The Pleistocene The final burst and then collapse

About 2 My ago, the world entered an even cooler period of alternating glacial and interglacial phases. The Pleistocene, which lasted until 10,000 years ago, is sometimes called the Ice Age, but the intense periods of cold were punctuated by warm episodes called interglacials, during which the climate was similar to that of the present. The warm periods were relatively short, lasting only a few thousand or tens of thousands years every 100,000 years or so. At peak glaciation, the polar ice sheets expanded; during the last full glaciation (20,00016,000 years ago), ice covered Canada, crept into the northern United States, and overran Scandinavia and the British Isles. Average global temperatures at higher latitudes were 5°C lower than present-day values. There have been 20 or more cold-warm oscillations since the beginning of the Pleistocene.

The frigid world of the Ice Age, with contrasting climate between the poles and the equator, had some of the most inhospitable habitats still seen today. Cold desert or tundra at higher latitudes and hot deserts in the tropical belt are symptomatic of the most arid habitats the globe has seen. Tropical forests disappeared altogether in places or were squeezed into pockets—the so-called Pleistocene refugia. At peak glaciation, the global sea level was much lower, providing land bridges across island archipelagos such as the Sunda shelf (connecting mainland Southeast Asia to Java, Sumatra, Borneo, and other islands). New opportunities were created for the migration of mammals and their subsequent evolution.

The start of the Pleistocene has been variously dated at 1.7 to 2 My ago or even earlier, depending on the evidence for the first glacial event in different parts of the world. The Pleistocene witnessed a great geographical expansion of the elephantids and a high rate of speciation and phyletic evolution. Rapid climatic change, habitat isolation accompanying the glacial events, and increased competition among species may have contributed to this rapid evolution. At the end, the curtains also came down swiftly on a bewildering array of proboscideans, from dwarf elephants not unlike the moeritheres in size to the towering mammoths.

A primitive stage of Loxodonta atlantica appeared by early Pleistocene in southern Ethiopia. The genus Loxodonta never left the African continent. As with L. adaurora, L. atlantica became scarce in eastern Africa with the rise of Elephas recki. By the middle Pleistocene, however, L. atlantica became the dominant elephant in northern and southern Africa. In the later Pleistocene, L. atlantica disappeared concurrent with the rise of E. iolensis, presumably a derivative of E. recki. It was only after the extinction of Elephas in Africa that Loxodonta reemerged as the dominant elephant in the two forms known today.

Elephas evolved more rapidly. Although it reached a dead end in Africa, with the last recorded species being E. iolensis, the genus continued its march across Eurasia. A derivative of one of the middle stages of E. recki, possibly E. recki atavus, probably migrated through the Middle East into Eurasia, where it appeared as E. namadicus during the later Pleistocene and the closely related E. antiquus in Europe. This lineage is also commonly known as Palaeoloxodon. Fossils of E. namadicus are widespread, from Germany, through India (Nar-mada and Godavari River valleys), and to Java, southern China, Mongolia, and Japan. Elephas namadicus showed further specialization in skull and dental characters. Its molars were narrower, higher crowned, and more progressive in number and spacing of plates.

It was, however, the earlier migration of Elephas into Asia that finally led to the modern Elephas maximus. The late Pliocene migration of a derivative of E. ekorensis resulted in new phyletic lines. The first species to appear in Asia was E. planifrons, well known from the Siwalik Hills in the Indian subcontinent. This species is also found in deposits over a wide area, from Bethlehem in the west to the island of Java in the east. Depending on its evolutionary stage, the molars of E. planifrons are variable. The species that was the direct ancestor of the modern Asian elephant is, however, E. hysudricus, also from the Siwalik beds. Some paleontologists considered E. hysudricus to be descended from E. planifrons, but their contemporaneous occurrence in the Siwaliks suggests that they had a common ancestor. According to Vincent Maglio, it seems probable that both E. planifrons and E. hysudricus had a common ancestry from the E. ekorensis-recki complex, with E. planifrons being more conservative, while E. hysudricus evolved rapidly, as did E. recki. The discovery of E. platy-cephalus from India poses a problem. This later Pleistocene elephant, with a flattened primitive skull and small tusks, was probably an offshoot of E. plani-frons.

Elephas hysudricus spans a relatively short period during the early Pleistocene in the Siwalik and upper Irrawady (Myanmar) deposits. The most striking feature of this elephant was the greatly expanded parietal and occipital regions of the skull. Being even taller than the modern Asian elephant, E. hysudricus thus had two very prominent domes or "bumps" on the top of the head, a feature that is retained, although less prominently, in E. maximus. Elephas hysudricus spread further east and evolved into E. hysudrindicus, known from Java. The last species converged toward the earlier E. recki type of skull, in some ways reversing the overall evolutionary trend. Elephas maximus is believed to be derived from E. hysudricus, after the mid-Pleistocene, by progressing further in dental characters. The number of plates more than tripled in E. maximus compared to the earliest form of this genus. The enamel became thinner and attained its maximum folding, while crown height quadrupled (over Primelephas).

Pleistocene evolutionary changes in the Mammuthus lineage were equally as, if not more, remarkable as those in Elephas. Mammuthus meridionalis, which had evolved from earlier mammoth forms during the late Pliocene, continued to thrive through the earlier Pleistocene in Europe and Asia. By about 1 My ago, the global cooling had further intensified, causing the woodland habitat of the ancestral mammoth to retreat and grassy landscape to take its place. Mammuthus meridionalis responded to this deterioration in climate and vegetation in Europe. Mammoth fossils dating to 750,000 years ago and later show progressively higher tooth crowns, thinner enamel ridges, and more enamel bands or plates per tooth.

The steppe mammoth, M. trogontherii, was clearly recognizable by this time in grass-dominated habitats across Europe. Newer interpretation from the work of Adrian Lister and Andrei Sher suggests that M. trogontherii did not evolve in situ in Europe as previously believed, but migrated from Siberia, where it had arisen earlier, perhaps as early as 1.2 My ago. A 1990 discovery of M. trogontherii at West Runton in England suggests that about 600,000 years ago this species still inhabited some woodlands, but elsewhere in Europe, its association with open habitat is clear.

The evolution of the mammoth culminated in the familiar woolly mammoth, M. primigenius, which first appeared in Europe about 200,000 years ago, but was again seen in Siberia perhaps 800,000 years ago. The early onset and persistent severity of glacial conditions in northeastern Siberia may have triggered this evolutionary trend.

Although smaller than the steppe mammoth, the woolly mammoth represented the most advanced adaptations among the proboscideans in skull and dental characters, no doubt driven by the most inhospitable habitat in which a proboscidean had ever survived. Mammuthus primigenius not only possessed molars with the highest crowns among the elephantids, as did M. trogontherii, but also had molars with even thinner enamel and up to 26 enamel ridges. To accommodate the higher-crowned teeth, the skull morphology changed through a deepening of the upper and lower jaws. The top of the skull also became more prominent (single dome, unlike the double-dome Elephas), thus providing greater area for attachment of the tendons and muscles needed to support its enormous tusks. To cope with the frigid cold of the last Ice Age, M. primigenius had a coat of thick fur, as entire frozen carcasses in the tundra have shown.

About 1.8 My ago, M. meridionalis or the advanced early Siberian M. tro-gontherii-like mammoth migrated across the Bering Strait during a period of low sea level into North America. Here, it underwent autochthonous speciation to give the Columbian mammoth, M. columbi. The Columbian mammoth was one of the largest of the mammoths, standing about 4 m high, but it lacked the dense fur of the woolly mammoth as it lived in a more pleasant southern habitat. However, it possessed the imposing, spirally curved tusks characteristic of the mammoths. The range of this mammoth extended south to Mexico, the farthest that Mammuthus was to reach after its migration out of Africa.

Some paleontologists describe two other mammoth species for North America, the imperial mammoth, M. imperator, with characters between M. meridionalis and M. columbi, and M. jeffersonii, which seems more advanced than M. columbi. These two are of uncertain validity; they may be considered subspecies of M. columbi.

The woolly mammoth, M. primigenius, which ranged widely across northern Europe and Siberia, also eventually migrated across Beringia about 100,000 years ago into North America. This represented the second and last mammoth migration into the New World. As in Eurasia, the woolly mammoth occupied the same northern range of latitudes in North America, maintaining reasonable separation overall from the Columbian mammoth, which had already established itself further south, although some overlap did occur.

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