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How did the reptiles give rise to the dinosaurs?

It was the diapsid group of the reptiles that eventually produced the dinosaurs. The jaw muscles in these reptiles were attached to the two openings on each side of the skull, giving their jaws better leverage and strength. Sometime in the Permian period, the diapsid line branched into two groups: lepidosaurs and archosaurs. The lepidosaur group evolved into todays lizards and snakes; the archosaurs eventually gave rise to the dinosaurs.

What were early archosaurs like?

One of the first archosaurs and probably typical of many archosaurs was the bigheaded Shansisuchus, a Middle Triassic period creature that lived in what is now China about 220 million years ago. It was about 7 feet (2.2 meters) long and had long back legs and short front legs.

What were other archosaurs like?

Archosaurs varied in many ways. Some, like the 1-foot- (30-centimeter-) long Gracilisuchus of the Middle Triassic could run on their hind legs over short distances; others, like the 12-foot- (4-meter-) long Chasmatosaurus, were heavy carnivores that walked on all fours. The 2-foot- (0.7-meter-) long Lagerpeton had very peculiar hind limbs: its feet had an elongated fourth toe that may have been used for perching.

What are thecodonts?

Thecodonts (socket teeth) were once thought to be a group living from the Late Permian to the end of the Triassic period a division within the archosaurs that eventually gave rise to the dinosaurs, and perhaps crocodiles, birds, and pterosaurs. It is now considered an obsolete term and there are definite reasons why.

Just like many fields of science, the study of early dinosaurs constantly changes not to mention that differences in opinion abound. By the mid-1980s, many scientists proposed that there was not a group called the thecodonts that evolved from the archosaurs. In fact, most scientists now believe Thecodontia as a group does not exist. Scientists cite that some of the animals listed in this group are more closely related to the crocodiles, some to dinosaurs, and some to the entire group of archosaurs. Other scientists believe that while thecodonts may not be a true group, the term was a handy way to describe certain creatures with socketed teeth within the archosaurs.

What was the next stage in the evolution towards true dinosaurs?

As time went on, another phase of dinosaur evolution took place. The animals skeletal structure changed, especially the hips, which gave many of the dinosaurs the ability to run on two legs. Euparkeria was a small lizard-like reptile that lived on land and walked on all fours, but it could run on two legs when in a hurry. Further along in time was Ornithosuchus, or bird crocodile, which was a two-legged predator. Its front limbs were too small to use for walking on all fours, and its thighs were nearly vertical. From this evidence, it appears that Ornithosuchus walked only on its hind legs.

What are some of the earliest known primitive dinosaurs?

Two of the earliest known primitive dinosaurs were both fast-running carnivores: the Eoraptor, or dawn hunter, a small, three-foot- (one-meter-) long dinosaur that weighed about 22 pounds (10 kilograms); and the Herrerasaurus, measuring from 9 to 18 feet (3 to 6 meters) long. Both lived approximately 230 million years ago in the area known today as Argentina. Still another earlier dinosaur was the Staurikosaurus, a Late Triassic (around 225 million years ago) carnivore found in Brazil. It was about 6.5 feet (2 meters) long, with a fully upright gait that allowed for speed. After the evolution of these early specimens, other dinosaurs evolved quickly, becoming more and more diverse, and reaching out into all ecological niches.

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