200-147 million years ago


During the evolution of the synapsid reptiles into the early mammals (a period of more than 150 million years), a number of changes evolved in the muscular system. The ancestral reptilian musculature was modified.

Early mammals evolved a number of modifications of their muscular systems. Epaxial musculature less segmented in nature and now included the sacrospinalis. The rectus abdominis produced the diaphragm which increased the amount of air which could be taken in with each breath. Caudal muscles were reduced.
The scapular origin of the deltoid developed from the scapular spine. Deep to the skin, a dermal panniculus carnosus developed from pectoralis muscles. The pectoralis muscle split to form the pectoralis major, pectoralis minor, pectoantebrachialis, and xiphohumeralis. The reptilian supracoracoideus divided to become the supraspinatus and infraspiantus. The intercostals became move developed. The multifidus developed as the mammalian remnant of transversospinalis muscles in reptiles. The ancestral single sheet of the latissimus dorsi divided to form the latissimus dorsi, the teres major and cutaneous maximus in mammals.
The biceps formed with two heads (from the fusion of two ancestral muscles). The reptilian gastrocnemius internus formed gastrocnemius medialis and flexor hallucis longus in mammals and the reptilian gastrocnemius externus formed gastrocnemius lateralis, soleus, and plantaris in mammals.
The hyoid arch musculature expanded to form facial muscles. Remnants of the cranial portions of the ancestral rectus abdominis in head region include the sternohyoid, sternothyroid, geniohyoid, omohyoid, and thyrohyoid. In mammals, the reptilian sphincter colli differentiated into facial muscles although they are poorly developed in most mammals. The outer layer of the sphincter colli (the platysma layer) forms the auricularis frontalis, orbicularis oculi, mentalis, quadratus labii inferiors, and zygomatic. The deep layer of the sphincter colli gives rise to muscles of the nose, the orbicularis oris, and buccinator muscles.