G. AXIAL SKELETON – THE SKULL AND VISCERAL SKELETON
1. Explain cephalization.
Many different animal lineages have undergone cephalization, basically “the evolution of a head.” Cephalization involves concentrating neurons into a brain at one end of the animal and evolving sensory organs at that same end.
2. Describe the neurocranium. What is its function?
The neurocranium is the portion of the skull surrounding the brain, including the elements that surround the olfactory, optic, orbital or sphenotic, and otic or auditory capsules and the anterior end of the notochord (endocranium) and the series of overlying dermal bones (dermocranium). Also called braincase. Its function is for protection.
3. Describe the chondrocranium. Describe the two cartilages that contribute to it and the formation of the ethmoid and basal plates.
The chondrocranium is the part of the neurocranium formed by endochondral ossification and comprising the bones of the base of the skull. Prechordal, or cartilage “in front of” the notochord, and parachordal, or cartilage “beside and behind” the notochord, contribute to the chondrocranium. Parachordal cartilages expand and along with the notochord form the basal plate. Prechordal cartilages expand and join to form an ethmoid plate.
4. Name the sense capsules that contribute to the braincase.
Olfactory capsules, otic capsules, optic capsules
5. Define occipital condyle and foramen magnum.
Occipital condyle – an articular surface on the occipital bone on each side of the foramen magnum by which the skull articulates with the atlas
Foramen magnum – large orifice in the base of the skull through which the spinal cord passes to the cranial cavity and becomes continuous with the medulla oblongata
6. Name the four ossification centers of the neurocranium. Approximate the location of these centers. (See Skull Handout.)
Occipital, otic, sphenoid, ethmoid (See handout for approximate location.)
7. Describe the dermatocranium and its origin. Name the four major categories of dermatocranium bones. Identify prominent bones in these categories and approximate their locations. (See Skull Handout.)
The dermatocranium consists of dermal bones that encase the chondrocranium and splanchnocranium and contribute to the braincase, jaws, and skeletal elements of the mouth (teeth). Phylogenetically, it is a membrane bone which probably evolved from dermal armor.
The four major categories of dermatocranium bones are roofing bones, upper and lower jaw bones, primary palatal bones, and opercular bones. Roofing bones include nasal, frontal, parietal, orbit, and squamosal. Upper and lower jaw bones include, premaxilla, maxilla, dentary, splenial, surangular, angular, coronoid, and prearticular. Primary palatal bones include vomer, palatine, and pterygoid. (See handout for approximate locations.)
8. Describe the splanchnocranium.
The splanchnocranium consists of visceral arches that support and move the gills and contribute to production of the jaws in gnathostomes. In more advanced vertebrates, parts of the splanchnocranium are modified to form derived structures such as jaws, ears and parts of the hyoid apparatus and pharyngeal cartilage.
9. List the four main portions of a typical visceral arch. Name the portion which may serve as a mid-ventral link of right and left arch.
Each arch, from dorsal to ventral, consists of cartilages or bars: pharyngobranchial, epibranchial, ceratobranchial, hypobranchial, and (connecting right and left sides) a series of basibranchial cartilages.
10. Discuss the evolutionary trends of visceral arches, including mandibular arch and hyoid arch.
Visceral arches originally evolved to support the gills. This part of the skull forms as cartilage and remains as cartilage in lampreys, sharks, and other cartilaginous fishes. In bony fishes and tetrapods, most or all of the cartilage of the visceral arches becomes ossified. The visceral arches play a key role in vertebrate evolution, because the anterior visceral arches eventually evolved into the mandibular arch and other parts of the splanchnocranium give rise to the hyoid apparatus that supports the tongue and the trachea and larynx.
11. Describe the head (neurocranium, splanchnocranium, and dermatocranium) of jawless vertebrates.
The chondrocranium, a cartilaginous neurocranium, protects the brain, otic, and nasal organs. The splanchnocranium is present as a continuous visceral skeleton, has a pharyngeal basket, and is branchial in function. The dermatocranium is present as a dermal head skeleton and varies depending on group.
12. Describe the head (as above) of placoderms.
In placoderms, the chondrocranium is usually cartilaginous. The first arch of the visceral skeleton, or splanchnocranium, form jaws. The dermal skeleton, or dermatocranium included an armored head and thoracic shields.
13. Address the jaw suspension of placoderms.
The splanchnocranium exhibits autostylic suspension, having the mandibular arch articulated directly to the cranium.
14. Address the jaw suspension of cartilaginous fish.
Primitive sharks exhibit amphistylic suspension, in which the second gill arch (hyoid) plays no part in the suspension of the lower jaw. Most sharks exhibit hyostylic suspenion, in which the upper jaw loses any major direct connection with the braincase and the upper and lower jaws are supported solely by the hyomandibula. Chimeras exhibit autostylic suspension, in which the upper jaw (palatoquadrate) articulates or is fused with the chondrocranium, lower jaw forms from the mandibular cartilage, and the jaw remains unsupported by the hyomandibula.
16. Describe the head of bony fish.
The chondrocranium of early fishes was already well ossified, of dipnoans was cartilaginous except exoccipitals, and of crossopterygians was somewhat cartilaginous with a jointed braincase. The splanchnocranium, or visceral skeleton, exhibits bone replacement instead of cartilage. The quadrate, or upper jaw, replaces the palatoquadrate. The only replacement bone in lower jaw is the articular. The second arch not branchial in function. The hyomandibula usually supports jaw except in dipnoans with autostylic suspension. Ray-finned fish are hyostylic. Crossopterygians are amphistylic. The splanchnocranium has branchial arches. The dermatocranium, or dermal skeleton, includes a pectoral girdle joined to skull and an operculum that covers the gills. Each major taxa has its own pattern of dermal bones.
17. Describe the head of amphibians.
The heads of ancestral amphibians had nearly a full complement of replacement bones with only the supraoccipital missing. Lissamphibians lack basioccipital and basisphenoid and have paired occipital condyles (exoccipitals). In the visceral skeleton, the quadrate articulates with the squamosal using autostylic suspension. The hyomandibula became the ear ossicle (stapes or columella). The hyoid arch elements support a large, fleshy tongue. The more posterior arches are generally reduced to three, and also support tongue and larynx (hyoid apparatus). In the dermatocranium, the operculum is lost, and the pectoral girdle is no longer joined to the skull.
18. Examine the jaw suspension of tetrapods.
In tetrapods, the upper jaw alone suspends the lower jaw. This condition is autostylic. This frees the hyomandibular of the hyoid arch from jaw suspension and it is incorporated into the ear.
19. Describe the head of reptiles.
The neurocranium has a decreased number of bones, one occipital condyle and is well ossified. The dermatocranium includes a full complement with some reduction in roof bones, a temporal region, and a secondary palate. It exhibits cranial kinesis. The splanchnocranium includes a quadrate with autostylic suspension, an articular, a stapes, a hyoid apparatus, a larynx, and tracheal rings. Differences occur in therapsids, having two occipital condyles, a synapsid skull, a free quadrate and articular, and a squamosal and dentary that articulate.
20. Explain how anapsid, synapsid, euryapsid, and diapsids differ. Explain the advantage of temporal fossae/fenestrae.
Anapsid – no temporal openings in the skull (turtles)
Euryapsid – one temporal opening bordered below by postorbital and squamosal (plesiosaurs, ichthyosaurs).
Synapsid – one temporal opening bordered above by postorbital and squamosal (mammals)
Diapsid – two temporal openings separated by postorbital and squamosal (archosaurs, Sphenodon)
The presence of temporal fossae or fenestrae allows the passage of chewing muscles.
21. Characterize a secondary palate. List and advantages it provides.
Paired vomers merge, migrate posteriorly and dorsally, the parasphenoid is lost, the pterygoids shorten, and shelf-like processes of maxillas and palatines grow to midline in front of shifting nares to form secondary palate. The palate allows simultaneous chewing and breathing.
22. Define cranial kinesis.
Cranial kinesis – movement between the upper jaw and braincase; provides a way to change the size and configuration of the mouth rapidly; optimize biting and rapid feeding.
23. How do therapsid heads differ from other reptiles? Why are these changes significant?
Differences occur in therapsid heads, having two occipital condyles, a synapsid skull, a free quadrate and articular, and a squamosal and dentary that articulate. Throughout the Mesozoic era, there was a series of successive radiations of therapsid reptiles, each of which tended to make a closer approach to the mammalian condition than its predecessors. By the end of the Mesozoic, the changes developed by this group had resulted in a form of body organisation that was typically mammalian.
24. Describe the head of birds. Categorize it according to the number of temporal fossae.
The neurocranium is larger, highly vaulted, incomplete dorsally, well ossified and fused, with one occipital condyle. The dermatocranium has one big opening as the supratemporal arch is lost, thus is it considered a modified diapsid. The beak is comprised of the premaxilla and the dentary. It exhibits cranial kinesis. The splanchnocranium is similar to that of reptiles.
25. Name the bones of the beak.
Premaxilla and dentary
26. Describe the head of mammals.
Neurocranium – larger, fusions, sutures, loss of bones, fontanels, sinuses, two occipital condyles
Dermatocranium – nasal structures with conchae, tympanic bulla, malleus, squamosal articulation with dentary bone, synapsid, decreased number of bones
Splanchnocranium – quadrate becomes incus, articular becomes malleus , hyomandibula previously evolved to become the stapes
27. Identify any unique changes to the mammalian skull.
Certain bones tend to fuse: postparietal with supraoccipital, basioccipital with exoccipitals, otic bones with squamosal (temporal), four sphenoid bones into one sphenoid.
Bones not found in mammals include: prefrontals, postfrontals, postorbitals, quadratojugals, parasphenoid.
All other dermal bones of lower jaw have been lost except the angular, prearticular, and articular, which have shifted away from lower jaw.
28. Describe the articulation between the upper and lower jaws in mammals.
In mammals, the dentary bone (mandible), the only bone in the lower jaw, articulates with the squamosal bone (part of temporal bone). No longer needed with this new jaw articulation, the articular and quadrate bones migrate into the middle ear to become the malleus and incus respectively, two of the three mammalian ear ossicles that conduct sound from the tympanic membrane to the inner ear.
29. Name the sole mammalian lower jaw bone.
30. Categorize the mammalian skull according to the temporal fossae.
Synapsid – one temporal opening with an infratemporal = zygomatic arch
31. Define suture, fontanel, paranasal sinus, conchae, turbinate, and tympanic bulla.
Suture – the line of junction or an immovable joint between two bones, especially of the skull
Paranasal sinus – any of the paired cavities, designated frontal, sphenoidal, maxillary, and ethmoidal, located in the bones of the face and lined by a mucous membrane continuous with that of the nasal cavity
Conchae – cartilaginous or slightly ossified scrolls that occupy the major portion of each half of the nasal cavity and are covered with nasal mucosa
Turbinate – a bone situated along the side wall of the nasal cavity covered by mucous membrane
Tympanic bulla – round bone that encloses the middle ear
32. Trace the evolution of the visceral arches and their derivatives. See Handout.
About 100 pairs of gill slits in some protochordates, about 5 to 15 pairs in early vertebrates, about 6 pairs in most jawed vertebrates.
In jawless vertebrates, gill bars not jointed. Gnathostomes had jointed visceral arches; usually 4 paired segments per arch. First visceral arch enlarges to become jaws, mandibular arch. Epibranchial forms upper jaw and is called the palatoquadrate. Ceratobranchial forms lower jaw and becomes the mandibular cartilage. Second visceral arch becomes the hyoid arch. Epibranchial becomes hyomandibula. Succeeding arches become branchial arches.
33. Define columella, stapes, malleus, incus, larynx, and hyoid apparatus.
Columella – a rod-like bone (ossicle) that picks up sound vibrations from the eardrum and transmits them to a membranous oval window in the inner ear; rudimentary stapes
Stapes – a stirrup-shaped small bone or ossicle in the middle ear which attaches the incus to the inner ear
Malleus – a hammer-shaped small bone or ossicle of the middle ear which connects with the incus and is attached to the inner surface of the eardrum
Incus – an anvil-shaped small bone or ossicle in the middle ear that connects the malleus to the stapes
Larynx – an organ in the neck of mammals involved in control of breathing, protection of the airway and sound production
Hyoid apparatus – an articulation of bones that acts to suspend the tongue and the larynx beneath the skull
34. Discuss some evolutionary trends of the skull.
Throughout human evolution there has been a tendency to change skull proportions in two ways: first to increase the size of the brain and second to decrease the size of the jaws, jaw muscles and teeth. This has changed the overall shape of the skull: taken together these changes produce a taller skull, with fewer ridges for muscle attachment and a shorter retrousse face. The position of the foramen magnum has also changed, decreasing the need for powerful neck muscles. Also, there has been a decrease in the number of bones and in the number of visceral arches.