359-299 million years ago

reptile skeleton

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).
In amniotes, the embryonic posterior cardinal veins remain in adults only as azygos veins (Romer 475). The renal portal system was reduced (Romer 475). Amniotes increase the number of veins which receive innervation (Prosser, p. 831).

While true lymph nodes exist only in mammals, lymphatic tissue and lymphatic cisterns may occur at sites in non-mammalian amniotes which contain lymph nodes in mammals (Kardong, 484). Amniotes developed pharyngeal tonsils. (Weichert, 1970, 247). IL2 was formerly called T cell growth factor. IL2 and 15 use the same ß and ? chains but differ in their a chains. They are only known in amniotes. IL2 stimulates T cell production (Kaiser, 2004). IL18 stimulates natural killer cells. It is involved in inflammatory skin reactions and graft vs. host disease. IL18 is only known in amniotes (Kaiser, 2004). Interferons are definitely known from amniotes (Magor, 2001).

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).

Ancestral amniotes evolved a more muscular tongue (Weichert, 1970, p. 164) and fleshy, movable lips and cheeks (Romer, p. 326). Amniotes lost ancestral palatal teeth and the location of taste buds was primarily limited to the tongue (Romer, p. 328). Parotid, sublingual salivary glands, and submaxillary salivary glands (Weichert, 1970, p. 161-2) evolved and amylase was produced in salivary glands (Stephens). In amniotes, teeth were embedded in sockets (thecodont condition) (Kardong, p. 497) and an additional layer of teeth known as cementum anchors the root of the teeth. (Weichert, 1970, p. 169) The hyoid apparatus became more closely associated with the larynx (Weichert, 1970, p. 230-1) and the hyoid was located more caudally (Webster, 1974, p. 329). In amniotes, the intestinal villi became more prominent (Romer, p. 386) and the large intestine develops a cecum and rectum (Romer, p. 389). In embryonic development, a large yolk sac distorts gut (even in mammals which lack yolk) (Romer, p. 374). The only cholinergic excitation of the gut came from the parasympathetic division of the ANS (Stevens, p. 274) and the primary bile salt became cholesterol (Stevens, p.167)

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).

In amniotes, keratins are only expressed in epithelia while other intermediate filaments, such as vimentin are expressed in mesenchyme. (Conrad, 1998).