Dinosaurs are related to birds.
Dinosaur . As such , birds were the only dinosaur lineage to survive the mass extinction event . While the dinosaurs ' modern-day surviving avian lineage ( birds ) are generally small due to the constraints of flight , many prehistoric dinosaurs ( non-avian and avian ) were large-bodied -- the largest sauropod dinosaurs are estimated to have reached lengths of 39.7 m and heights of 18 m and were the largest land animals of all time .