The Anura probably originated among temnospondylous labyrinthodonts,
through a line represented approximately by _Eugyrinus_, _Amphibamus_,
and the Triassic frog _Protobatrachus_, as shown by Watson, Piveteau and
others. The known Paleozoic lepospondyls do not show clear indications
of a relationship with Urodela, but _Lysorophus_ may well be on the
ancestral stem of the Apoda.
Between Urodela and Anura there are numerous resemblances which seem to
indicate direct relationship through a common stock: (1) a similar
reduction of dermal bones of the skull and expansion of palatal
vacuities; (2) movable basipterygoid articulation in primitive members
of both orders; (3) an operculum formed in the otic capsule, with
opercularis muscle; (4) many details of cranial development, cranial
muscles, and thigh muscles, especially between _Ascaphus_ and the
Urodela, as shown by Pusey and Noble; (5) essentially similar manner of
vertebral development, quite consistent with derivation of both orders
from Temnospondyli; (6) presence in the larva of _Leiopelma_ of a
salamander-like gular fold, four limbs, and no suggestion of
modification from a tadpole (Stephenson).