Simplify the following text for a Grade 10 native English-speaking person.
Original: There is a wide consensus among paleontologists that birds are the descendants of theropod dinosaurs . Using the strict cladistical definition that all descendants of a single common ancestor must be included in a group for that group to be natural , birds would thus be dinosaurs and dinosaurs are , therefore , not extinct .
Simplified: Birds are the descendants of theropod dinosaurs ; all the terrestrial dinosaurs are extinct .