Dinosaurs: Facts about the reptiles that roamed Earth more than 66 million years ago

Dinosaurs were once thought to look like modern lizards, covered in green scales. However, some dinosaur species were actually covered in feathers, just like birds today.

We know this because some fossils from creatures such as Archaeopteryx and Sinosauropteryx — both theropod dinosaurs — have preserved feather imprints. And some Velociraptor fossils have tiny bumps on their arm bones where feathers were attached, just like in modern birds.

At first, paleontologists thought only theropod dinosaurs had feathers. However, scientists have found traces of downy feathers on fossils of a plant-eating dinosaur, suggesting that feathers may have been more common in dinosaurs than we thought.

Some dinosaurs may have used feathers to stay warm, court mates or even fly. For instance, paleontologists think a tiny chicken-size dinosaur named Microraptor had feathers on all four limbs to help it glide or even fly.

Feathers may have been very colorful in some species. Fossils of Sinosauropteryx even had preserved pigment structures named melanosomes, which allowed scientists to figure out it had an orange and white striped tail. These bright colors could have been used for mating displays, or intimidation of others, just like how modern birds use their colorful feathers today.

Many species of dinosaurs were enormous. The largest group of all the dinosaurs were the titanosaurs, which included Argentinosaurus and Patagotitan.

Patagotitan is one contender for the longest dinosaur discovered so far, measuring 37.5 meters 123 feet (37.5 m) long, weighing (62 tons) 57 metric tons, and standing 8 meters (26 feet) tall. Argentinosaurus may have been heavier though; it reached lengths of 115 feet (35 m) and weights of up to 110 tons (100 metric tons). Some other dinosaurs, such as Supersaurus and Giraffatitan, may have grown even longer or taller than these species.

It's not clear why some dinosaurs were so big, though scientists think that their upright posture, wide hips, light bones and unique foot structure may have made it easier to grow so large.

Dinosaurs also probably had very efficient breathing thanks to air sacs within their light bones. These sacs helped them take up oxygen during both inhalation and exhalation. This may have contributed to their growth to such gigantic sizes.

And the long necks of titanosaurs allowed them to graze on food right on the tops of trees where no others could reach, meaning they could consume lots of food without moving much and burning much energy.

Most dinosaurs suddenly went extinct about 66 million years ago after an asteroid struck Earth. You can see evidence of this impact at the Chicxulub crater on the Yucatán Peninsula in Mexico, which measures about 110 miles (180 km) across.

The asteroid was likely between 6.2 and 9.3 miles (10 and 15 km) wide, and its collision with Earth triggered intense wildfires, tsunamis, and massive earthquakes around the world. The impact also threw huge amounts of dust and debris into the atmosphere, blocking sunlight for years. With less sunlight, temperatures dropped, plants died, and food chains collapsed. Additionally, sulfur in the atmosphere from the asteroid impact then fell as acid rain, killing off tiny plants that drift in the ocean known as plankton, which many animals relied on for food. These effects wiped out 75% of animals on Earth in a short period of time.

While a lot of volcanoes were also erupting around the same time, most scientists think the asteroid caused the mass extinction.

Only small theropod dinosaurs survived the extinction; the ancestors of modern-day birds. Our small mammal ancestors probably survived extinction because they could burrow and eat a wide range of foods.

Dinosaurs still exist today — as birds. Modern birds evolved from the small, feathered theropod dinosaurs that survived the mass extinction event 66 million years ago.

Unlike giant dinosaurs, most early birds were small, which meant they required less food to survive. After the asteroid impact, food became scarce, but smaller birds could survive on seeds and insects. And unlike ground-dwelling dinosaurs, birds could fly to new areas to find food and shelter, giving them a huge survival advantage when ecosystems collapsed.

With all the larger non-avian dinosaurs dead, the earliest birds rapidly evolved, and today there are about 11,000 bird species. Modern birds share many traits with dinosaurs, including hollow bones and wishbones.

Early birds were also better able to survive the cold conditions after the asteroid strike than many reptiles. This is because like mammals, birds are also warm-blooded, meaning they can regulate their body temperature during extreme climate changes.

Some animals around today are not considered dinosaurs, but stayed quite similar since the age of the dinosaur. Both crocodiles and dinosaurs belong to a group called archosaurs and share a common ancestor that lived around 250 million years ago. However, crocodiles have changed very little in the last 200 million years or so, making them one of the closest things we have to prehistoric reptiles.