What species are today descendants of dinosaurs?

Dinosaurs ruled the world for about 165 million years, dominating most other land animals. They first appeared in the Triassic around 250 million years ago and rose to prominence in the Jurassic that followed. But after the six-mile-wide asteroid Chicxulub hit Earth at the northern end of what is now the Yucatan Peninsula at the end of the Cretaceous around 66 million years ago, they went extinct.

Or did they? The truth is that all birds that walk, swim or fly on our planet are technically dinosaurs, descendants of giant creatures from the same family as Tyrannosaurus Rex.

“In the same way that humans are apes, birds are dinosaurs,” says Eugenia Gold, a paleontologist at Suffolk University in Boston.

Close cousins

Not all of the creatures we often think of as dinosaurs are dinosaurs – the entire lineage of flying creatures known as pterosaurs, for example.

The group of reptiles, including pterodactyls, evolved before the dinosaurs. They started out as smaller flying creatures and gradually evolved into a larger species with wingspans over 33 feet, like the Hatzegopteryx thambema and Quetzalcoatlus nordopi. This wingspan is roughly the size of a four-seat Cessna aircraft.

Some of these early pterosaurs may also have had feathers, such as the Tupandactylus imperator fossils found in Brazil revealed. But the pterosaur lineage did not result in any direct living descendants today – asteroid Chicxulub drove the pterosaurs to extinction.

Read more: Pterosaur fossil reveals evolutionary origin of feathers

Other reptiles, besides birds, that still exist today mostly predate the dinosaurs in the evolutionary tree. Crocodilians are the most closely related group. They evolved before dinosaurs and experts place crocodiles in the larger family group, archosaurs. Strictly speaking, birds are the only direct descendants of extinct giant dinosaurs, and crocodiles and alligators are close relatives.

In fact, crocodiles are more closely related to birds than to lizards and snakes, Gold says. This almost throws the outdated, but still widely held idea, that birds are a separate species from reptiles, into a conundrum.

As for lizards and snakes, they are closely related to each other and both evolved earlier than crocodiles and dinosaurs. Experts still wonder where the evolution of turtles fits among these groups of reptiles. And the last remaining order of reptiles – the tuatara — are more closely related to lizards and snakes than to birds and crocodiles, Gold says.

Up early

Although we can say that all birds are certainly dinosaurs, we cannot say that all dinosaurs were birds. Birds are classified as theropod dinosaurs – the same clade that includes species like T. rex and Velociraptor.

Other large groups of dinosaurslike sauropods which include creatures like Brontosaurus, and Ornithischians which include creatures like stegosaurusare now completely extinguished.

The first ancestors of modern birds appeared in the Jurassic from a clade of theropods called Screens. “[Theropods] get smaller, their arms get smaller, their center of mass shifts,” Gold says of the evolution of birds over time.

For a long time, scientists believed that Archeopteryx was the oldest ancestor of birds. They date to around 150 million years ago in the Late Jurassic period, based on a well-preserved specimen complete with feathers that researchers found in Germany, in the 19th century. But at the same time Archeopteryx is also from Screens group, a doubt where Archeopteryx fits into the family tree. Archeopteryx had a mobility of the shoulders different from that of the birds, and with teeth, it had a long well-developed tail, all the birds contrasting.

Scientists also haven’t stopped at when feathers began to appear on dinosaurs. If pterosaurs had feathers, some suggest that all dinosaurs had some form of feathers, even if they were so few or not prominent, like the hairs of elephants and whales. With other theropods this is uncertain due to the fragile nature of the feathers.

“Probably the T. rex also had feathers, they just aren’t preserved,” Gold says.

In the Cretaceous, toothless birds began to appear. Paleognaths may have first evolved around this time. This clade – the oldest surviving bird alive today – includes ostriches, kiwis, cassowaries and emus.

“When you see an ostrich running, it’s very dinosaur-like,” Gold says, though she adds that their ancestors used to be able to fly, but lost that ability at some point.

The clade of poultry, including ducks and ground fowl like chickens, and neoaves – a clade that includes almost all other birds today, including penguins – began to diversify rapidly just as of the major extinction of all other dinosaurs 66 million years ago.

In other words, experts suggest that birds are the living descendants of dinosaurs. But, they are more likely the surviving branch of dinosaurs.

“Dinosaurs are the name of a group, and birds are one of them,” Gold says.

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