Archosauria (the "ruling reptiles")
is a major group of diapsids, differentiated from the other diapsids by the presence of single openings in each side of the skull, in front of the eyes (antorbital fenestrae), among other characteristics.
This continues the tetrapod trend of the reduction of skull bones by
the fusion of multiple bones and the opening of fenestrae in the skull.
This helps to lighten the skull, provides more room for muscles and other
tissues, and allows more skull flexibility (kinesis) when eating. Other
typical archosaurian characteristics include another opening in the lower
jaw (the mandibular fenestra), a high narrow skull with a pointed snout,
teeth set in sockets (called thecodont tooth implantation), and a modified
The ancestral archosaurs probably originated some 250 million years or so ago, in the late Permian period. Their descendants (such as the dinosaurs) dominated the realm of the terrestrial vertebrates for a majority of the Mesozoic Era. Today, only the birds and crocodilians exist to provide a glimpse into the past glory of archosaurs.
The archosaurs ('ruling reptiles') were the direct ancestors of the dinosaurs. They evolved from more primitive reptiles in the Triassic, following the Permian mass extinction. The evolution of the archosaurs is a very significant event in the history of life on land, since they not only led to the evolution of dinosaurs and birds, but also to the pterosaurs and crocodiles.
The early archosaurs took over the habitats left vacant by the large herbivores and carnivores that died out at the end of the Permian. Early forms such as the carnivore Proterosuchus (about 1m long) had a sprawling walk (similar to modern lizards). Some archosaurs such as Vjushkovia and Erythrosuchus were large carnivores (up to 5m long) with a more erect gait, that fed on other large plant-eating reptiles.
Later in their evolution, the archosaurs split into two 'branches' on
the evolutionary tree. This split occurred in the middle to late Triassic
period (about 220 million years ago). One of these branches led to the
evolution of the crocodiles, while the other led to the evolution of the
dinosaurs and birds. It has been suggested that the evolution of the dinosaurs
from archosaurs was related to changes in the way the beasts moved, from
a sprawling walk similar to lizards, to a semi-erect posture as in modern
crocodiles, to the fully erect stance of the dinosaurs.
Tetrapoda: "four feet"
Terrestrial vertebrates have a worldwide distribution. The earliest members of this group were moderately large (1-2.5 m body length). The oldest known skeletal remains of terrestrial vertebrates were found in the Upper Devonian of East Greenland (Clack, 1994). The presence of Lower to Middle Devonian trackways in Australia has led to suggestions that this group may have originated in the Lower Devonian, at least 400 million years ago (Warren et al., 1986), but digits are not visible in these impressions, so these trackways may have been left by other sarcopterygians.
The largest group of terrestrial vertebrates is Tetrapoda. Tetrapoda means "four feet", and the group was so-named as its members primitively had four limbs, as opposed to fins. This taxon includes about 3000 extant species of amphibians (frogs, salamanders, and caecilians) and approximately 18100 extant species of amniotes (mammals, reptiles, and birds). The number of extinct species of tetrapods is of course unknown, but about half of the currently known species of tetrapods are extinct (Carroll, 1988).
Tetrapods originated no later than the Mississippian (about 350 million years ago), the period from which the oldest known relatives of living amphibians are known. Relatives of amniotes must have been present at that time, but they have not been found so far. The fossil previously reported (Smithson et al., 1994; Carroll, 1995) as an early Mississippian amniote or anthracosaur (Westlothiana) is probably either a stem-tetrapod or an early amphibian (Laurin, 1998a). Stegocephalians (see section on classification below for a definition of this group) originated no later than the Upper Devonian.
Tetrapods range from 9.8 mm (in the frog Psyllophryne didactyla) to 30 m (in the blue whale) in overall length. They have a worldwide distribution and inhabit all major habitats. Most are terrestrial, but several have returned to the aquatic environment in which our distant ancestors lived. Aquatic tetrapods include various salamanders (sirenids, cryptobranchids, proteids, etc.), frogs (pipids), some caecilians (typhlonectids), leatherback turtles, sea snakes, pinnipeds (seals and walruses), and whales. Some tetrapods are capable of flight (birds and bats), while others glide, such as flying squirrels, dermopterans (sometimes called "flying lemurs", even though they are not primates), and the flying dragons (Draco volans).
Therapsids: "mammal-like reptiles"
They are an order of synapsids. Traditionally, synapsids were referred to as reptiles. However, when the term is used cladistically, the taxon also includes the mammals, which are descended from the cynodont therapsids. The evolutionary trend of the therapsids began in the Lowere Permian, this trend starts with a group of pelycosaurs, the Sphenacodontia, a lineage that gave rise to Dimetrodon and its family, gave rise to therapsids. Evidence in favor of this link was their anatomical features such as the skull, and the vertebrae. Therapsids became the dominant land animals in the Middle Permian, which replaced the pelycosaurs. Therapsid temporal fenestrae were larger than those of the pelycosaurs. Thermal regulation (warm-bloodedness) in therapsids probably evolved by the Middle or Upper Permian. Therapsids probably had skin glands, similar to the mammals, rather than scales as in reptiles. Early therapsids did not have fur; the theriodonts are probably the only therapsids known to have had fur, which evolved in the Late Permian. Therapsida consists of three major clades, the dinocephalians, the herbivorous anomodonts and the mostly carnivorous theriodonts, with the carnivorous biarmosuchians as a paraphyletic assemblage of primitive forms. After a brief burst of evolutionary diversity, the dinocephalians died out in the later Middle Permian but the anomodont dicynodonts and the theriodont gorgonopsians and therocephalians flourished, being joined at the very end of the Permian by the first cynodonts.