Dromaeosauridae

Dromaeosauridae is a family of feathered, coelurosaurian theropod dinosaurs. Their name comes from Greek words meaning "running lizards."

These carnivores were generally small to medium-sized animals. However, some species grew to be quite large. They flourished during the Cretaceous Period. They lived until the Cretaceous–Paleogene extinction event 66 million years ago.

Some fossil teeth suggest they may have existed as early as the Middle Jurassic. Definitive body fossils have been found in North America, Europe, and Asia.

The physical build of a dromaeosaurid is very distinct. They had relatively large skulls and narrow snouts. Their forward-facing eyes provided binocular vision. Most had an S-curved neck and a short, deep trunk. Their arms were long and could be folded against the body.

These arms ended in large hands with three long fingers. The middle finger was the longest, while the first finger was the shortest. Each finger ended in a large claw. They also featured a large pubic boot on the hip structure.

One of the most famous features is the specialized foot. Dromaeosaurids were bipedal, meaning they walked on two legs. Most theropods walked with three toes touching the ground. Dromaeosaurids practiced functional didactyly. This means they held the second toe off the ground in a hyperextended position.

This toe bore a large, recurved, sickle-shaped claw. Scientists believe this claw helped them catch prey or climb trees. In the species Balaur bondoc, even the first toe was modified with a similar claw.

The tail played a vital role in their movement and balance. Many dromaeosaurid tails were long and slender. Some species had bony, rod-like extensions called prezygapophyses.

John Ostrom proposed these features stiffened the tail like a rigid lever. However, a Velociraptor specimen shows a tail that could bend side to side. This suggests the tail could act as a stabilizer or counterweight. In Microraptor, the tail ended in a diamond-shaped fan of feathers.

This might have worked like a rudder for gliding.

Feathers are a defining characteristic of this family. Evidence shows they had long, pennaceous feathers on their arms and tails. They also had shorter, down-like feathers covering their bodies.

The first dromaeosaurid with definitive feather evidence was Sinornithosaurus, reported in 1999. Some fossils show quill knobs, which are attachment points for wing feathers. Even large species like Zhenyuanlong had a full feathered coat.

This suggests that feathers were a common trait across the whole family.

Size varied greatly among the different genera. Smaller members like Velociraptor were relatively tiny. In contrast, Utahraptor, Dakotaraptor, and Achillobator approached or exceeded 5 meters in length.

Large size actually evolved at least twice in this group. It appeared in the dromaeosaurines, such as Achillobator. It also appeared in the unenlagiines, like the 1.5-meter-long Austroraptor.

There may have even been a third lineage of giants on the Isle of Wight.

The relationship between dromaeosaurids and birds is a major topic in science. They share many features with the clade Avialae, which includes birds. Some researchers argue they are so bird-like they should be classified as birds. Others suggest they are cousins to birds. There is also a theory called "Birds Came First." This hypothesis suggests dromaeosaurids descended from flying ancestors.

In this view, large dromaeosaurids became flightless, much like modern ostriches.