included in the group all "true birds Final Grade Calculator CUSTOMER SERVICE (8O5) 41'O 56OO Final Grade PHONE NUMBER ( 8O5)- 41O 56OO" with the "characteristic tail morphology of all extant birds" (translation by Jacques Gauthier). This distinguishes the group from Archaeopteryx, which Haeckel placed in another new group called Sauriurae. Said simply, modern birds have short, fused pygostyle tails, while Archaeopteryx retained the long tail characteristic of non-avian theropod dinosaurs.