7 Major Vertebrates and their Skulls | Chordata | Zoology

In all the vertebrates neurocranium, dermatocranium and splanchnocranium are the same. But still there are differences in form and detailed structure of skull.

Vertebrate # 1. Cyclostomata:

In cyclostomes the skull is cartilaginous and very incomplete. In hagfishes it consists of a plate of cartilage on which brain rests. The otic capsules are joined to the posterior part of this plate.

In lamprey sides are also present, but a roof is lacking and the brain is protected dorsally only by fibrous connective tissue. Jaws are lacking in cyclostomes. Gill area is supported by a complicated cartilaginous basket and not homologous with gill supports of other vertebrates.

Vertebrate # 2. Chondrichthyes:

The skull is cartilaginous and brain is roofed. Both the olfactory and otic capsules are incorporated into the skull. No true bone is present in living cartilaginous fishes. The optic capsules remain free. The first or mandibular arch is modified to form an upper and lower jaw, known respectively as the palatoquadrate cartilage and Meckel’s cartilage.

The upper jaw is loosely attached to the skull and is supported posteriorly by the upper or hyomandibular element of second arch (hyostylic suspension). In chimaeras, upper jaw is firmly fused to the cartilaginous skull (autostylic suspension).

Vertebrate # 3. Osteichthyes:

In lower ganoids such as sturgeons, the chondrocranium is essentially unossified. However, there are a number of dermal bones, presumably derived from scales that have sunk beneath the surface, that cover the root of skull.

In higher bony fishes dermal bones are numerous, forming an armour around the skull, and parts of the chondrocranium are replaced by bone in occipital, sphenoidal, otic, and ethmoid regions. Jaw suspension in bony fishes may be either hyostylic (Holostei and Teleostei) or autostylic (Dipnoi).

The mandibular cartilage or its derivatives become invested with dermal bones. The dentary is anterior and dorsal in the jaw with one or more splenials, an angular, a supra-angular and a coronoid. Head of Meckel’s cartilage may be replaced by a bone, the articular which articulates with the quadrate.

Vertebrate # 4. Amphibia:

The skull of amphibians shows a reduction in the number of bones and a flattening of the skull. In the modern amphibian skull, the principal bones are paired premaxillaries, maxillaries, nasals, frontals, parietals and squamosals. Much of the upper surface of the head lacks a bony covering especially anurans. Parts of the chondrocranium still remain unossified.

The underside of brain case is covered by dermal bone, the parasphenoid. The length of the occipital region is much reduced. The embryonic chondrocranium persists considerably in the adult, only some of it is replaced by cartilage bones. The basioccipital supraoccipital, basisphenoid and presphenoid are absent. The otic capsule has a ventral membrane covered aperture, the fenestra ovalis into which the stapedial plate of the columella fits.

The dorsal surface of the skull is solidly covered by dermal bones leaving opening for nares and eyes. Exoccipitals are ossified and bear two occipital condyles one in each exoccipital. The basisphenoid, presphenoid and alisphenoid are cartilaginous. Epipterygoid is never ossified in amphibians and incompletely ossified in urodeles and fused with the otic region. Articular of lower jaw is bony in caudates and cartilaginous in anurans. A dentary, splenial and angular may be present.

Vertebrate # 5. Reptilia:

In the skull two more segments have been incorporated than in Amphibia, hence, the hypoglossal nerve emerges from the skull. The entire chondrocranium is ossified except the naso-ethmoidal region. There are more dermal bones than in amphibians. The parietals are fused together (except Chelonia) and an interparietal foramen is prominent in lizards.

Between the orbits is an interorbital septum which is imperfect in lizards. Behind the orbits are one or two temporal fossae (except Chelonia) for muscles of the jaws. In Chelonia, the roof of skull is solid having no temporal openings.

Parasphenoid begins to disappear and is fused with the basisphenoid. In the occipital region all four bones surround the foramen magnum, and there is a single occipital condyle. The prootic, epiotic, and opisthotic remain separate in the otic region. A stout ectopterygoid or transverse bone is added between the maxilla and pterygoid (except Chelonia), and a vertical epipterygoid runs from the prootic to the pterygoid.

The lower jaw in each half has one cartilage bone and five dermal bones, the Meckel’s cartilage becomes very slender. The quadrate forms a movable union with the squamosal on one side and on the other side the quadrate articulates with the lower jaw, such a suspension is known as streptostylic. In turtles, crocodiles and tuatara, quadrate is fused with the skull.

A quadratojugal is absent the hyoid arch forms a columella of the middle ear from its hyomandibular, the lower part of the hyoid arch and third and fourth visceral arches form the hyoid. The remaining visceral arches form cartilages of larynx and trachea. Secondary palate is well developed in crocodiles, less developed in turtles and absent in others.

Vertebrate # 6. Aves:

The skull is made on the same plan as in reptiles and it resembles the skull of lizards. The bird skull differs in being larger and very light due to pneumatic bones. Its bones are fused together so that there are practically no sutures. There is a large, arched cranium due to a greater development of the brain which lies in the posterior part. The orbits are immense and lie in front of the cranium.

They are separated by a thin interorbital septum, thus, pushing the brain case posteriorly, developed better than in reptiles. Orbits are very large and each is continuous with a temporal fossa. Each orbit is surrounded by a ring of thin dermal bones called sclerotics. There is a large pointed beak formed by the premaxillae and dentaries. There are no teeth in modern birds.

There is a single occipital condyle formed from basioccipital and exoccipitals, shifted to the ventral side so the skull is at right angles to vertebral column. Foramen magnum is surrounded by all four occipital bones. Auditory capsules are sunk inwards, their bones fuse together and also with adjoining bones. The Meckel’s cartilage is much reduced, it has one cartilage bone and four ensheathing dermal bones.

There is a freely movable quadrate articulating with the lower jaw, the suspension is streptostylic in which the lower jaw comes to have two movable joints, one at each end of the quadrate. Like reptiles, hyomandibular forms the columella of the middle ear, the rest of the hyoid arch, along with the first branchial arch forms the hyoid apparatus.

Vertebrate # 7. Mammalia:

There is much variation in the skulls of different mammals, yet there are certain common features. In the evolution of the skull from fishes to mammals there has been a considerable reduction in the number of skull bones by loss and fusion, (usually 35 in number). The prefrontals, postfrontals, postorbitals and quadratojugal are lacking.

In mammals four occipital bones are fused together. Presence of two occipital condyles, one in each exoccipital. Various degrees of fusion occur in the sphenoidal area. The presphenoid, orbitosphenoids, basisphenoid and alisphenoids may be separate or all fused into a single bone. Due to the formation of a large brain the cranium is very large having expanded dorsally and laterally.

Its cavity is closed in front by a cribriform plate. The petrosal may fuse with the squamosal to form the temporal bone. The tympanic encloses the ear ossicles, the quadrate of the upper jaw forming the incus, articular of the lower jaw forming the malleus and hyomandibula or columella of reptiles forms the stapes.

The bony palate in mammals is formed from the premaxillae, maxillae and palatines, in some the pterygoids also contribute to the bony palate.

The lower jaw in each half is made of a single dentary, with no trace of Meckel’s cartilage in the adult. It articulates with a glenoid fossa in the squamosal. The major part of the hyoid arch forms a hyoid apparatus. The remaining visceral arches form the thyroid, epiglottis, arytenoids, cricoid in the larynx and also the rings of the trachea.