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The superordinal clade Afrotheria is composed of elephant shrews, elephants, sea cows, hyraxes, aardvark, golden moles and tenrecs. Molecular evidence demonstrates their relationship and they share a number of anatomical features including a large yolk sac and allantois (Oduor-Okelo, 2004; Madsen, 2001). The Afrotheria lineage seems to have diverged from that of other mammals at about the time when Africa separated from other continents, about 100 million years ago ( Roca, 2005). Some of the groups within Afrotheria are more closely related to each other, forming the group Tethytheria.
The name Tethytheria is the name given to a group of related mammals: elephants (Proboscidea), sirens (Sirenia) and an extinct group of mammals called the Demostylia. The first members of this group seem to have been semi-aquatic, including the ancestors of elephants. (Gaeth, 1999). Virtually all placental mammal males have testes which are permanently descended into the scrotum so that sperm can be stored at a lower temperature than body temperature. While some mammals have secondarily returned the testes to the body (such as whales and dolphins), very few placental mammals keep their testes inside the abdomen (such as a few insectivores and in some bats where the testes only descend into the scrotum during mating season). In elephants and sirens, the testes are permanently located inside the abdomen and there is no indication that they evolved from ancestors with a scrotum. This seems to indicate that the tethytheres were a very early offshoot of placental mammalian lineages.
A number of sirens evolved
which include the modern manatee and the recently extinct Steller’s sea cow. The
Steller’s sea cow could reach 10 tons in weight and lengths of 25 feet.
The bones of a manatee's flipper are modified versions of those found in all mammalian arms.
The bones of a manatee's flipper are modified versions of those found in all mammalian arms.
Although there are only 2 modern elephants (proboscideans), there is a great diversity of fossil species which are classified in at least 40 genera and 164 species and subspecies. These fossil forms include mastodons, mammoths, deinotheres and moeritheres, gomphotheres, and the ebrithopods. Many species were large, including some larger than modern elephants, but some were small, standing only 1 meter tall at the shoulder (such as the earliest known proboscidean from the late Paleocene, Phoshatherium). Extensions of the nostrils and upper lips fused to form a trunk (in elephant fetuses today the upper lip and trunk begin as distinct structures which later fuse) and the middle incisors enlarged to form tusks (which are the largest teeth of any known animal). In the earliest species, the incisors are equal in size.
. Anthracobunes include several genera which are commonly considered to be early proboscideans or a closely related group. They had a tooth formula of 3,1,4,3/3,1,4,3 (incisors, canines, premolars, and molars in the upper/and lower jaws).
Moeritherium was an early proboscidean with a small trunk (and possibly a mobile upper lip) and only moderately enlarged second incisors. It varied in size from that of a large pig to a small tapir and possessed spaces (diploe) in skull characteristic of elephants. The loss of some teeth gave it a tooth formula of 3,1,3,3/2,0,3,3. Moeritherium is known from the Late Eocene to the Early Oligocene (Shoshani, )
|Numidotherium from the Middle Eocene had spaces in its skull bones, a higher opening for its nostrils, and a large second incisor. The various species of Barytheres varied in size from that of a tapir to that of a small elephant. Their tooth formula was 2,0,3,3/2,0,3,3 (Gheerbrant, 1996).|
The species Eritreum melakeghebrekristos is known from the late Oligocene (27 million years ago) in East Africa. It is transitional between the earliest proboscideans and the lineage which led to mastodons, gompotheres, stegodons, mammoths, and elephants. It is estimated to have stood 1.3 meters tall at the shoulder with straight tusks 20 cm long (although the fossil specimen had probably not attained full size). Like Gomphotherium angustidens, the tusks were oriented horizontally. Phiomia and Palaeomastodon represent lineages which diverged from that of Eritreum and higher elephants (Shoshani, 2006).
Deinotheres possessed a flat cranium and a tooth formula of 0,0,2,3/1,0,2,3. Their only tusks were curved incisors from the lower jaw which may have been useful in digging for roots. Some reached heights of 4 meters (Shoshani, ).
Mastodons are first known from Africa (near Egypt) from about 30 million years ago. By 18 million years ago they had spread throughout Europe and Asia and by 13 million years ago they had reached North America (and later traveled to South America). Ancestral mastodons possessed tusks in both their upper and lower jaws. One groups, the zygodont mastodons gradually reduced the size of the tusks on the lower jaw. The American mastodon, which could reach a shoulder height of about 10 feet, represented the latest living member of this group. The cusp-like teeth of the mastodon were adapted for browsing leaves, as were the ancestral elephant teeth. Digestive tract contents of mastodonts indicated that they fed on conifers, leafy trees, grasses, aquatic plants, and moss. They were apparently covered with coarse brown fur (Kurten,1988; 1980).
Mammoths and modern elephants possess teeth which are adapted for grazing on grasses. The tusks on the lower jaws may have helped strip material off trees. While mastodons ranged over much of North America, they were most common along the eastern region and seemed to prefer wetland habitats. (Kurten, 1988).
The relationships between many of the fossil elephants are known. Zygolophodon
was ancestral to Miomastodon which was ancestral
which was ancestral to the mastodon Mammut americanum. Mastodons
Below are a few photos of the Sugarloaf mastodon, Sugar.
The first time that the bones of extinct elephants were correctly identified was by early 18 th century African slaves living in California. In 1739, mastodon fossils were found in Ohio. After they were identified as belonging to elephant relatives, Georges Cuvier named them “Mastodon” (which means “breast tooth”) because of the raised portions of the teeth. Because the difference between mammoths and mastodons was not originally appreciated, the scientific name for the American mastodon is Mammut americanus. (Kurten, 1988).
George Washington obtained a mastodon tooth. While some told tales of fierce, carnivorous mastodons, Benjamin Franklin pointed out how its teeth identified it as a vegetarian. The last mastodons are thought to have gone extinct between 9,000 and 12,000 years ago, in part at least because of hunting from early Americans (Kurten, 1988) The first mastodon remains were found in
After this first migration from
gomphotheres such as Platybelodon and Ambeledon had shovel-like inferior
Bunodont mastodons produced a variety of forms including some with shovel-shaped lower jaws and many which retained prominent tusks on their lower jaws. Stegomastodonts were the last North American members of this group and could reach 8 feet in height at the shoulder (Kurten, 1988). The subfamily Stegotetrabelodontinae included 2 tusked elephants such as Stegodibelodon and 4 tusked elephants such as Stegotetrabelodon.
The second elephant migration consisted of
gomphotheres and stegodontids. The second and third migrations of elephants
consisted primarily of grazing animals since climate change and the spread
of grasslands were driving the evolution of many mammal groups. There
are six known North American gomphotheres. At least 4 genera of gomphotheres
In the family Elephantidae,
there was a shift in tooth action from grinding and shearing to horizontal
shearing (by moving the jaws forward and backward). Elephas (the Asian elephant) and
Mammuthus (the mammoth)
are more similar to each other than either is to Loxodonta (the African elephant).
Vestigial premolars are known in some primitive species of this
family. This family spread throughout
the world in the third migration of elephants and even colonized a number
of islands with dwarf species. Dwarf elephants from the genera Elephas, Mammuthus, and Hesperoloxodon
are known from islands of the
Mammuthus is the only genus of the modern family Elephantidae (which includes the African and Asian elephants) which can inhabit cold regions so it was the only genus which migrated to North America across the Beringia land bridge. The earliest mammoth fossils in North America are dated at 1.4 million years ago. The earliest mammoth, Mammuthus subplanifrons is known from Africa from fossils 4 million years old. Another African mammoth, Mammuthus africanavus, descended from this species. Mammoths apparently swam to a number of islands and evolved smaller sizes such as the 4-6 foot tall Mammuthus exilis and 3 foot tall Elephas falconeri. The Columbian mammoth is only known from North America and its range stretched as far south as Mexico, farther south than any mammoth since their migration to Europe.
The ancestral European mammoth, Mammuthus meridionalis, had migrated from Africa to Europe by 1.5 million years ago.
The two lineages of its descendants include the steppe mammoth (Mammuthus trogontherii) and its descendant the woolly mammoth (Mammuthus primigenius) and the Columbian mammoth Mammuthus columbi (depicted below).
While ancestral mammoths probably fed primarily on leaves, later forms were specialized grazers which could feed on grasses. Steppe mammoths could reach heights of 14 feet at the shoulder and weigh 10 tons. The wooly mammoth probably evolved in Siberia and from there migrated eastwards to the East Coast of North America and westwards to Ireland. Gestation required an estimated 22 months before birth (Kurten, 1988; Lister, 1994).
Mammoths and modern elephants have highly modified teeth. The teeth are large (mammoth teeth could measure more than afoot and weight 4 pounds). Each jaw only possessed one exposed tooth at a time. Six teeth would be produced in each jaw over the course of a lifetime and replace the older tooth as it slid into place. The sixth and final molar erupted around 30 years of age and was the largest with the greatest number of ridges. The sixth molars would service their owners for the remainders of their lives.
The largest known tusk of any elephant relative is that of a Columbian mammoth which measured 16 feet. The largest wooly mammoth tush measured almost 14 feet and weighted 185 pounds One fossil find is of a pair of mammoth skulls with their tusks intertwined, perhaps representing a tragic end to intermale combat (Lister, 1994).
The several species of mammoth which reached North America included the smaller Mammuthus meridionalis which stood about 12 feet tall, possessed simpler teeth, lacked the high back and domed cranium of later mammoths, and may have lacked thick body hair. This species is known from Europe and from sites throughout the North America ranging from Idaho to Arizona and Florida. Mammoths were apparently able to thrive in plains habitats where the browsing mastodons were not well adapted. From this species evolved the larger Mammuthus columbi which stood about 13 feet tall, possessed teeth with a greater number of ridges, and possessed a sloping back and more of a domed cranium. The wooly mammoth, Mammuthus primigenius typically stood about 10 feet tall and evolved from the much larger Mammuthus armeniacus (which had evolved from Mammuthus meridionalis). Their adipose layer of 3 ½ inches helped to insulate them. Tusks of the wooly mammoth are often worn in a way to suggest that they could clear snow in order to reach their food (Kurten, 1988).
Contrary to some erroneous statements, frozen Siberian mammoths were not preserved in a single catastrophic event. These mammoths were frozen as isolated incidents during a period from earlier than 44,000 years ago to about 11,400 years ago and many were already partially decomposed at the time in which they froze. Mammoths were hunted by early inhabitants of Europe and Asia and more than 360 artistic representations of mammoths are known. Mammoth ivory was often used as a material for carving. Both Neanderthals and modern humans used mammoth bones as building materials to construct huts (Lister, 1994). Archaeological evidence indicates that Paleoindians hunted both mammoths and mastodons. Most, if not all, North American mammoths and mastodons were extinct by 11,000 years ago (Kurten, 1988).
There was considerable skull variation between mammoth species and even within a mammoth species.
The earliest true elephants included Primelephas and Stegotetrabelodon from about 6 million years ago (Lister, 1994). There are three modern species of elephant: the Asian elephant, the African forest elephant, and the African savannah elephant ( Roca, 2005).
Elephant cheek teeth can be enormous, weighing up to 5 kg. There is not enough room in the cheek for all the cheek teeth at any one time and, as a result, an elephant will have 5-6 sets of teeth over the course of its life. The penis can measure 1 meter, the clitoris 4 cm. Elephants can vary their skin/body temperature within a few degrees of their average body temperature.
The gland between the eye and ear (called the temporal gland, known only in elephants) produces secretions which relay information about dominance and can be detected by the vomeronasal organ(Shoshani, ).
Elephant tusks are lateral maxillary incisors. A tush forms as the deciduous tooth equivalent to the tusk, but it does not erupt and is resorbed after the development of the tusk (Raudenheimer, 2000). In female Asian elephants, the tusks may be small, vestigial, or even absent while tusks may be of equal length in both genders of African elephant. One hybrid is known to have been born between African and Asian elephants, although it died after 10 days (Shoshani, p. 52).
At least 12 genera of elephants from
four families are known from