· The internal or external hardened tissue lodges to form the skeleton.
· Organism will remain small and slow-moving if there had been no skeletal support and to serve as levers on which muscles act.
· Skeleton of invertebrates is most often rated on the surface, forming a lifeless or dead whereas the skeleton of vertebrates develops often underneath the surface forming a living or growing endoskeleton.

Types of the Vertebrate Skeleton

· There are three types of skeletons developed in vertebrates.

1. Epidermal horny exoskeleton
· These are hard and horny or keratinized derivatives of one layer of skin, such as claws of reptiles, feathers of birds, and hairs, horns and hoofs of mammals etc.
· All living amphibians lack an exoskeleton.

2. Dermal bony skeleton
The dermal bony skeleton is derived from the dermis of the skin.
· It includes bony scales and plates or scutes of reptiles, fin-rays of fishes and antlers of mammals.

3. Endoskeleton
· Greater part of the vertebrate skeleton lies more deeply, forming the endoskeleton.
· It develops from mesenchyme.
· At the early embryonic stage, the endoskeleton is composed of cartilage, which is replaced by bone in most adult vertebrates.
· Such bones, deposited in place of pre-existing cartilages are cartilage replacement bones.
· Thus, they are extinguished from the dermal or membrane bones which directly form more superficially in the dermis without any pre-existing cartilage.

Functions of Endoskeleton

· The chief functions of the vertebrate endoskeleton can be enumerated as follows:
1. To provide physical support to the body by forming a firm and rigid internal framework.
2. To give definite body shape and form.
3. To protect by surrounding delicate internal organs like brain, heart, lungs, etc.
4. To permit the growth of huge body size (whale, elephant, extinct dinosaurs), since it is living and growing.
5. To provide a surface for the attachment of muscles.
6. To serve as levers on which muscles can act.
7. To manufacture blood corpuscles in the bone marrow.
8. To aid in hearing (ear ossicles).
9. To help in breathing (tracheal rings, ribs).

Subdivisions of the vertebrate endoskeleton
· The endoskeleton of vertebrates is further subdivided into 3 major categories on the basis of their location in the body: axial, appendicular and heterotypic. 
· Each of these categories includes several elements.
· According to another scheme, the endoskeleton can be divided first into somatic and visceral skeletons, as follows:
1. Somatic skeleton (Skeleton of body wall)

(a) Axial skeleton: It includes vertebral column, ribs, sternum and most of the skull (neurocranium and dermatocranium).
(b) Appendicular skeleton: It includes girdles and limb bones.
2. Visceral skeleton: (Skeleton of the pharyngeal wall (splanchnocranium)

· The skeletal structure forming the framework of the vertebrate head is called the skull.
· It is an important structure that is derived from three major embryonic components:
(1) neurocranium or chondrocranium,
(2) dermatocranium and
(3) splanchnocranium.

1. Neurocranium or Chondrocranium

· It includes:
(1) the cranium or brain box that houses the brain and
(2) three pairs of sense capsules form a complete cartilaginous roof over the brain.
· Some openings remain uncovered for cranial nerves and blood vessels.
· The largest of all is the foramen magnum at the posterior end of the chondrocranium for the spinal cord.

2. Development of splanchnocranium
· It develops partly from neural crest cells and partly from splanchnic mesoderm.
· It includes a visceral or pharyngeal skeleton consisting of a series of horseshoe-shaped paired cartilaginous arches (usually 7 pairs) encircling and supporting the pharynx between gill-clefts.
· The arches remain united and interconnected ventrally but are free dorsally.
· In jawed vertebrates or gnathostomes, the first or mandible arch on either side is divided into a dorsal palatopterygoquadrate cartilage forming the upper jaw, and a ventral Meckler's cartilage forming the lower jaw.
· The second or hyoid arch on either side gives out dorsal hyomandibular cartilage to support and connect jaws to chondrocranium below auditory region, and ventrally forming the hyoid apparatus supporting tongue.
· The remaining or branchial arches support the gills or larynx.

Skull in Different Vertebrates

· A comparative study shows that the basic architectural pattern of the three major components of the skull (viz. neurocranium, dermatocranium and splanchnocranium) is essentially the same in all vertebrates.

1. Cyclostomata

· Skull is the most primitive among living cyclostomes.
· It retains cartilaginous embryonic neurocranium with an imperfect fibrous roof without dermal plates or bones.

2. Chondrichthyes
· In elasmobranches, neurocranium is cartilaginous.
· The brain is completely roofed.
· Olfactory and otic capsules are fused with chondrocranium, but optic capsules remain free.
· Dermal bones are absent.

3. Osteichthyes
· In ganoids or primitive bony fishes, such as gar, sturgeon, spoonbill etc. and in earlier crossopterygians, neurocranium is flat, completely roofed, cartilaginous and partially ossified forming many sculptured dermal bones by the fusion of dermal scales.
· In some primitive teleosts (Trout, Salmon), chondrocranium is mostly cartilaginous but in higher teleosts, the skull is highly specialized, laterally compressed and well-ossified.
· Cartilaginous visceral arches have been changed to bones or replaced by dermal bones.
· Palatoquadrate cartilages do not meet anteriorly.
· Upper jaw is formed by premaxilla and maxilla, which are dermal bones.
· Lower jaw (Meckel’s cartilage) has three bones: dentary, angular and articular-the last hinging on the quadrate which attaches to the cranium.

4. Amphibians
· Modifications in skulls of amphibia over that of fishes are correlated with a shift from water to land.
· There are fewer bones much more embryonic cartilage in skulls of modern amphibians, which is markedly platybasic and flattened.
· Hyomandibular becomes Comella of the middle ear which articulates for transmitting sound waves.
· Two occipital condyles (dicondylic) one on each ex-occipital, are present.

5. Reptilia
· In modern reptiles, neurocranium shows extensive ossification except in nasoethmoidal region.
· There is one occipital condyle (monocondylic) and more dermal bones than in Amphibia.
· Skull is tropibasic and two temporal fossae occur behind the orbits, in Chelonia.
· Pineal foramen is lost except in sphenodon and many lizards.
· Prootic, epiotic and opisthotic of the otic region remain separate.
· A quadratojugal is absent.
· Quadrate is movable at both ends showing streptostylism.
· There is a tendency to form a turbinal element in the nasal passage and to form a secondary palate.
· A transverse ectopterygoid and a vertical pterygoid are present.
· Hyomandibular is modified into columella of the middle ear.
· Lower jaw exhibits a large-toothed dentary, angular, supraangular, splenial, coronoid and articular bones.

6. Aves
· Bird skull is essentially reptilian in structure.
· Neurocranium is well-ossified.
· A single occipital condyle (monocondylic) occurs.
· Modifications are associated with flight and altered feeding habits.
· Skull is large, pneumatic and light, with very thin dermal bones and practically without sutures.
· Pre-maxillary and dentary are elongated to form a toothless beak necessary for feeding.
· Cranium is large and its roof is domed to accommodate the larger brain.
· Orbits are large, separated by a thin interorbital septum, and each with a ring of dermal sclerotic bones. Foramen magnum faces downwards.
· Like reptiles, there is a columella in the middle ear.
· Quadrate is streptostylic.
· Lower jaw has one cartilage bone (articular) and four dermal bones.

7. Mammalia
· Mammalian skull has two occipital condyles (dicondylic), a condition inherited from ancestral synapsid reptiles.
· All lower jawbones except dentary are absent.
· Occipital bones fuse into a single piece enclosing foramen magnum.
· Middle ear cavity has 3 ear ossicles: malleus (articular), incus (quadrate) and stapes (columella or hyomandibular).
· A complete secondary palate is present.
· Teeth are heterodont and present on premaxillae, maxillae and dentaries.
· Pterygoids are insignificant.
· Lower jaw on either side is made of a single dentary, there is no trace of Meckel’s cartilage.
· Hyoid arch mainly contributes to hyoid apparatus.
· Remaining visceral arches contribute to the thyroid, epiglottis, arytenoids, cricoids, tracheal rings, etc.

Suspensoria or Jaw Suspensions

· The vertebrate skull has three major parts: neurocranium, dermatocranium and splanchnocranium.
· The splanchnocranium includes the visceral arches.
· The first or mandibular arch consists of a dorsal palatopterygoquadrate bar forming the upper jaw, and a ventral Meckel’s cartilage forming the lower jaw.
· The second or hyoid arch consists of a dorsal hyomandibular that supports and suspends the jaws with the cranium and a ventral hyoid proper.
· The remaining arches that support the gills are known as branchial arches.
· Thus splanchnocranium plays an important role in the formation of jaws in gnathostomes and in their suspensions with the chondrocranium. The method of attachment of suspension of jaws from the chondrocranium is termed jaw suspension or suspensorium.

· There are principal variants or types of suspensoria as follows:
1. Autodiastylic
· This condition was found in some earliest gnathostomes such as acanthodians.
· The jaws are attached to the cranium by anterior and posterior ligaments.
· Hyoid arch remains completely free or independent and does not support the gill cleft in front of the hyoid arch bears complete gills and does not form any spiracle.

2. Amphistylic

· This is a rather primitive nor its arrangement is found in Crossopterygii, a primitive shark (e.g. Heptanchus) quadrate or the basal and otic processes of the upper jaw (mandibular arch) are attached by a ligament to chondrocranium.
· Similarly, the upper end of the hyomandibular (hyoid arch) is also attached to chondrocranium while the two jaws are suspended to its other end.
· This arrangement makes a double suspension.
· Sign (Amphi= both + style= bracing) since both first and second arches participate in bracing jaw against the chondrocranium.

3. Hyostylic
· It is found in most elasmobranchs and all bony fishes.
· Upper jaw palatoquadrate is loosely attached by anterior ethmoid-palatine ligament and posterior spiracular ligament to the cranium.
· Both the jaws are braced against the hyomandibular, the upper end of which fits into the auditory region of the skull.
· Since only the hyoid arch braces or binds the two jaws against cranium, this jaws suspension is termed hyostylic.
· It provides the jaws with a wider movement and helps in swallowing larger prey.

4. Autostylic

· This condition is found in extinct placoderms, chimaeras, lungfishes and most tetrapods (amphibians, reptiles and birds).
· Hyomandicular does not participate but becomes modified into columella or stapes of middle ear for transmitting sound waves.
· Upper jaw (palatoquadrate) is directly and intimately bound to the cranium by investing dermal bones (auto = self).
· The articular of the lower jaw articulates with the quadrate of the upper jaw.
· Autostylic suspensorium is widespread and has at least three variations or sub-types.

a. Holostylic

· In Holocephali (Chimaera), the upper jaw is firmly fused with the skull and the lower jaw is suspended from it.
· The hyoid arch is complete, independent and not attached to the skull.

b. Monimostylic

In many tetrapods, hyomandibular forms columella and articular articulates with the quadrate.
· However, the quadrate remains immovably attached to the skull.

Types of Jaw Suspensoria in Vertebrates

c. Streptostylic

· In some reptiles (lizards, snakes) and birds, quadrate is loosely-attached and is movable at both ends, a condition is known as streptostylism.

d. Craniostytic
· This type of jaw suspension is the characteristic of mammals and some consider it as a modification of autostylic suspension.
· Upper jaw fuses throughout its length with the cranium, and hyomandibular forms the ear ossicle, the stapes.
· But articular and quadrate also become modified into ear ossicles: malleus and incus, respectively.
· Consequently, two dermal bones, the dentary of the lower jaw and squamosal of the skull provide the articulation between jaws.

Vertebral Column

· Notochord: In all chordate embryos, the first axial endoskeleton to appear is a slender, stiff, unsegmented gelatinous rod, the notochord.
· It is present below the nerve cord and above the digestive tract.
· Its ancestral predecessor is not known but it probably originated from endoderm.
· Typically, the notochord is covered by inner and outer elastic fibres connective tissue sheaths, called elastic interna and elastic externa, respectively.
· In protochordate (Amphioxus) and Cyclostomes (Lamprey), notochord persists throughout the life and continues to grow with the animal.
· But in fishes and higher types, notochord, later on, is surrounded by cartilaginous or bony rings, called vertebrae.
· In most fishes and aquatic amphibians, the adult notochord is constricted within each vertebra.

· Backbone or vertebral column of all vertebrates is formed of a metameric series of many small and essentially similar pieces, called vertebrae.
· Thus, a vertebra is the unit of the vertebral column.
· Vertebrae are named after the region of the body in which they occur.
· Vertebral column of fish comprises only trunk (abdomen) and caudate (tail) vertebrae.
· In tetrapods, the vertebral column includes five regions: cervical, thoracic, lumbar, sacral and caudal, each having usually several vertebrae.
· Amphibians have a single cervical (Atlas) and only one sacral (9th) vertebra.
· Morphologically, vertebrae differ in different vertebrates or even in different regions of the same vertebrate, but all vertebrae are built according to a similar basic pattern.

· The basic structure of a vertebra:
· Typically a vertebra has a cylindrical spool-like body or a centrum which encloses or replaces the embryonic notochord.
· Above the centrum is a neural arch produced dorsally into a neural spine.
· Successive neural arches enclose a vertebral or neural canal in which the spinal cord lies.
· The caudal vertebra in fishes also has a ventral haemal arch enclosing a haemal canal through which the caudal artery and vein pass.
· Haemal arch also carries a ventral haemal spine.

Types of Processes:
· Various kinds of processes (apophyses) arise from the arches or centra of vertebrae.
a. Zygapophyses:
· Invertebrates, from anterior and posterior faces of neural arch project paired articular facets, the pre and postzygapophyses.
· These serve for the articulation between adjacent vertebrae.
· Zygapophyses do not occur in the fish vertebra.

b. Transverse processes:
· Lateral transverse processes arise from the centrum and serve for the attachment of ligaments and muscles.

c. Diapohyses:
· Each project laterally from the centrum or neural arch and articulates with dorsal head or tuberculum of thoracic rib.

d. Parapophyses:
· Each project laterally from the centrum and articulates with the ventral end (capitulum) of the rib.

e. Basapophyses:
These project ventrolaterally from the centrum or haemal arch or meet ventrally to form a haemal arch.

f. Pleurapophyses:
· These are lateral transverse processes fused with short ribs at the tip.

g. Hypapophysis:
· It is a single prominent mid-ventral projection of certain vertebrae.

Types of Centra and Vertebrae

· Intervertebral disc or intercentrum is often present between centra of successive vertebrae in embryos.
· This may fuse with the anterior or posterior end of the centrum changing its shape to convex or flat.

· On the basis of the particular shape of centra, the following main types of vertebrae occur:
a. Procoelous (pro = in front + coelous -hollow)
· The anterior face of the centrum is concave whereas the posterior face is convex.
· Example: typical vertebrae of amphibians and most reptiles.

b. Opisthocoelous (opistho = at the back)
· Centrum is concave posteriorly and convex anteriorly.
· Example: cervical vertebrae of some large ungulates.

c. Amphicoelous (amphi = both)
· Centrum is concave at both ends. Example: vertebrae of most fishes and tailed amphibians, 8th vertebra of the frog.

d. Acoelous or amphiplatyn (a =absent, amphi- both, platy- flat)
· Centrum is flat at both ends, without any convexity and concavity at both ends.

· Example: vertebrae of mammals.

e. Bicovex (bi- both)
· Centrum is convex at both ends.

· Example: sacral or 9th vertebra of the frog.

f. Heterocoelous (hetero = asymmetrical)

· The ends of the centra are shaped like a saddle.

· Example: vertebrae of modern birds

Types of Vertebra based on Shape of Centra

Also, Read our Other Notes Important for Entrance Exams:

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