Birds are the only living dinosaur lineage.
While the dinosaurs ' modern-day surviving avian lineage -LRB- birds -RRB- are generally small due to the constraints of flight , many prehistoric dinosaurs -LRB- non-avian and avian -RRB- 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.
