359-299 million years ago
The early amphibians evolved a number of changes such as a separation of the atria, new blood vessels to the limbs and modified blood vessels in the head, salivary glands, a separation of the intestines, laryngeal cartilages, and new ducts of the male reproductive tract.
Ancient amphibians diversified as they adapted to terrestrial environments. Balanerpeton is close to the ancestry of the group which would give rise to modern amphibians in addition to many fossil groups. Before that lineage separated from that which would evolve into anthracosaurs and reptiles, a number of changes in had occurred which would be shared among modern tetrapods.
The amphibian interatrial septum completely separated the ancestral atrium
into right and left atria. Amphibian blood vessels were modified as well.
The aortic arches were reorganized: the common carotid formed from the
ventral aorta which formerly connected aortic arches III and IV; the internal
carotid, together with aortic arch III and part of the dorsal aorta, joined
it and the carotid body formed at the terminus of the common carotid (Kardong,
Romer). The subclavian veins emptied into the anterior vena cavae (Romer
478). There was further development of limb vessels to produce axillary,
brachial, radial, ulnar, femoral, saphenous, popliteal, and tibial arteries
and veins. The first two aortic arches disappeared early in development.
In amphibians, the archinephritic ducts were only used to transport
sperm (Romer, p. 414) and male amphibians evolved a ductus defererns and
an epididymis. (Weichert, p. 319). In frogs, SOX9 is expressed in both
genders, seemingly involved in the development of both ovaries and testes,
unlike its role in testis formation in mammals (Takase, 2000). Amphibians
evolved new peptide signals such as neurokinin A and PP in tetrapods (Larhammar,