Troodon

The discovery of this small flesh eating dinosaur, Troodon and its relatives, was really a milestone in changing thinking about the behavior of dinosaurs. This fierce and intelligent hunter is pronounced Troo-o-don and its name means "wound tooth."
This fascinating carnivore was likely to be closely related in lifestyle to the more well known "raptors" and were more or less of the same size category or slightly smaller. They were likely to be ground-dwelling protobirds. Fossil remains are still limited and we will know more as more are discovered.
It is likely that they were fast, intelligent, pack hunters, and possibly nocturnal. The best approximation is that of a pack of small wolves or perhaps better yet coyotes. Remains suggest that they were social and raised their young in a family atmosphere. They had the sickle-clawed toes found in the velociraptors and likely lived a very similar life although they may have hunted different prey much like coexisting leopards and cheetahs.
The tail was likely used as a rod to control high speed maneuvers. They possessed three fingers and an opposable thumb. And they had very large eyes which could have been used for nocturnal hunting and no doubt had great vision at all times. What is perhaps most important however, is their large brain cavity. While still small by current mammal comparisons they never the less had a much larger size in comparison than current day reptiles and were highly evolved hunters.
One of their favorite prey items were young hadrosaurs as teeth and teeth marks are often found on baby duckbill fossils. In all likelihood they were also one of the major predators of early mammals and were on their way to single-handedly keeping mammals from evolving.