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The first true mammals, themselves a specialized subgroup of Therapsids, also evolved during this period, as well as the first flying vertebrates, the pterosaurs, who like the dinosaurs were a specialized subgroup of archosaurs.The vast supercontinent of Pangaea existed until the mid-Triassic, after which it began to gradually rift into two separate landmasses, Laurasia to the north and Gondwana to the south.Eighteenth century reports describe the displacement of black rats by the more aggressive and larger Norway rats all over Europe (Grzimek 1968, Krinke 2000).
Norway rats arrived in Europe several centuries after the black rats, though the exact date of their arrival is unknown.
Norway rat bones have been discovered at the medieval settlement of Klein Freden near Salzgitter in Northern Germany, which was occupied from the 9th to the 13th centuries AD (König, 2007), and at Bodenteich castle in the district of Uelzen, Lower Saxony, dating to the medieval and post-medieval period.
Selection for tameness leads to changes in hormone chemistry, physiology, and development.
These changes have wide-ranging effects, including changes in coat color.
White rats of European origin were brought to America shortly after this and became the foundation stock of American laboratory rats (Castle 1947).
The oldest strain of inbred rats dates from 1856, when the Jardin des Plantes reported a feeder colony of black hooded rats.
The Wistar rat, and to a lesser extent, the Sprague-Dawley rat, gradually became the most popular rat strains for laboratory research.
The Triassic began in the wake of the Permian–Triassic extinction event, which left the earth's biosphere impoverished; it would take well into the middle of this period for life to recover its former diversity.
True rodents are widely considered to have originated in Asia. Rattus underwent two intense periods of speciation. Black rats originated further south in the in the Indo-Malayan region (Krinke 2000, Walker 1964).
Rodents first appear in the fossil record at the end of the Paleocene and earliest Eocene in Asia and North America, about 54 million years ago (Meng et al. These original rodents were themselves descended from rodent-like ancestors called anagalids, which also gave rise to the Lagomorpha, or rabbit group. The split between these two species occured around 0.5 million years ago (Verneau et al. Today, there are 51 species within the genus Rattus. One about 2.7 million years ago (MYA), another about 1.2 million years ago which may still be ongoing. Both species of rats began to live in human homes, buildings, and ships, in a human-dependent association called commensalism.
Albino rats were brought into laboratories for physiological studies as early as 1828.