416-359 million years ago


As the fins developed into limbs over millions of years, the muscles which had previously serviced the fin were enlarged and reorganized.

In amphibians, the external oblique produced additional muscles such as the serratus anterior, levator scapulae, and rhomboid muscles (Romer, p. 287). The hypobranchial musculature produced glossus and hyoid groups of muscles (Kardong, p. 378; Romer, p. 288-9). The arms were moved by pectoralis, latissimus dorsi, coracobrachialis, biceps brachii, brachialis muscles (Romer, p. 295), and dorsalis scapulae and procoracohumeralis longus (acromiodeltoid and scapulodeltoid in mammals) (Kardong, p. 386). The muscles of the forearm included extensor digitorum communis, extensor carpi radialis, extensor carpi ulnaris, extensores digitorum breves, supinator, dorsal interossei, flexor carpi radialis, palmaris longus, flexor carpi ulnaris, pronator profundus (quadratus), flexor palmaris (digitorum) profundus (Romer, p. 302-3).
The legs were moved by puboischiofemoralis internus (which became the psoas, iliacus, and pectineus muscles in mammals) (Kardong 387; Romer, p. 297) ambiens/iliotibialis (sartorius in mammals) (Kardong, p. 387; Romer, p. 297), triceps femoris of ampibians (iliotibialis and femortibialis in reptiles; quadriceps in mammals), gluteal muscles (iliofemoralis in reptiles) (Romer, p. 297), puboischiofemoralis externus (which develops into obturator externus and quadratus femoris in mammals) (Kardong, p. 387; Romer, p. 297), ischiotrochantericus (develops into obturator internus and gemellus in mammals) (Kardong, p. 387; Romer, p. 297), gracilis muscles (puboischiotibialis in reptiles) (Romer, p. 298), gastrocnemius (Romer, p. 300), and the iliofemoralis (tensor fascia latae, pyriformis, and gluteus muscles of mammals) (Kardong, p. 387). The muscles of the lower leg included the tibialis anterior, peroneus longus, extensor digitorum longus, peroneus brevis, extensor digitorum brevis, caudofemoralis (Kardong, p. 386)

The operculum was lost and the hyoid arch musculature expanded into the neck (Romer, p. 309). Tongue muscles developed from hypobranchial muscles (from gill arches) and anchored to the hyoid (which is composed of the fused cartilage of several gill arches) (Romer, p. 328). Amphibians evolved a depressor mandibulae (which is retained only as the stapedius in mammals) (Kardong, p. 393), levatores arcuum (sternomastoid and cleidomastoid in mammals (Kardong, p. 386). Interspinalis muscles also developed (Webster, 1974).