The Deinonychosauria carnivores are known for their switchblade-like second toes. This clade is divided into two further, more familiar clades: the Dromaeosauridae and Troodontidae. The dromaeosaurs (or dromaeosaurids) are dinosaurs the general public associates with the term “raptor.” These dinosaurs were members of the maniraptorans, and therefore may share a common ancestor with birds. Some paleontologists even speculate that birds are highly evolved dromaeosaurs.

This group ranged widely in size, starting at just about the size of a large dog and up to 30 feet (9 meters) long. These dinosaurs had large, clawed, grasping hands; a stiffened tail that may have acted as a stabilizer; muscular, toothy jaws; and a large, retractable, slashing claw on the second toe of each hind foot. They were probably agile, predatory dinosaurs. The discovery of their remains revolutionized the way we view dinosaur metabolism and behavior.

The troodontids are a small group with only a few fossil remains found, including several incomplete specimens from North America and Mongolia. These dinosaurs were about the size of a small adult human. They had long skulls; unique recurved and saw-edged teeth; large, flexible hands; and long, slender legs. The troodontids were probably fast, agile hunters. Although they had an enlarged claw on their back feet, it was not as large as those of the dromaeosaurs, and was probably not used in the same way.

The most unique characteristic of the troodontids was the size of their brain case: it was the largest, relative to body size, of all the dinosaurs. This may indicate that the troodontids were the most intelligent of all the dinosaurs, although this is only speculation. The skull had large eye openings (orbits), and there appears to be evidence for well-developed brain centers for sight and hearing. In fact, the ears of troodontids were also unusual among theropods, having extremely enlarged middle ear cavities, possibly indicating acute hearing ability.