (d) Miocene Gomphotherium; (e) evolution of elephant molars from the low mounded teeth of the Miocene Gomphotherium (bottom), through the more incized teeth of the Pliocene Stegodon (middle), to the deeply ridged teeth of the living Elephas; teeth are shown in lateral, occlusal and section views; enamel is black, cementum heavy stipple and dentine light stipple. [Figures (a, b, d) after Andrews 1906; (c) after Flower and Lydekker, 1891; (e) after Savage and Long, 1986.]

Oligocene of Egypt, and the elephantoids. Elephantoids show many modifications in the skull (Tassy, 1990), including the loss of more premolars, modification in the shape of the molars to become long and replacement of the teeth from the back with wear. The elephantoids radiated rapidly during the early Miocene and they include several distinctive groups, a paraphyletic assemblage sometimes known as mastodonts, the mammutids, the gomphotheres and the stegodontids. All of these groups died out in the Pliocene or Pleistocene. A later radiation of elephantoids, the

Elephantidae, occurred in the Late Miocene and this family survives.

Some Plio-Pleistocene lineages of mammutids retained the primitive pattern of rounded mound-like cusps on the cheek teeth. Mastodonts arose perhaps in central Asia and they spread rapidly over Asia, Europe and Africa, and reached North America in the early Miocene. The Miocene gomphotheres (Figure 10.24(d)) have four short tusks. They spread from Africa to Europe, Asia, North America and even South America.

These clades all show trends to larger size, few functional teeth in the jaw at any time, tusks and a trunk. These changes appear to be linked. As the elephantoids became taller (modern elephants are up to 3.5 m at the shoulder), the head became heavier not least because of the large tusks. The vast head is supported on a very short neck and so the modern elephant cannot reach the ground with its mouth. Hence, the short trunk of the early proboscideans became much longer.

Modern elephants have long lives, up to 60 years, and this leads to problems of tooth wear by abrasive plant material. Whereas Moeritherium had all six cheek teeth in each jaw, as in other mammals, the modern elephant has only one or two in place in each jaw at a time. They still have six cheek teeth, but the first three are milk molars, occurring in the young animal up to age 15. The remaining three adult molars come into use as follows: number 4 at age 18-28, number 5 at age 40-50 and number 6 at age 50 or so. This final tooth remains in the mouth and old elephants die when this last tooth is worn to the bone. The teeth are replaced by a process of drift, whereby teeth push forward in the jaw as they erupt, a process shared evolutionarily with sirenians and convergently with kangaroos.

The elephantids (mammoths and elephants) elaborated their teeth to enhance their efficiency in grinding tough plant food (Figure 10.24(e)).The ridges and valleys, covered by hard crystalline enamel, become extremely deep, and they increase in number to 10-30

transverse lines of fused cusps. The valleys between the cusp rows are filled with cement, so that a worn tooth is made from an alternating series of transverse lines of enamel, dentine, enamel, cement, enamel, dentine and so on. The hard enamel forms ridges and the whole tooth appears like a row of parallel shears in cutting vegetation.

Mammoths, the most potent images of the Pleistocene Ice Ages (Figure 10.25), spread from Africa over much of Europe and Asia, and later, North America (Lister and Bahn, 2000). All these mammoths appear to form a monophyletic group and to be most closely related to the African elephant, based on molecular analysis of preserved DNA (Thomas et al., 2000). The woolly mammoth is known from many bones, as well as near-complete carcasses preserved for thousands of years in the frozen tundra of Siberia and Alaska. These show a 2.8-m-tall elephant, covered with an 80-mm-thick fat layer and shaggy dark brown or black hair. The broad sweeping tusks may have been used in fighting, in breaking through the ice to get water, or to clear snow from the grasses and low plants that they ate. It is sometimes said that the flesh of mammoths can still be eaten,but this is unlikely. Nevertheless, the preservation is often good enough to yield the remnants of their last meal in the stomach or even in the mouth. Mammoths lived side-by-side with early humans, and died out only 12,000 years ago in Europe and 10,000 years ago in North America, although recent discoveries have

by John Long in Savage and Long, 1986.)

revealed some very late-surviving dwarf mammoths on Wrangel Island in the Russian Arctic up to 4000 years ago.

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  • Tom
    What is the difference between the palaeomastodon and elephant?
    3 years ago
  • Luwam
    Which part become modified as tusk of elephanto option canine premolar second incisor molar?
    3 years ago

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