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.
Ampibians evolved tissue homologous to pharyngeal tonsils (Weichert, 1970, p. 247) and salivary glands (Romer, p. 329). The esophagus became more prominent and smooth muscle was incorporated into its muscularis layer (Romer, p. 378). The stomach developed a glandular cardiac region (Stevens, p.16). The intestine lost the intestinal spiral valve which is present in gnathostome fish (Romer, p. 387), developed an ileocecal valve separating a small and large intestine (Romer, p. 388), and specialized the anterior portion of the small intestine to become the duodenum (Kardong, p. 512). The intestine coiled, as did the mesentery which surrounded it (Romer, p. 318-9). The intestines evolved villi, lacteals within villi, and glandular crypts of Lieberkuhn (Stevens, p.18). New digestive enzymes included maltase, isomaltase, and trehalase (Stevens). Amphibians evolved sacral innervation of GI tract (Stevens, p. 274).
Amphibian lungs possessed simple squamous epithelia, collagen, and smooth muscle (Dutta, p. 248). Connecting the lungs to the pharynx, there developed a short trachea and larynx (Romer, p. 370; Dutta, p. 246). The cartilage around amphibian nostrils gave them the ability to change the size of nostril openings (Weichert, 1970). Amphibians evolved conchae and cricoid and arytenoids cartilages of the larynx (Weichert, 1970, p. 204).

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, 1993).