By the start of the Middle Triassic Epoch, 245 million years ago, three evolutionary lines of amniotes had become well established. The fundamental anatomical trait used by paleontologists to differentiate these groups is the absence or presence of different numbers of temporal fenestrae—the small holes, or "windows"—in the temple region on the sides and top of the skull. These openings in the skull provided a spot for complex groups of jaw muscles to attach and made the skull lighter. The presence of these temporal fenestrae contributed to the adaptability of some amniotes to varied food sources and improved their innate swiftness without sacrificing bone strength. The result was a blossoming of amniote types and adaptations that ranged from bulky herbivores such as Lystro-saurus (South Africa, India, China, Russia, and Antarctica), with jaws adapted for more efficient chewing of plants, to lightweight but deadly predators such as Euparkeria (South Africa).
The roots of all of today's terrestrial amniotes—vertebrates that protect the embryos of their offspring within the sealed environment of an amniotic egg—are found in three lines of reptiles that originated in the Late Paleozoic and diversified in the Meso-zoic Era.
Anapsida. These are amniotes with no temporal fenestrae, including the earliest reptiles. This group includes the early reptiles Hylonomus and Paleothyris and several other extinct Late Carboniferous-to-Triassic reptiles such as pareiasaurs and procolo-phonids. The group also includes living tortoises and turtles.
Synapsida. These are amniotes (but not reptiles) with one temporal fenestra on each side of the skull, positioned somewhat behind and below the orbit, or eye opening. This group includes all mammals as well as extinct "mammal-like reptiles." Synapsids first appeared in the middle of the Carboniferous Period.
Diapsida. These are amniotes with two temporal fenestrae on each side: a lower one like the one seen in synapsids, and another one just above it, on top of the skull and behind the orbit. Lizards, snakes, crocodylians, and birds are included in this group, as are extinct dinosaurs and pterosaurs (flying reptiles). Diapsids first appeared in the Late Carboniferous Period. The diapsids also include extinct marine reptiles such as nothosaurs, plesiosaurs, and ichthyosaurs, most of which thrived in the Mesozoic Era. Marine reptiles secondarily, and independently of other diapsids, lost one of their temporal fenestrae.
Of these four groups, the Diapsida became the most prevalent amniote group of the Mesozoic. The biggest reptilian success story of all time encompassed the rule of the dinosaurs and their kin. The remainder of this chapter examines the roots, diversification, and relationships of the early archosaurs that led to the rise of the dinosaurs.
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