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of these adaptations is lacking, the presence of many modern amphibian species that retain their eggs within their bodies for long periods of time, especially in times of drought, attests to the feasibility of this evolutionary scenario. Additionally, some modern salamanders, exemplified by plethodontids, lay eggs and their embryos develop completely in non-aquatic environments without any larval stage; among their preferred habitats are inside moist tree trunks or logs. Interestingly, skeletal remains of the Carboniferous amniotes Westlothiana, Hylonomus, and Paleothyris were all discovered within Carboniferous fossil tree trunks, so they may have occupied the same niche as modern plethodontids.

Amniota as a clade is synonymous with the older Linnaean (gradistic) classification of Class Reptilia, but with some qualifications. Under gradistic classification, reptiles, such as lizards, snakes, turtles, and crocodiles, were traditionally regarded as scaly vertebrates; most have four legs (except snakes, of course) and reproduce by laying enclosed eggs. This classification excludes mammals and birds, but cladistics recognizes shared derived characters, meaning that amniotes include all descendants from an ancestral amniote. As a result, Amniota, which includes reptiles, mammals, and birds, is a monophyletic clade. In contrast, reptiles actually comprise a paraphyletic group, not a clade, because it does not include all of its descendants, such as mammals and birds. The term "reptile" has been long associated only with lizards, snakes, turtles, alligators, crocodiles, pterosaurs, extinct marine reptiles (such as euryapsids, discussed later), and dinosaurs, among others. Nevertheless, change is a part of science (Chapter 2) and part of that change is exemplified through new classification schemes. So now most paleontologists recognize that mammals and birds are also "reptiles" in a cladistic sense.

Once amniotes had developed by the Carboniferous Period, their adaptation into numerous terrestrial ecosystems and consequent diversification was relatively rapid and impressive. The major basis for recognition of their diversification is seen in the arrangement of skull bones, specifically the presence and positions of temporal fenestrae. Amniotes can be subdivided into three major clades on this basis and other characteristics - Anapsida, Synapsida, and Diapsida (Fig. 6.5).

Fossil and modern turtles best represent anapsids, which lack temporal fenestrae. The first reptiles had anapsid skulls, thus turtles exhibit a plesiomorphic trait for amniotes in general. Synapsids and diapsids evidently evolved from a common

FIGURE 6.5 Three skull types, with positions of temporal fenestra outlined, characterizing the Anapsida, Synapsida, and Diapsida in the context of a cladogram, showing their hypothesized evolutionary relationships.

Eye socket

Fenestrae

Eye socket

Eye socket

Fenestrae

Eye socket

FIGURE 6.5 Three skull types, with positions of temporal fenestra outlined, characterizing the Anapsida, Synapsida, and Diapsida in the context of a cladogram, showing their hypothesized evolutionary relationships.

Amniota

FIGURE 6.6 Dimetrodon, a Permian synapsid and pelycosaur that was carnivorous, but definitely was not a dinosaur. Denver Museum of Science and Nature, Denver, Colorado.

FIGURE 6.6 Dimetrodon, a Permian synapsid and pelycosaur that was carnivorous, but definitely was not a dinosaur. Denver Museum of Science and Nature, Denver, Colorado.

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