359-299 million years ago
The first amniotes evolved a number of new muscles, completely separated their pulmonary and systemic blood circuits, reduced the heart's conus arteriousus and sinus venosus, elongated the trachea, subdivided the lungs, evolved pleural cavities around the lungs, evolved salivary glands and a cecum, and modified their reproductive tracts.
In ancestral amniotes, the dorsalis trunci muscle divided into medial, intermediate, and lateral groups (Romer, p. 283; Kardong 382). In the amphibian ancestors of amniotes, extensions of the oblique muscles form the intercostal and scalene muscles of amniotes. (Webster, 1974; Weichert, 1970, p.512; Romer, p. 284). Subvertebral muscles become more developed (Romer, p. 284) and a levator palpebrae superioris moved the eyelid (and nictitating membrane in some) (Romer, p. 291). The sternomastoid and cleidomastoid develop from trapezius (Romer, p. 309). Amniotes evolved an interhyoideus (forms digastric in mammals) (Kardong, p. 393; Romer, p. 310), a longissimus coli and levatores costarum (Webster, 1974; Weichert, 1970, p.512) and a sphincter colli deep to the integument of the head (Weichert, 1970). The latissimus dorsi and deltoid became more prominent (Romer, p. 293) and additional muscles supported the arm such as the subcoracoscapularis (subscapularis in mammals) and scapulohumeralis (teres minor in mammals) (Romer, p. 294). The musculature of the dorsal forearm became fairly standardized (Romer, p. 294). The legs contained an adductor femoris, flexor tibialis externus and internus (hamstrings in mammals), a pubotibialis (adductor longus in mammals), and a caudofemoralis (Romer, p. 298).
Ancestral amniotes evolved several biochemical traits of red blood cells
and the occasional loss of nuclei from red blood cells (Mauro, 1997).
Hematopoeisis in adults was limited to the red bone marrow and spleen
(Torrey). As they became better adapted to breathing air, a completely
separate pulmonary circuit evolved without a ductus arteriosus connecting
pulmonary and systemic circuits in adults (Kardong). In the heart, the
sinus venosus became smaller but it still contained the SA node. A pulmonary
trunk and at least one systemic aortic trunk leave the heart (Kadong 467).
Partial division of the ventricles aided in the separation of oxygenated
and deoxygenated blood (Kardong 467). Amniote hearts created higher pressure
and allowed a greater cardiac output (Kardong 467). Amniotes evolved a
conduction system in the heart, replacing the wave-like contractions of
more primitive vertebrates (Romer, 482). The conus arteriosus is present
in amniote embryos only although it composes portions of vessels leaving
the heart in adults (Romer 488). In amniotes, the adrenal medulla participates
in the regulation of the heart (Porges, 1988).
Some amniotes evolved pleural cavities around their lungs (Romer, p. 320) and vocal cords with a resultant voice (Romer, p. 370). Amniotes increased the internal complexity of their lungs with septa further dividing the lungs (Romer, p. 363; 366), the trachea lengthened and divided to form bronchi, and ribs attached to the sternum to function in pulmonary ventilation (Romer, p. 370). Intercostal and abdominal muscles were used in breathing negative pressure used to inflate lungs (Kardong, p. 421-2).
In amniotes, the renal corpuscle reduced its size to conserve water (Romer, p. 400) and the metanephros became the primary kidney in adults (Kardong, p. 532)
Males no longer retained Muellerian ducts (as in some amphibians and lungfish) (Romer, p. 427) and develop an epididymis (Romer, p. 434), corpora cavernosa, and glans penis (Romer, p. 441). The corpora cavernosa becomes engorged with blood during sexual activity (Kardong, p. 562). In females, the ovary joined the oviduct (Romer, p. 427), portions of oviduct specialized to become uterus and vagina (Romer, p. 429), oviducts develop from the Mullerian ducts rather than archinephric ducts (Weichert, p. 288), a clitoris develops (Romer, p. 441), and ova moved in oviducts through ciliary action and muscle contraction (Weichert, p. 294). Reptilian homologs of AMH, DAX1, SF1, SOX9, and WT1 seem to function similarly to those proteins in mammals where they are involved in sexual development (Pleau, 1999; Shimada, 1998).