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Parrots, eagles, penguins, chickens … they’re all dinosaurs. Birds are actually specialised coelurosaur theropods. Clearly, not all dinosaurs became extinct – over 10,000 species live among us today.

One of the earliest known birds is the 150-million-year-old Archaeopteryx, but birds had evolved before then. A range of birds with more advanced features appeared soon after Archaeopteryx. One group gave rise to modern birds in the Late Cretaceous. So, for a time, bird-like dinosaurs, primitive birds and early modern birds all co-existed.

Only a small number of advanced birds (‘neornithines’) survived the end-Mesozoic extinction event. These are the ancestors of today’s birds. Non-avian dinosaurs and primitive birds, such as Archaeopteryx, did not survive.

Why did some birds make it when so many others perished? Advanced birds were smaller in size and grew faster than primitive birds. They also had higher metabolic rates and more efficient flight systems. Some or all of these features may have been the key to their survival.

The idea that birds evolved from dinosaurs is not new. It was first suggested in the 1860s after the famous fossil discoveries of the primitive bird Archaeopteryx and the small theropod Compsognathus. Gradually falling from favour, the theory was resurrected by a series of new discoveries including spectacular feathered dinosaurs found in the 1990s. In the same way that we evolved from a mammal ancestor and are mammals, birds evolved from a dinosaur ancestor and are dinosaurs.

Archaeopteryx  lithographica

Bird fossil Archaeopteryx lithographica

Origin: Germany, late Jurassic

155-150 million years ago

Image: Carl Bento
© Australian Museum

Bird or dinosaur?

Birds are distinctive today as they are the only animals with feathers, wishbones and specialised joints in the wings. But they weren’t so distinctive in the past. Many of these features were present in non-avian dinosaurs. Early birds also had teeth, long bony tails and clawed hands like their closest dinosaur relatives.

To separate birds from other dinosaurs we use a definition based on ancestry – birds are all the descendants of the common ancestor of Archaeopteryx and modern birds. Birds can be called ‘avian dinosaurs’ but it is still correct to use the term ‘bird’ when talking about the feathered animals we see today. Dinosaurs that are not birds are often called ‘non-avian (meaning non-bird) dinosaurs’. Two key examples are:

  1. Archaeopteryx lithographica: Discovered in the 1860s, Archaeopteryx was the first fossil evidence linking birds to dinosaurs. It had feathers like modern birds and a skeleton with features like a small non-avian dinosaur. Although it is the earliest and most primitive bird known to date, it is not considered the common ancestor of all birds. Classification: Theropoda; Aves.
  2. Compsognathus longipes: Compsognathus was a small non-avian dinosaur with many similarities to Archaeopteryx. Discovered in the 1850s, it was a key fossil in discussions about birds and dinosaurs. Some Archaeopteryx remains were even mistakenly identified as Compsognathus. Classification: Theropoda; Compsognathidae.

Fossil cast Caudipteryx zoui
China, Cretaceous 130-125 myo Image: Robert Jones
© Australian Museum