Evolution of Amniotes

The first amniotes evolved from amphibian ancestors approximately 340 million years ago during the Carboniferous period. The early amniotes diverged into two main lines soon after the first amniotes arose. The initial split was into synapsids and sauropsids. Synapsids include all mammals, including extinct mammalian species. Synapsids also include therapsids, which were mammal-like reptiles from which mammals evolved. Sauropsids include reptiles and birds, and can be further divided into anapsids and diapsids. The key differences between the synapsids, anapsids, and diapsids are the structures of the skull and the number of temporal fenestrae behind each eye (Figure 1).

The illustration compares three different skull types. All three skulls are elongated and similar in shape; the only difference between them is the number of holes behind the eye. The anapsid skull (left) has no openings. The synapsid skull (middle) has one opening, and the diapsid skull (right) has two openings, one on top of the other.

Figure 1. Compare the skulls and temporal fenestrae of anapsids, synapsids, and diapsids. Anapsids have no openings, synapsids have one opening, and diapsids have two openings.

Temporal fenestrae are post-orbital openings in the skull that allow muscles to expand and lengthen. Anapsids have no temporal fenestrae, synapsids have one, and diapsids have two. Anapsids include extinct organisms and may, based on anatomy, include turtles. However, this is still controversial, and turtles are sometimes classified as diapsids based on molecular evidence. The diapsids include birds and all other living and extinct reptiles.

The diapsids diverged into two groups, the Archosauromorpha (“ancient lizard form”) and the Lepidosauromorpha (“scaly lizard form”) during the Mesozoic period (Figure 2). The lepidosaurs include modern lizards, snakes, and tuataras. The archosaurs include modern crocodiles and alligators, and the extinct pterosaurs (“winged lizard”) and dinosaurs (“terrible lizard”). Clade Dinosauria includes birds, which evolved from a branch of dinosaurs.

The trunk of the amniote phylogenetic tree is the ancestral amniote. Initially, the tree branches into diapsids, anapsids, and synapsids. Synapsids give rise to mammals, which are therapsids. Anapsids are all extinct. Diapsids are subdivided into two groups, lepidosaurs and archosaurs. Lepidosauria includes plesiosaurs, ichthyosaurs, Sphenodontia and Squamata, which includes lizards and snakes. Archosauria branches into Crocodilia, pterosaurs, dinosaurs, and birds.

Figure 2. This chart shows the evolution of amniotes. The placement of Testudines (turtles) is currently still debated.

Practice Question

Members of the order Testudines have an anapsid-like skull with one opening. However, molecular studies indicate that turtles descended from a diapsid ancestor. Why might this be the case?