Amniotes include most of theland-dwelling vertebrates alive today, namely, mammals, turtles, Sphenodon,lizards, crocodylians and birds. It is a diverse clade with over 20000 living species. Amniotes include nearly all ofthe large plant- and flesh-eating vertebrates on land today, and they liveall over the planet in virtually every habitat. They also sport disparateshapes - chameleons, bats, walruses, Homo sapiens, soft-shelled turtles,ostriches and snakes are but a few examples - and they include some of thesmallest (sphaerodactyline geckoes) and largest (mysticete whales)vertebrates (Figs. 1 and 2). Although fundamentally land dwellers, several cladessuch as ichthyosaurs, plesiosaurs, pinnipeds and cetaceans have returned tothe sea. A few forms are gliders - the Flying Dragon lizards, HoneyCreepers, and Flying Squirrels - and powered aerial flight has originatedthree separate times, first in pterosaurs, then in birds, and later stillin bats.
Figure 1. Two extant amniotes (diapsids). The rattlesnake can detect its prey atnight using an infrared-sensitive organ that allows it to detect the warmbody of small mammals. It then kills its prey with it poisonous fangs.Parrots eat nuts and fruits. Pictures copyright © 1996 Michel Laurin.
An extensive fossil record documents the origin and early evolution ofAmniota, and that record has played a key role in understandingphylogenetic relationships among the living amniotes (Gauthier et al.,1988b). The oldest amniotes currently known date from the MiddlePennsylvanian locality known as Joggins, in Nova Scotia (Carroll, 1964).The relationships of these fossils indicate that amniotes first divergedinto two lines, one line (Synapsida) that culminated in living mammals, andanother line (Sauropsida) that embraces all the living reptiles (includingbirds). One Joggins fossil, the "protorothyridid" Hylonomus, appears to bea very early member of the line leading to Sauria (Crown-clade diapsids),the clade encompassing all living diapsids. This suggests that the more inclusive clade of which turtles (Testudines) are part (Anapsida) in most morphological phylogenies had diverged as well, even though its current record extends back only to the Lower Permian (Laurin & Reisz, 1995).
An older amniote (from the Lower Carboniferous) was reported (Smithson, 1989). However, more recent studies suggested that it was only a close relative of amniotes (Smithson et al., 1994), and the latest study even suggested that it was more likely to be a stem-tetrapod or an early amphibian than a relative of amniotes (Laurin & Reisz, 1999).
Figure 2. More extant amniotes. The insectivorous elephant shrew (a synapsid) resembles vaguely the earliest placental mammals. The omnivorous terrestrial box turtle Terrapine (an extant anapsid) eats mushrooms, fruits, insects, and worms. Like all turtles, it is not affected by senescence but eventually succumbs to a disease, an accident, or a predator. Pictures copyright © 1996 John Merck.
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