Why did certain animals survive and other animals did not?

No one is really sure why certain animals died out and others did not. In some ways, the animal extinctions at the end of the Cretaceous period were very selective.

Are mammals surviving relatives of the dinosaurs?

No, mammals are not the surviving relatives of the dinosaurs. The earliest mammals were descendants from certain types of reptiles, but they are not in the same line as the dinosaurs.

What mammals lived at the end of the Cretaceous period?

Mammals had been around for millions of years before the end of the Cretaceous period; in fact, the first group of true mammals, the morganucodontids, evolved in the late Triassic period. They were a successful group of animals for about 150 million years before the dinosaurs became extinct.

By the end of the Cretaceous period, some mammals had developed many innovations vital to their survival. Many stopped laying eggs and were able to deliver live young. Various mammal species eventually grew specialized teeth for a variety of tasks, such as cutting, gnawing, and grinding, for the better processing of food. They developed better ways to compete for food, such as having more energy in proportion to their size, or adapting to changing diets by becoming omnivores (plant- and meat-eating animals).

The therian mammals (marsupials and placentals) became the apparent heirs to the land the dinosaurs and other organisms left behind. Some mammal subgroups had already disappeared before the demise of the dinosaurs; others made it through the end of the Cretaceous period; and some even survive to this day.

Why did the mammals come to dominate in the Cenozoic era?

Mammals came to dominate the Cenozoic era (our present time) because there was suddenly little competition. The larger predatory reptiles had disappeared, so the mammals were able to quickly fill the available ecological niches.

Are mammals the most abundant animals on modern Earth?

No, mammals are not the most abundant animals in terms of species or individuals on our planet. There are many more kinds of fish, reptiles, and birds; and there are even more invertebrate species on Earth, including insects and mollusks.

What are the closest living relatives to the dinosaurs?

The closest living relatives to the dinosaurs are thought to be certain modern reptiles and birds.

What is a reptile?

Modern reptiles include the alligators and crocodiles, turtles, lizards, and snakes. They all have several typical characteristics: They have a protective covering of scales or plates, they lay eggs, they are cold-blooded, they breathe with lungs instead of gills, and they have five clawed toes on each foot (with exceptions, of course, such as snakes; and alligators only have four toes on their back feet, but five on their front feet).

When did modern reptiles evolve?

The earliest turtles evolved during the Triassic period, but they probably could not withdraw into their shells like modern turtles. Lizards and snakes have poor fossil records. This is probably due to the animals tendencies to live in dry uplands, far from the areas that are most likely to produce fossils (most animal bones survive if they are quickly buried with sediment, such as along riverbanks). It is thought that lizards appeared in the Late Triassic; the earliest remains of snakes are found in the Late Cretaceous (in North America and Patagonia, South America).

How well did reptiles fare at the end of the Cretaceous period?

Todays reptiles, like many animals at the end of the Cretaceous period, are almost like the survivors of a shipwreck.

After the Cretaceous period, most reptiles were wiped out. About 6,000 reptiles species exist today, fewer in numbers and much smaller in size than their ancestors, but greater in diversity.

Modem snakes belong to the group of animals ailed reptiles; all reptiles have scales, lay eggs, and are coldblooded (iStock).

What reptile species survived past the end of the Cretaceous period?

One remarkable group of reptiles also close relatives of the dinosaurs are the crocodiles. They appear to have evolved from archosaurian ancestors during the Late Triassic, but unlike most of their contemporaries they survived through to the present day. They are also remarkable for the fact that these moderate- to large-sized, semi- aquatic predators have remained relatively unchanged since the Triassic period.

What are the two types of modern crocodiles?

There are two types of modern crocodiles found in tropical and subtropical environments. The gavialids are found in India; they eat fish and have slender snouts. The crocodylids are found almost worldwide and consist of crocodiles and alligators. They have long bodies and powerful tails used for swimming or defense. Their limbs allow the animals to maneuver and steer in the water; on land, they use their limbs to walk with a slow gait, with their bellies held high off the ground. These animals survive off a wide variety of food, including fish, large vertebrates, and carrion.

What is the difference between an alligator and a crocodile?

Although there is some overlap of habitats with alligators and crocodiles, it is rare to see both together. The best way to tell the difference between the two animals is by checking the size and head: Crocodiles are slightly smaller and less bulky than an alligator. In addition, the crocodile has a larger, narrower snout, with a pair of enlarged teeth in the lower jaw that fit into a notch on each side of the snout. The alligator has a broader snout, and all the teeth in its upper jaw overlap with those in the lower jaw.

Both crocodillians, an alligator (top) has a number of physical characteristics that are different from a crocodile (bottom), including the shape of the snout, arrangement of teeth, and body size (iStock).

How might dinosaurs have evolved if they had not gone extinct 65 million years ago?

Dale Russell, curator of fossil vertebrates at the Canadian Museum of Nature in Ottawa, Canada, believes that dinosaurs were evolving toward more human-like features toward the end of the Cretaceous period, including the development of a larger brain, forward-focused eyes, and bipedalism. Extrapolating these tendencies, he evolved a dinosaur called a Troodon. He came up with a bipedal creature he called a Dinosauroid, which, though reptilian in many ways, including its extremities and somewhat scaly skin, also looked very humanoid.

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