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摘要
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Abstract
Amniota is a clade noted for the extraordinary ecological and taxonomic diversity of its more than 23,000 living species, comprising mammals and reptiles, including birds. thetaxon is named for the characteristic egg structure in which the amnion membrane forms a PuidAlled cavity that surrounds the developing embryo. theamniote egg is considered one of the key adaptations in the evolution of vertebrate terrestrial life history (Fig. 1). 7is shared-derived character is generally accepted to be one of several key adaptations for enduring the terrestrial life adopted by our amniote ancestors, thereby facilitating the remarkable evolutionary success of the group. Considerable debate has been focused on precisely when our most recent common amniote ancestor lived (1). Tightly constrained radiometric analysis of fossils places the divergence between the Arst undisputed synapsids (mammals which have only one temporal fenestration in the skull) and diapsids (reptiles with two temporal fenestrations) at 306.1 ± 8.5 Ma. 7is dating technique generally exhibits about 1% error (2). theupper bound of this narrow estimate for bird–mammal divergence overlaps with a widely cited value of 310 Ma, based on the approximate methods of Benton (3) and employed by Kumar and Hedges (4) to calibrate a comprehensive molecular timescale for vertebrate evolution. theamniote timetree considered here includes six terminal taxa: Mammalia (mammals), Sphenodontia (the tuatara), Squamata (lizards and snakes), Testudines (turtles), Crocodylia (alligators and crocodiles), and Aves (birds). A combination of molecular and fossil evidence suggests that the major lineages of living reptiles likely originated in the Permian and Triassic (3–5) although exact divergence time estimates vary among the molecular studies used in the present synthesis.
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