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
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.