The archosaurs, or "ruling reptiles," represent a formidable line of tetrapods that includes dinosaurs, pterosaurs, crocodiles, and birds. The archosaurs are distinguished from other diapsids by a number of anatomical features; these included an additional opening in the side of the skull, just in front of the orbit. Some lines of archosaurs also developed a more upright posture over time, with forelimbs and hind limbs positioned more squarely beneath the body. One of the first known archosaurs is Protorosaurus, from the Late Permian of Germany, a 6.5-foot (2 m) long reptile that was already beginning a trend for large size. Protorosaurus had a long neck, a long snout, and a narrow jaw filled with small, sharp teeth. In many respects, it resembled a prototype of the modern-day monitor lizard.

During the first half of the Triassic Period, archosaur predators grew larger, fiercer, and more widespread; they effectively took over the ecological niche left vacant by the synapsids at the end-Permian extinction. Archosaurian body plans diverged in many directions. The largest land predators included Erythrosuchus and its kin, from the Early Triassic of South Africa. These animals grew to 16 feet (5 m) long and were equipped with a deep, bulky head with massive jaws and bone-crushing teeth. Other archosaurian predators of the Early to Middle Triassic included the crocodilelike Chasmatosuchus (South Africa and Asia), which measured about 6.5 feet (2 m long).

Some archosaurs were plant eaters, and they, too, developed into fairly large animals by the Late Triassic.

As was the case with the early diapsids, archosaurian evolution is also known for some extraordinarily strange experiments. Tanys-tropheus, an archosaur from the Middle Triassic of Israel, Germany, and Switzerland, had a short body and a neck so long that it almost defies believability. The animal was about 10 feet (3 m) long, six feet (2 m) of which consisted of a long, stiff neck. The neck of Tany-stropheus is itself a puzzle, as it is composed of only 10 elongated vertebrae. The mouth of Tanystropheus was equipped with small, carnivorous teeth. Many paleontologists assume that this animal lived in a near-shore environment, where it probably could take to the water to support its gawky body.

Another bizarre early archosaur is Longisquama, from the Early Triassic of Asia. This small creature measured only six inches (15 cm) long and was most akin to a lizard except for its unusual dorsal spines. Mounted on the back of Longisquama were a dozen or more stiff, bony frills arranged in a single row above the spine. They resembled a row of upright hockey sticks, and their purpose is not fully understood. The most likely explanation is that the frill row had no more function than that of display, either to ward off a predator, to attract a mate, or both. Some paleontologists have also suggested that these frills might have aided in heat exchange, could somehow have been used in gliding, or may have been some form of proto-feathers.

Another notably small archosaur was Euparkeria, a two-foot (60 cm) long creature found in Early Triassic rocks of South Africa.

Euparkeria was another important archosaur. Because it had hind legs that were one-third longer than its front legs, this tiny reptile could sometimes scamper on two feet—showing an early appearance of bipedalism.

With long and powerful hind legs, a strong neck, and a mouth full of recurved, biting teeth, Euparkeria had many affinities with the first dinosaurs that arose in the Late Triassic. Having hind legs that were one-third longer than its front legs suggests that this tiny powerhouse could sometimes scamper on two feet, an early appearance of bipedalism. This was another trait later perfected by its archosau-rian relatives, the meat-eating dinosaurs.

The roots of the archosaurs, as represented by these earliest representatives, provided an evolutionary workshop for the spectacular rise of crocodiles, flying reptiles (pterosaurs), and dinosaurs, beginning in the Late Triassic Epoch.

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