The Phylogeny and Ontogeny of Facial SMAS

Introduction

The superficial musculoaponeurotic system (SMAS) is a fascial layer that plays a crucial role in the anatomy and function of the human face, with its origins and development deeply intertwined with the evolutionary and developmental history of facial structures. The facial SMAS has evolved over millions of years as a specialized part of the facial muscles and connective tissue. Its origins can be traced to the early tetrapods, where similar structures helped enable complex facial movements and expressions. As the facial structures became increasingly important for social communication, feeding, and other critical functions in higher vertebrates, the SMAS continued to evolve, becoming more differentiated and integrated with the underlying musculature and bony structures.[1]

The SMAS emerges from the mesenchymal tissues during the early stages of facial development, with the individual components, such as the superficial and deep fascial layers, gradually taking shape and becoming distinct structures.[2] [3]

The maturation of the SMAS anatomy is closely tied to the development of the facial muscles, nerves, and other anatomical features, as the system plays a crucial role in the coordinated movement and expression of the face.[4] Understanding the phylogenetic and ontogenetic origins of the facial SMAS is essential for clinicians and researchers seeking to optimize the use of injectable neuromodulators and soft-tissue fillers, as the system's intricate structure and function must be taken into account to achieve safe and effective aesthetic outcomes. It is equally important for surgeons involved in facial rejuvenation procedures, as the SMAS is a key target and consideration in many of these interventions.[2] [3] [4] [5] Phylogeny and ontogeny of the SMAS help in understanding the anatomy of SMAS and the reason for the variation in the appearance and behavior of the SMAS with aging,[6] [7] [8] as well as the reason why the anatomy is the way it appears.[6] [7] [9] This deep understanding of the phylogenetic and ontogenetic origins of the facial SMAS is invaluable for clinicians, researchers, and surgeons who work with the face, as it allows them to leverage this knowledge to optimize the safety and efficacy of their intervention.

Research has shown that the facial SMAS is a complex and multifaceted system that has evolved over time to serve essential functions in the human face.[3] From an evolutionary perspective, the SMAS can be seen as a specialized adaptation of the connective tissue and musculature that has emerged over millions of years to facilitate the complex movements and expressions required for social communication, feeding, and other critical functions.[2]

At the ontogenetic level, the SMAS undergoes a complex developmental process, with the system's components first appearing in the embryo and undergoing further specialization and integration throughout childhood and adolescence.[3]

The Evolutionary Origins

Tracing the evolutionary origin of the human face, the tale begins with the fish and reptilia and their primitive facial musculature.[10] Development of the face and neck is from branchial arches.[11] As evolution progressed into mammals, facial bony structures also evolved, and the maxilla and mandible became smaller and were positioned under the temporal bone. With the development of the brain, the frontal bones started becoming prominent anteriorly. Along with this, voluntary muscles like the masseter and buccinator were shifted deeper.[12]

This evolutionary process led to the complex facial musculature and connective tissue that we observe in modern humans, including the SMAS. As the primate evolution progressed, the facial structures became increasingly important for social communication, feeding, and other critical functions, leading to the continued differentiation and specialization of the SMAS system.[13]

The muscles of the face developed from the interhyoideus and cervicomandibularis of nonmammalian tetrapods,[12] evolved and specialized in mammals. This evolutionary process has shaped the complex SMAS system we see in human faces today, with its superficial and deep fascial layers, and integral role in facial movement and expression.[14] [15]

Facial muscles are only present in mammals.[12] The mesoderm of the second arch developed into the constrictor muscles of the neck—sphincter colli profundus, supplied by the nerve of the second arch—the seventh cranial nerve.[16] Mammalian facial muscles developed from the interhyoideus and cervicomandibularis of the nonmammalian tetrapods.[17] Sphincter colli profundus develops from interhyoideus and later develops into zygomaticus major and minor, orbito-temporo-auricularis, frontalis, auriculo-orbitalis, temporoparetalis, auricularis anterior and superior.[17] Sphincter colli profundus is a strong muscle covering the whole neck, and its contraction tightens the skin and is similar to platysma[12] [18] and is supplied by the seventh nerve. The sphincter colli profundus starts disappearing with the development of platysma myoides.[18] As primates evolved, the sphincter colli started disappearing, leaving behind Zygomaticus major and minor[10] [11] as well as the muscles closer to the lips and modiolous.[10] [19]

From Monotremes, such as the platypus, a platysmal layer is seen superficial to the sphincter colli ([Fig. 1]). This is a part of the Panniculus Carnosus layer of muscle, a thin layer of striated muscle closely attached to the skin and fascia. In mammals such as rodents, cats, dogs, horses, and whales, the panniculus carnosus, which covers an extensive body surface area including limbs, abdomen, back, and chest, facilitates voluntary twitching of facial skin, aiding in wound healing and repelling insects.[20] In primates, the panniculus carnosus largely atrophies, leaving behind the craniofacial muscles and the platysma. The platysma exists as two separate muscles: platysma cervicale—the muscle connecting the back of the neck (the nuchal region) to the face, different from platysma myoides connecting the front of the neck and pectoral region to the mouth[21] ([Fig. 2]).

Fig. 1 Platypus with platysma cervicalis and platysma myoides with sphincter colli. Original Illustration by ©Bev Shiner.

Fig. 2 Arrangement of platysma cervicalis, platysma myoides, and sphincter colli profundus in lemur: early primate. Original Illustration by ©Bev Shiner.

The platysma cervicale is commonly present in earlier primates, but diminished or absent in adult orangutans, chimpanzees, and humans. The platysma cervicale originates from the transversus nuchae, covers the posterior part of the neck, passes above the superior edge of the platysma myoides, below the pinna, and reaches the mid face. From Lemurs to Chimpanzees, the platysma cervicale starts atrophying and gradually disappearing, leaving behind a fibrous sheet ([Figs. 2] and [3A–C]). Remarkably, while the platysma cervicale appears early in human development, it typically integrates into other structures in later stages.[19] ([Fig. 4])

Fig. 3 (A) Muscle structure in a gibbon. Sphincter colli profundus has atrophied, leaving behind zygomaticus major and minor. Platysma cervicalis has started atrophying and has been replaced with fascia. (B) In the Macaque, the platysma cervicalis has atrophied, and auricularis inferior is seen. (C) Chimpanzee: platysma cervicalis and auricularis inferior have atrophied. Original Illustration by ©Bev Shiner.

Fig. 4 Early embryo, 37mm, shows fleeting appearance of platysma occipitalis, same as platysma cervicalis. Original Illustration by ©Bev Shiner.

Contrasting the platysma cervicale, the platysma myoides typically remains distinct in adult members across the later primates, chimpanzee, orangutan, and humans[22] ([Figs. 3A–C] and [5]). This muscle encompasses the upper chest, anterior neck, and extends into the lower face. The origin of the risorius is controversial; different authors believe it to develop from the platysma cervicale, platysma myoides, or sphincter colli profundus. Lightoller believes this muscle developed from the sphincter colli profundus and passes under the platysma cervicale in the face. The evolution of platysma myoides as a distinct muscle in later primates is a key evolutionary development in the facial musculature, and with this, the sphincter colli gradually disappears.[18]

Fig. 5 SMAS in an adult human. Platysma myoides persists. Sphincter colli profundus has atrophied, leaving behind zygomaticus major and minor. Platysma cervicalis has atrophied and remains as a fascia, with dense adherence to deeper tissue except for a persisting part of the sphincter colli profundus. This attachment is denser in the area of the atrophied auricularis inferior. The deeper attachment of Platysma cervicalis to the anterior border of the sternomastoid forms the cervical retaining ligament. Original Illustration by ©Bev Shiner.

Platysma myoides remains as a muscular layer in humans, and it has loose areolar tissue separating it from massetric fascia ([Fig. 6]). Hence, the easy dissection of SMAS and platysma from massetric fascia at the lower and mid massetric space, as well as from the deep fascia of the neck. Anteriorly, the platysma myoides joins with the same layer as the orbicularis oris. Platysma cervicalis atrophies in humans and changes to the fascia. This has a denser attachment to deeper tissue. Hence, sharp dissection is required for the upper masseteric space. This area is similar to the subzygomatic space described by Gassner et al[23] ([Fig. 6]). Inferiorly, it is continuous with the platysma myoides and may be represented by risorius. Superiorly, it is in the same layer as the orbicularis oculi. Where it crosses below the auricle, there is dense attachment as the platysma auricular adhesion. The firm attachment of the deeper surface can be seen in its relation to the parotid fascia, the relatively immobile part of the SMAS, as well as the anterior border of the sternomastoid as a cervical retaining ligament. It is quite possible that this layer is densely adherent to the deep fascia over the posterior triangle.

Fig. 6 Lower, mid, and upper masseteric space. The deeper attachment is denser in the upper massteric space and requires sharp dissection. This is deeper to the SMAS fascia, a remnant of Platysma cervicalis.

Gassner et al have suggested a bilayered structure of the SMAS.[23] This may be possible from the atrophied sphincter colli profundus as a deeper layer and the platysma myoides and atrophied platysma cervicalis as a superficial layer. The double layer is more obvious in the prezygomatic space, between the zygomatic major and minor, evolved from sphincter colli on the deeper surface, and the remnant of the platysma cervicalis and orbicularis oculi superficially. ([Fig. 7])

Fig. 7 Cadaver dissection showing plane between zygomaticus major and SMAS. This plane represents the plane between Platysma cervicalis and Sphincter colli profundus. The zygomatico cutaneous ligament and prezygomatic space are shown.

Early primates have an orbito-temporo-auricularis muscle, developed from the sphincter colli. This muscle later evolves into the frontalis, auriculo-orbital, and auricularis superior muscles. Muscles like the auricularis anterior and auriculo-orbital muscle aid in independent sight and hearing without head movement, and are advantageous for hunting and evading predators.[16] [24] As the evolution reached humans, this muscle remains as the temporoparietalis and auricularis superior and anterior. ([Figs. 5], [8], and [9])

Fig. 8 An 80 mm embryo. SMAS arrangements are almost like adults. Original Illustration by ©Bev Shiner.

Fig. 9 Development of the facial nerve in the second branchial arch in front and above the otic capsule. As the brain develops, it moves the otic capsule into its adult position. Original Illustration by ©Bev Shiner.

The function of the facial muscles in mammals was also associated with flight or fight when facing an adverse situation. Snarling and threatening exposure of teeth were accomplished with the help of these muscles. This also helped communicate with members of the same or other species. As primates developed into humans with more social and communication skills, these muscles came into more use to express more subtle socially important messages. Asymmetric use of facial muscles is also a feature of humans. The muscles of the upper face have less chance of asymmetric movement, but the lower two-thirds muscles show easy asymmetric movement.[12]

 

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The Embryological Origins

Facial muscles derive from the second arch mesoderm. These muscles are supplied by the nerve of the second arch—the seventh cranial nerve. During human embryonic development, myocytes from the second arch form buds and migrate to surrounding areas. The platysma cervicale briefly appears as the platysma occipitalis before disappearing and becoming a fibrous sheath, except possibly the risorius[19] ([Figs. 4] and [8]).

The facial nerve has a complex development, initially lying dorsal to the otic placode and vesicle.[25] The developing brain carries the otic capsule superiorly and medially and positions it in the petrous temporal bone. The facial nerve now has a twisted course through the bone and later courses through the stylomastoid foramen, entering the face ([Fig. 9]). Here, it faces the budding parotid gland and is trapped in the substance of this gland ([Fig. 10]).

Fig. 10 Adult course of the facial nerve related to the labyrinth. As it comes out of the temporal bone, it stays between the developing parotid buds. In front of the parotid it stays under the buccinator fascia, which is a continuation of the parotid fascia. Original Illustration by ©Bev Shiner.

Here it branches, and as it comes out of the gland, it is placed underneath the masseteric fascia, which is a continuation of the parotid fascia. Here, the branches of the facial nerve maintain a connection with tissues of the second arch, which is seen as a soft tissue “mesentry”[26] ([Fig. 11]). Beyond the masseter muscle, the facial nerve's branches diverge to supply muscles derived from the second pharyngeal arch.

Fig. 11 While under the buccinator fascia, the fascial nerve maintains its relation to the SMAS from the second arch as soft tissue folds, as seen for the lower and upper buccal branches at the edges of the lower and mid massetric spaces. Original Illustration by ©Bev Shiner.

 

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Publication History

Received: 03 August 2025

Accepted: 07 October 2025

Accepted Manuscript online:
08 October 2025

Article published online:
28 October 2025

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