ABSTRACT
As successful as amphibians are and have been, they nonetheless require access to water, minimally for reproduction if not also for other phases of life history. Vertebrates would not become a dominant presence on land unless that constraint was alleviated. The solution was the amniotic egg, one with a protected embryo that could be deposited on dry land and that eliminated the larval stage. The amniotic egg underpins the great phyletic and ecological diversity of nonamphibian tetrapods—the turtles, lizards, snakes, birds, and mammals.
Stem amniotes arose in the Late Carboniferous about 320 million years ago. Two major amniote groups emerged—the Sauropsida and the Synapsida. The former includes turtles and the basal and derived groups of reptiles and birds. The latter comprise the mammals and their ancestors.
Turtles are thought to belong to a lineage that differs from diapsid reptiles in respects other than the trademark shell. Temporal fenestrations are absent, for example. However, recent morphological evidence that includes fossils argues that turtles might be diapsids after all.
The shell limits where turtles can live, what they can do, and thus restricts diversity and ecological opportunity. Living turtles belong to either of two groups defined by their method of head retraction into the shell. Plerodira, the side-necked turtles, and Cryptodira, the hidden neck turtles, which are the dominant lineage and includes those species known from Southern California, both terrestrial (four species) and marine (four species).
