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Lab 9 - Reptiles and Birds

Lab 9 - Reptiles and Birds - LAB 9 REPTILES EARLY SYNAPSIDS...

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LAB # 9 – REPTILES, EARLY SYNAPSIDS AND BIRDS CLASS REPTILIA The oldest-known reptiles are found in Pennsylvanian-age deposits from Nova Scotia, Canada. These "stem reptiles" ( cotylosaurs ) possessed labyrinthine infolding in their teeth and assumed the sprawled posture of their labyrinthodont amphibian (Seymouriamorph) ancestors. However, these reptiles showed vast improvements over the amphibia in their adaptation to land- dwelling through the evolution of internal fertilization and development of the amniotic egg . This type of egg could be laid on land and through its development the reptiles freed themselves from dependence on the aquatic environment. The cotylosaurs were ancestral to the other reptiles. By middle Mesozoic times, reptiles completely dominated terrestrial environments and other members of the group reinvaded the oceans and developed powers of flight. Reptile evolution involves a number of distinct fossil and modern groups. The classification of the reptiles into major categories is primarily based on the position and numbers of temporal openings on the sides of the skull. The anapsids , which include the turtles and primitive reptile groups such as the cotylosaurs, lack temporal openings. Diapsids , including dinosaurs, the ancient flying pterosaurs and all modern reptiles (except turtles) primitively have two temporal openings. The synapsids , including the fin-backed pelycosaurs and mammal-like reptiles (therapsids), had a single lower temporal opening. The euryapsids , including the ancient swimming plesiosaurs and ichthyosaurs, had a single upper temporal opening. Aquatic Reptiles Several groups of reptiles invaded aquatic habitats. The most fish-like in appearance were the ichthyosaurs (Triassic- Cretaceous), whose habits were probably very similar to those of modern dolphins. These marine reptiles showed advancements over other types in the bearing of live young. Plesiosaurs (Jurassic- Cretaceous) were long-necked, large (15-40 feet) marine reptiles which propelled themselves through the water by means of large flippers. Another group of marine reptiles were the mosasaurs , whose fossils are restricted to Cretaceous rocks). Mosasaurs were a group of giant aquatic lizards which evolved from primitive varanid lizards. They had large jaws equipped with impressive teeth and were carnivores, a habit of most living and extinct aquatic reptiles. The most successful aquatic reptiles are the turtles (Triassic- Recent). Modern members of this group are found in marine, freshwater and terrestrial environments and are quite common fossils. Terrestrial Reptiles A number of various reptile groups have "adapted" to a land existence. During the Triassic, the primitive reptile groups were replaced by the "ruling reptiles", or archosaurs . This group includes the dinosaurs, pterosaurs, crocodiles and primitive ancestral types termed the thecodonts . The thecodonts dominated the Middle and Upper Triassic but during the close of the period were replaced by the dinosaurs and crocodilians. The thecodonts were ancestral to these "ruling reptiles".
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