The Neogene

Abstract

The Neogene Period consists of the Miocene (23–5Ma) and Pliocene (5–2.6Ma) epochs, the second half of the Cenozoic Era. Originally, it also included the Pleistocene (2.6Ma to 10,000years ago) and the Holocene (the last 10,000years), although there is currently no consensus on this terminology. During the Miocene, the continents continued and finished their movement from their origin in Pangea in the Permian to reach their present positions. India continued to press into Asia, uplifting the Himalayas and the Tibetan Plateau to their present elevation by the Miocene, if not sooner. Africa pushed against Europe, uplifting the Alps and producing huge faults and folds. Eventually, the Tethys Seaway from Gibraltar to Indonesia was completely destroyed when Arabia docked with Asia in the middle Miocene, enclosing the Mediterranean. In the late Miocene, the Mediterranean dried up completely and sea level dropped over 2.5km (8500ft); this was the Messinian Salinity Event. It accumulated enormous volumes of salt and gypsum, carving deep canyons beneath the Nile and Rhône rivers, before catastrophically flooding and filling with normal seawater by the beginning of the Pliocene. In South America much of the eastern part of the continent (including the Amazon basin) flooded during the Miocene, while the Andean volcanic chain built up to its present size. The closure of the Panamanian land bridge in the mid-Pliocene not only ended the isolation of South American land mammals and allowed the invasion of North American groups (which soon came to dominate and drove the natives to extinction), but also cut off the flow of Caribbean water into the Pacific. This amplified the warm-water flow to the Gulf Stream, strengthened the global ocean conveyor belt, and triggered the growth of the Arctic ice cap in the mid-Pliocene. Meanwhile, North America switched from Andean-style subduction to a peculiar tectonics of the San Andreas transform, which produced a mantle bulge behind it that helped create the Basin and Range Province, the uplift of the Rocky Mountains and Colorado Plateau, and the south-to-north shutoff of the Cascade Range. Early Miocene marine faunas were adapted to warmer waters than those of the Oligocene, culminating in the peak of marine diversity during the Middle Miocene Climatic Optimum. But, at 14Ma, the warm early-middle Miocene abruptly ended with a cooling event that expanded the East Antarctic ice sheet and cold bottom waters, and severely affected the warm-water marine organisms. The cause of this cooling is controversial, although it clearly involved a large drop in atmospheric carbon dioxide, possibly caused by the weathering of the rapidly rising Himalayas and Tibetan Plateau, which absorb carbon dioxide in the weathering process. It also caused the spread of C4 plants with large areas of savanna grasslands. In the Old World, these grasslands were dominated by cattle and antelopes, giraffids, mastodonts, diverse horses, hippos, rhinos, and pigs, and preyed on by cats and hyenas, much like the modern East African savannas. In North America, grasslands also developed, but they were inhabited by North American ecological equivalents that convergently evolved on the African-Eurasian grassland mammal, including mastodonts instead of elephants, camels performing the roles of giraffes and antelopes, pronghorns instead of true antelopes, three-toed horses instead of zebras, rhinos shaped like hippos as well as some shaped like modern rhinos, peccaries instead of pigs, and predators dominated by “false cats,” dogs performing the roles of hyenas, and the first bears and raccoons. This all came to an end in the Miocene-Pliocene transition, when many of these savanna lineages vanished during the cooling and drying triggered by the Messinian event, and a much less diverse fauna of horses and camels survived into the steppes of the Pliocene in North America.