Approximately 66 million years ago, one of the most significant mass extinctions in Earth’s history occurred. Paleontologists refer to it as the “Cretaceous-Paleogene extinction,” but most people know it as the “Dinosaur Extinction.” It is widely believed that this group of reptiles vanished from the face of the planet forever. However, that is not entirely true.

If you ask a specialist, they will tell you that all non-avian dinosaurs disappeared. But birds, which modern research classifies as theropod dinosaurs, survived the extinction and are thriving today. It may feel strange to think of birds as living dinosaurs, but let’s explore whether birds have any unique characteristics that their dinosaur ancestors lacked.

The first thing we associate with birds is feathers.

Feathers have been discovered on a wide range of theropod dinosaurs. Even the ancestors of Tyrannosaurus had feather-like scales, although the tyrant lizard itself did not have feathers. However, developed plumage was typical for small predators like dromaeosaurids. The most famous of these is Velociraptor, and the most unusual is Microraptor. The feathers of this small, crow-sized dinosaur had an asymmetrical structure—a crucial feature indicating the ability to fly. Interestingly, Microraptor had this plumage not only on its forelimbs but also on its hind limbs.

The skeleton of birds also shares many features with dinosaurs. Consider Archaeopteryx, discovered in Germany in the 19th century. If it weren’t for the imprint of feathers, it could easily be mistaken for a small predatory dinosaur.

Let’s take the goose as an example and see which features of its skeleton are of dinosaur origin.

1. Lightweight Skeleton with Hollow Bones: This feature is typical of dromaeosaurids and other dinosaurs. The complex sacrum is another shared trait.
2. Shoulder Girdle: The bird’s shoulder girdle includes a long scapula, similar to dinosaurs, and a furcula (wishbone), which is a fused pair of clavicles. Such a furcula was present in large predators like Allosaurus, in dromaeosaurids, and in Archaeopteryx.
3. Reduced Forelimbs with Three Fingers: The reduction of the forelimbs is characteristic of bipedal predatory dinosaurs, as is the reduction of fingers to three. In some cases, the reduction went even further, as seen in Tyrannosaurus with its tiny arms and only two fingers.
4. Bird’s Leg with Tarsometatarsus: The bird’s leg features a distinctive element—the tarsometatarsus, a long bone between the foot and the tibia, making it seem like birds’ knees bend backward. In reality, the tarsometatarsus is formed by fused metatarsal bones, with the knee joint located higher. This fused tarsometatarsus first appeared in Jurassic maniraptors and was inherited by birds.
5. Tail: Perhaps the only significant difference between birds and dinosaurs is the short tail section of the spine. The long reptilian tail was lost during evolution, replaced by tail feathers. These tail feathers help birds maneuver in flight, take off, and land.

Teeth: Birds also lost their teeth to reduce skull weight. However, as late as the Cretaceous period, fully developed birds like Ichthyornis still had teeth.

Birds are warm-blooded animals with a very fast metabolism. Active flapping flight requires a great deal of energy, meaning metabolism and body temperature must be maintained at high levels.

It is still challenging to determine whether dinosaurs were warm-blooded. However, it is highly likely that they were, particularly small predators like Velociraptor and other dromaeosaurids. It is believed that feathers first evolved in their ancestors to maintain a constant body temperature, allowing them to remain active at any time of day. These quick, agile predators led an arboreal lifestyle, jumping from branch to branch. The presence of feathers allowed them to increase the length of their jumps by gliding and cushioned their falls. Thus, in addition to providing warmth, feathers gradually evolved into tools for flight.

Another important aspect of life is reproduction. And here again, there’s nothing new. Birds, like dinosaurs, lay eggs. There is no known example of live birth in birds. However, it is worth noting that in terms of caring for their offspring, birds have advanced significantly. While some examples of nest building, egg incubation, and offspring rearing are known among predatory dinosaurs, such behavior was probably not typical of all species. In contrast, there are no known birds that abandon their eggs.

As we can see, the traits we consider uniquely avian actually first appeared in dinosaurs. The features listed here are just the basics. In reality, the similarities between dinosaurs and birds are much stronger, as confirmed by genetic data. Of course, we don’t have dinosaur DNA for comparison. But working with the genetic material of various birds allows us to identify genes responsible for traits like long tails and teeth. These genes haven’t disappeared; they’ve just been “switched off.” If we were to reactivate them, we would see a bird with a distinctly dinosaur-like appearance.

All these fascinating facts allow us to see the world around us in a new light. Not all dinosaurs went extinct 66 million years ago. As early as the Jurassic period, one group of dinosaurs mastered flight, successfully competing with pterosaurs. By the Cretaceous period, these flying dinosaurs had diversified into a wide range of species. Today, we classify them as a separate class, calling them birds.

This article is based on an interview with paleontologist Dr. Pavel Skutschas, PhD in Biology, Associate Professor at St. Petersburg State University, and a specialist in Mesozoic vertebrates.