In the Late Jurassic, lifelike dinosaur model it seems that yet another huge meteorite crashed into the earth, but again dinosaurs show no ill effects. The continents, still under a mild semiarid climate, were reseparating at that time. As a result, new oceans, such as the namow Protoatlantic, began to appear, and more provincial faunas began to develop. The advanced avetheropods had become preeminent, perhaps because of their more sophisticated, birdlike features. one of these was the air sac ventilated lung system which will be described .why advanced theropods developed such sophisticated lungs, which today are found only in birds, is not clear. Certainly not for flight. Besides, bats fly perfectly well with ordinary lungs. The one clear-cut advantage of the avian lung is its ability to extract o4ygen at high altitudes-some birds can fly five miles high.8 But theropods and even early birds were not specialized for high-altirude flight, so it could not have evolved for this purpose.
So yet another question remains unanswered.
Having seen that some predatory dinosaurs became gigantic, we can ask why. why, when mammalian land predators never reached such sizes? In the early Tertiary, some of the fast-running mammalian predators such as Wyoming’s Pochyhyaeno, animatronic dinosaur for sale a member of a primitive group of predators called mesonychids, reached bear dimensions. Only 10,000 years ago a 600-kg, shortfaced bear, Arctodus, lived in North America. Its rather catlike head and long limbs suggest that it was more carnivorous and fleet-footed than living species. Brown and polar bears can reach 800 kg, but the first is mainly herbivorous, and the latter more aquatic than land-dwelling. Dinosaur costume The biggest of all known mammalian land predators lived in Late Tertiary Africa. This was Megnstoth’ erium, a primitive creodont which massed perhaps 900 kg, still not in the truly gigantic category. With a head over two feet long, and nearly as broad, it may have hunted elephants by crushing and cutting their lower limb muscles. Then there wasMegalanio, the giant Komodo-like monttor hzard of fairly recent Australia. Some estimates that it was as heavy as two tonnes, as big as most tyrannosaurs, cannot be right because Megolonia did not reach such dimensions: one tonne is more likely.