The Skull Chapter 7 Introduction The skeleton gives

The Skull Chapter 7

Introduction • The skeleton gives the body its basic shape, supports weight, offers levers for movement, and protects soft parts of the body. • Because of its make-up bones are often the only part of animals left behind for fossilization. • The skeletal system is composed of an exoskeleton and an endoskeleton. – Exoskeleton is from integumentary elements, – The endoskeleton from mesoderm deep within the body.

• During the course of vertebrate evolution, most bones of the exoskeleton stay within the integument and protect surface structures. • Other bones have sunk inwards, merging with the deeper bones and cartilage of the endoskeleton into a composite structure. – For this reason it is difficult to divide the skeletal system based on embryonic origin and function. • Therefore it is divided into two regions, the Skull and the Post Cranial Skeleton.

• Although merged into a harmonious unit, the skull (or cranium) is actually a composite structure formed of 3 distinct parts. – Each from a separate phylogenetic source. 1. The splanchnocranium, which first arose to support pharyngeal slits, 2. The chondrocranium, which underlies and supports the brain and is composed of endochondral bone, cartilage, or both. 3. The dermatocranium, which forms the outer casing of the skull and has its origins from dermal bone.

Chondrocranium • Elements of the chondrocranium appear to lie in series with the base of the vertebrae. • This has led anatomists to hypothesize that the back wall of the skull may represent several ancient vertebral elements. • In elasmobranchs, the expanded and enveloping chondrocranium supports and protects the brain. – It does not ossify, instead cartilage grows over the brain to complete the protective walls and roof of the braincase. • However, in most vertebrates the chondrocranium is primarily an embryonic structure serving as the scaffolding for the developing brain and sensory organs. – The remaining chondrocranium partially or fully ossifies

Splanchnocranium • The splanchnocranium an ancient chordate structure. • In early chordates it is associated with the filter-feeding structures. • Among vertebrates it generally supports the gills and offers attachment for respiratory muscles • In gnathostomes it also contributes to the jaws and hyoid apparatus.

• Pharyngeal bars in protochordates arise from the mesoderm and for the unjointed branchial basket. • Pharyngeal arches of aquatic vertebrates are usually associated with their respiratory structures. • Because of this they are referred to as branchial arches, or gill arches. • Each arch is composed of a series of up to 5 articulated elements per side; – Pharyngeobranchial, epibranchial, ceratobranchial, hypobranchial, and basibranchial.

• Branchial arches that support the mouth are called jaws. • The first fully functioning arch of the jaw is the mandibular arch, composed of the paloquadrate (dorsally) and Meckel’s cartilage (ventrally)

Origin of Jaws • Jaws arose from one of the anterior pair of gill arches. • Evidence of this comes from several sources: 1. Embryology in sharks suggests that jaws and branchial aches develop similarly in series and both arise from the neural crest. 2. Nerves and blood vessels are distributed in patterns similar to branchial arches and jaws. 3. The musculature of the jaws appears to be transformed and modified from branchial arch musculature.

• It is unknown if jaws originated from the a combination of any of the 1 st four branchial arches. – The serial theory is the simplest view and holds that the 1 st (or 2 nd) arch gave rise exclusively to the mandibular arch, the next arch exclusively to the hyoid, and the remainder are the branchial arches of gnathostomes. – The composite theory states that 10 branchial arches existed in primitive species that underwent a complex series of losses or fusions of between selective parts of several arches that came together to produce a single composite mandible.

Serial Theory Composite Theory

Types of Jaw Attachment • Evolution of the jaw is often traced by how the mandible attaches to the skull. – Agnathans represent the earliest paleostylic stage in which none of the arches attach to the skull. – Euautostylic attachment is the earliest gnathostome condition, in which the mandibular arch is suspended from the jaw itself. – Early sharks exhibit amphistylic jaw suspension, in which the jaw is attached through two primarily articulations. • Modern sharks still have a variation of this attachment type

– Most modern bony fishes, jaw suspension is hyostylic because the mandibular arch is attached to the braincase primarily through the hyomandibula. – Amphibians, reptiles, and birds have metautostylic attachment of the jaw. • Jaws attach directly through the quadrate bone. • The hyomandibula plays no part; instead it gives ride to the columella (stapes) involved in hearing. • Other parts of the third arch give rise to the hyoid apparatus that supports the tongue. – In mammals, jaw suspension is craniostylic: the entire upper jaw is incorporated into the braincase. • The lower jaw, suspended by the squamosal bone, is composed entirely of dentary bone. • The paloquadrate and Meckel’s cartilage still develop, but remain cartilaginous except at the end where the incus and malleus develop.

Dermatocranium • Dermal bones that contribute to the skull comprise the dermatocranium. • Phylogenetically, these bones arise from the bony armor of the integument of early fishes and sink inward to become part of the developing skull. • Dermal bones first become associated with the skull in ostracoderms. • In later groups, additional bones of the overlying integument also contribute. • The dermatocranium forms the sides and roof of the skull to complete the protective bony case around the brain. • Teeth that arise within the mouth are also of dermal origin.

Parts of the Dermatocranium • Dermal elements in modern fishes and living amphibians have tended to be lost or fused so that the number or bones present in the skull is reduced. • In amniotes, bones of the dermatocranium predominate, forming most of the braincase and lower jaw. • The dermal skull may contain a series of bones joined firmly at sutures in order to enclose the brain, these can be grouped in series and include: – Facial, Orbital, Temporal, Vault, Palatal, and Mandibular

Phylogeny of the Skull

Agnathans • The earliest vertebrates, known from soft tissue impressions only, do not possess any elements of the skull. • Ostracoderms possessed a head shield formed from a single piece of dermal bone. • Modern Cyclostomes lack bone entirely and have a braincase composed entirely of cartilage.

Fishes • Placoderms – Up to ½ of the body is covered in bony plates of dermal bone that enclose the pharynx and braincase. – Dermal plates in the head were joined into a cranial shield. – The braincase was heavily ossified, and the upper jaw attached to it. – In most a well defined joint existed between the braincase and first vertebra.

• Chondrichthyans – Cartilagenous fishes possess almost no bone. – Denticles are present embedded in the integument. – The dermatocranium is absent, instead the chondrocranium has been expended upward. – Modern sharks lack strong, direct attachment between the paloquadrate and the hyomandibula. • Instead, it is suspended by the Meckel’s cartilage and a strong ligament. – The first gill is reduced, forming a spiracle, or absent. – During feeding sharks may use suction to capture prey, more often they attack prey directly. • Upper and lower jaws articulate with each other, and are suspended from the hyoid arch. • This attachment allows the jaws to swing down and forward. – Jaw protrusion may assist in synchronized meeting of the upper and lower jaw. – Retraction of the jaws following feeding restores hydrodynamic properties.

• Actinopterygians: – Early members had long jaws extending to the front of the head, numerous teeth, and an operculum that covered the gills • These are lost in tetrapods – Within actinopterygians an extensive radiation occurred that continues to the present. • This makes it difficult to generalize about trends within the skull. • If a common trend exists it is for increased liberation of bony elements to serve a more diverse role in food procurement. – Most living actinopterygians are suction feeders. • Negative pressure sucks water and prey into the mouth. • Compression of the buccal cavity forces excess water out of the gills. • Suction feeders possess a well muscularized buccal cavity and powerful, kinetic jaws.

• Sarcopterygians: – In early lungfish. The upper jaw was fused to the ossified braincase. – This suggests that these fishes fed on hard foods. – Bone of the dermatocranium resemble those of actinopterygians, the palatoquadrate articulates with the nasal cavity and maxilla. – Unlike actinopterygian fishes, the braincase is typically ossified as two distinct units.

Nasal Capsules • Nasal capsules hold the olfactory epithelium in the form of a paired nasal sac. In actinopterygians, the nasal sac typically doesn’t open into the mouth, – Instead, its incurrent and excurrent openings establish a one way flow across the epithelium. • Each nasal sac of tetrapods opens directly into the mouth via internal naris, or choana. – Each nasal sac opens to the exterior by way of the external naris (nostril). – This helps establish a respiratory flow with the lungs.

Early Tetrapods • Early tetrapods arose from rhipidistian ancestors and retained many of their skull features. • Beginning in tetrapods, the hyomandibula ceases to be involved in jaw suspension and instead becomes dedicated to hearing. • The opercular series of bones is lost. • The pectoral girdle loses its attachment to the back of the skull. • Roofing bones and the chondrocranium become more tightly associated, reducing the mobility of the snout. • The stapes aids in hearing, but in early tertapods it is a robust bone that also acts as a support between the braincase and palatoquadrate.

• The skull of modern amphibians is much simpler that their fossil ancestors. • Many of the dermal bones are lost or fused into composite bones. • The splanchnocranium is reduced. • The hyomandibula plays no role in jaw suspension. • Taken over exclusively by the articular and quadrate bones. • The branchial arches of the hyobranchial apparatus support external respiratory gills in larva, but become reduced to the hyoid apparatus that supports the tongue. • On land amphibians typically use a sticky tongue to capture prey. • It is propelled by muscles, over short distances, and musclehydrostatic fluid projection, at a distance.

Primitive Amniotes • The skull roof, similar to early tetrapods, is formed from the dermatocranium. • The palatoquadrate of the mandibular arch is reduced into a epipterygoid and sperate quadrate. • The hyoid arch becomes the stapes and, in association with the lower jaw, assists in sound transmission.

• Temporal Fenestrae, openings in the outer dermatocranium are present in some amniotes. – Turtles are anapsid, having no openings – The diapsid skull, found in crocodilians, has two temporal fenestrae. • Birds and modern reptiles have a modified diapsid skull – Therapsids and mammals have synapsid skulls that contain a single opening.

Modern Reptiles • In lizards, loss of the lower temporal bar produces the modified diapsid skull. – This serves to partially liberate the posterior part of the skull from the snout, producing the mesokinetic part of the skull. • Without these kinematic linkages, jaw closure would be scissor –like, and law closing forces on the prey would have a forward component. – In the skull of many lizards, rotation of the linkages permits changes in the geometric configuration of the skull. • As a consequence lizards can alter the position of the tooth row

• In snakes, the frontal and parietal roofing bones have grown down around the sides of the skull to form most of the walls of the braincase. • Snake skulls are prokinetic, there are no bony cross connections which allows them to rotate each side of the jaw independently. • This is especially important during swallowing. • Crocodilians possess both cranial bars, and no evidence of kinesis is present. • Also, modern crocodiles possess a secondary palate which separates the nasal passages from the mouth.

Birds • Birds are also ancestrally diapsid, but show considerable modification. • Their braincase is enlarges to compensate for the larger brain within. • The palatal bones are varied, but show signs of reduction. • Birds are toothless and their jaw is covered by a keratinized sheath, or beak. • The skill is prokinetic, and a strong postorbital ligament extends behind the eye to the lower jaw.

Mammals • The skull of mammals represents a highly modified synapsid pattern. • Various dermal elements are lost. • Fusion of separate centers of ossification produce composite bones in the skull of placental mammals. • The occipital bone defines the foramen magnum and closes the posterior wall of the braincase. • A ventrally located occipital condyle articulates with the atlas, the first vertebra to produce the neck joint.

• On the side of the braincase, behind the orbit, a large temporal bone is formed by fusion of all 3 part of the skull. • The splanchnocranium has been modified to include the malleus, incus, and stapes of the inner ear. • Mammals possess 3 sets of turbinates within the nasal passage. • They increase the respiratory epithelial surface and aid in scent reception and heat loss.

Middle Ear Bones • Two profound changes in the lower jaw mark the transition from therapsid to mammal. 1. The loss of the postdentary bone of the lower jaw. 2. The presence of middle ear bones.

• In early synapsids, the lower jaw includes the tooth-bearing dentary bones. • In derived synapsids this set of bones has been lost and the dentary has enlarged to assume the exclusive role of lower jaw function. • Their removal from the jaw joint permits their more specialized role in transmitting sound. – This is believed to have occurred to assist in hearing. – Or, changes in feeding style freed these bones from their attachment to the jaw and secondarily allowed them to aid in hearing. • Either way, changes in the jaw were accompanied by changes in the method of food preparation prior to swallowing. – Mammals are the only group to chew, or masticate, their food. – Mastication requires changes in the jaw and tooth structure to prevent damage to either.

Secondary Palate and Akinesis • In addition to changes in the lower jaw, the presence of the secondary palate is also related to mastication. • The secondary palate includes a hard palate of bone and a posterior, fleshy, soft palate. – This effectively separates the food chamber form the respiratory chamber above. – Similarly, it completes the form roof of the food chamber, so the pumping action of the throat of an infant creates negative pressure within the mouth without interfering with respiration.

• Mastication has been accompanied by precise tooth occlusion. • Precise occlusion requires a firm skull, so mammals have lost all cranial kinesis, leaving an akinetic skull. • As a further consequence of precise occlusion the pattern of tooth replacement differs from most other mammals. • In lower vertebrates teeth wear and are replaced continually (polyphydont), so the tooth row is always changing. • To avoid disruption of occlusion, mammalian teeth are only replaced once (diphydont) with young, “milk teeth” being replaced with “permanent” teeth in adults

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