Some of the most fascinating life-forms ever to live on Earth evolved during the Mesozoic Era. One group of organisms you’re familiar with – the dinosaurs – appeared during this geologic era.
The Mesozoic Era, or era of middle life, began about 245 million years ago. At the beginning of the Mesozoic Era, all continents were joined as a single landmass called Pangaea. Pangaea separated into two large landmasses during the Triassic Period. The northern mass was Laurasia, and Gondwanaland was in the south. As the Mesozoic Era continued, Laurasia and Gondwanaland broke up and formed the present-day continents.
Species that survived the mass extinctions of the Paleozoic Era adapted to new environments. Recall that the hard scales of a reptile’s skin help to retain body fluids. This trait, along with the hard shell of their eggs, enabled them to readily adapt to the drier climate of the Mesozoic Era. They became the dominant animal life-form in the Jurassic Period. Some of the reptiles evolved into archosaurs, the common ancestor of crocodiles, dinosaurs, and birds.
What were the dinosaurs like? Dinosaurs ranged in height from less than one meter to enormous creatures like Apatosaurus and Tyrannosaurus. One species of tyrannosaur stood as tall as a two-story building. Some dinosaurs ate meat, whereas others ate only plants.
The first small dinosaurs appeared during the Triassic Period. Larger species appeared during the Jurassic and Cretaceous Periods. Throughout the Mesozoic Era, new species of dinosaurs evolved as other species became extinct.
Recent studies indicate that dinosaurs may not have been cold-blooded, as are present-day reptiles. Tracks left in the mud by reptiles are usually close together. This indicates that reptiles generally move very slowly. Some dinosaur tracks that have been found indicate that they were much faster than most of the cold-blooded reptiles. This faster speed would be expected of warm-blooded animals, which need speed to be successful in hunting. Gallimimus was four meters long and could reach speeds of 80 km/h, as fast as a modern race horse.
Good Mother Dinosaurs
The fossil record indicates that some dinosaurs nurtured their young and traveled in herds with adults surrounding their young. One such dinosaur is Maiasaura. This dinosaur built nests in which it laid its eggs and raised its offspring. Nests have been found in clusters, indicating that more than one family of dinosaurs built in the same area. Some fossils of hatchlings have been found very close to the adult animal. This has led some scientists to hypothesize that some dinosaurs nurtured their young. In fact, Maiasaura hatchlings may have stayed in the nest while they grew in length from about 35 cm to more than one meter.
Other evidence that leads scientists to think that dinosaurs may have been warm-blooded has to do with their bone structure. Cross sections of the bones of cold-blooded animals exhibit rings similar to growth rings in trees. The bones of some dinosaurs don’t show this ring structure. Instead, they are similar to bones found in birds and mammals. These observations indicate that dinosaurs may have been warm-blooded, fast-moving, nurturing animals somewhat like present-day mammals and birds. They might have been quite different from present-day reptiles.
The first birds appeared during the Jurassic Period of the Mesozoic Era. The animal Archaeopteryx had wings and feathers like a bird but teeth and claws like a meat-eating dinosaur. Archaeopteryx may not have been a direct ancestor of today’s birds. But modern birds and Archaeopteryx probably share a common ancestor. Scientists think that birds evolved from dinosaurs. Some scientists even think that birds are dinosaurs, evolved from the advanced theropod called Troodon. Theropods form a group of meat-eating dinosaurs that walked mainly on their hind legs.
Mammals first appeared in the Triassic Period. Mammals are warm-blooded vertebrates that have hair or fur covering their bodies. The females produce milk to feed their young. These traits have enabled mammals to survive in many changing environments.
Please answer in one to two complete sentences.
1. What does the term Mesozoic mean? How long ago did it begin?
2. What did the continents look like at the beginning of the Mesozoic Era? What was this landmass called?
3. What happened to this landmass? What was in the North? In the South?
4. What is unique about reptiles that allowed them to adapt to the drier climate of the Mesozoic Era?
5. Where did the archosaurs come from that eventually evolved into crocodiles, dinosaurs, and birds?
6. When did the first small dinosaurs appear (what period?)
7. Why did scientists feel that dinosaurs may not have been cold-blooded like reptiles of today?
8. Why did scientists feel that some dinosaurs may have been good mothers that nurtured their young and traveled in herds with adults surrounding their young?
9. When did the birds first develop? Where do scientists think that birds evolved from?
10. When did mammals first appear? What makes mammals different?
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