Simplify the following text for a Grade 12 native English-speaking person.
Original: They can be defined in traditional , Linnaean terms , in which case they are a suborder of mammal-like reptiles that lived from the Middle Permian to the Middle Cretaceous , or in Cladistic terms , in which case they include not only the traditional theriodonts but also their descendants the mammals as well ( in the same way that , cladistically speaking , the theropod dinosaurs include the birds as a sub-clade ) .
Simplified: In cladistic terms , they include not only the traditional theriodonts , but also their descendants the mammals as well ( in the same way that theropod dinosaurs include the birds as a sub-clade ) .