Exam 2 - Skull Flashcards
Functions of skull
- Protects the brain
- Holds some of the sense organs
- Feeding
- All of these functions involve modifications of the skull
Types of bone
- Endochondral or replacement bone
i. Bone derived from a cartilage model during development
ii. cartilage is converted into bone throughout life - Dermal bone
i. Bone develop directly without a series of intermediate steps
ii. Originated fin the dermis of the skin
• Vertebrate cranium is composed of three distinct elements:
chondrocranium, splanchnocranium (visceral cranium) and dermatocranium
Chondrocranium
i. Underplays and supports the brain and cranial sense organs
ii. Endochondral bone
iii. Five groups or elements:
1. Notochord
2. Trabeculae
3. Para chordal cartilages
4. Neural arches of cranial vertebrae
5. Sense capsules
Visceral cranium (Splanchnocranium)
- Arose to support the pharyngeal gill arches
- Gill arches gave rise to different structures
i. First arch enlarges and becomes the jaw
a. Epibranchial becomes the palatoquadrate
b. Ceratobranchial becomes the mandible
ii. Second arch becomes the hyoid apparatus
iii. Remaining arches function as gill arches
Dermal elements ‒ dermatocranium
- Outermost casing of the skull
- Dermal bone
‣ Also called membrane bone - Forms the majority of the skull in tetrapods
- Probably derived from the dermal armor of early fishes
Agnathas
- Chondrocranium
i. Braincase is cartiliginous
ii. Single medial nostril and a pineal opening
iii. Lacks bone - Visceral cranium ‒ cartiliginous basket that joints he chondrocranium
- Dermal elements are not homologous with other vertebrates
Chondrichthyans
- Chondrocranium
i. Almost entirely lacks bone
ii. Chondrocranium has expended upward and over the top of the random to for, the brain case
iii. Better developed than in any other vertebrate - Visceral cranium
i. Jaws are not all that strong
ii. Upper jaw and lower jaws articulate with each other and are suspended from the hyoid arch
◦Upper jaw is not attached to skull (weak jaws are an adaptation because they provide flexibility) - Dermatocranium is absent
Bony fishes
- Skull is varied and complex
- Largest group of vertebrates have many niches, reason for the diversity
- Chondrocranium
i. Have large eyes and small nasal capsules
ii. Crossopterygians have mostly cartilage and skull is very complete - Visceral cranium
i. Jaws have lots of teeth
ii. Operculum covers the gill arches
iii. Hyoid arch increased to support the mandibles so the jaws are stronger
iv. Palatoquadrate may be replaced by several bones
a. Quadrate (dermal) forms the upper part of the jaw articulation in bony fish, amphibians, reptiles and birds
b. Quadrate is lost in mammals
v. Articulate is the only endochondral bone in the lower jaw, it forms the part of the jaw articulation - Have a complete dermal skeleton
i. Pectoral girdle is attached to the skull by a series of bones
ii. First appearance of the operculum, diagnostic characteristic
Crossopterygians
i. Upper jaw (palatoquadrate) fused to the ossified brain case, which it self was a single unit
ii. Teeth were small and pointed with the walls extensively folded
iii. Teeth are called labryinthodont
Amphibians
- Arose from the crossopterygian fishes
- Subclass Labyrinthodontia was probably the first group and retains many crossopterygian skull features
- Early amphibians had labyrithodont teeth
- Most of the bones of the chondrocranium are retained in ancestral forms
- Ancestral forms had one occipital condyle; extant forms have two
- Visceral skeleton changes
i. Articular (lower jaw) and quadrate bones (upper jaw) involved in the jaw articulation
ii. Quadrate articulates with the squamosal one the brain case
iii. Jaws are suspended directly from the chondrocranium, no involvement of the hyomandibula
iv. Hyomandibula became involved in hearing; stapes - Most bones of the dermatocranium are retained
- Bones of the snout are reduced, distinct nasal bones are present
- Bones associated with the operculum are lost
- Pectoral girdle loses its attachment with the skull (neck)
- Optic notch is present and is the location of the ear drum
Reptiles and Birds
- Well developed chondrocranium
i. Bones are fused in adult birds
ii. Both groups have one occipital condyle - Visceral skeleton in both groups retains many amphibian characteristics
i. Quadrate and articular form the first arch
ii. Stapes form the second arch
iii. Hyoid apparatus forms the second, third and fourth arches
iv. Larynx and tracheal rings form the sixth and seventh arches - First visceral arch was highly modified, quite significant
i. Dentary bone was enlarged
ii. Articulate and quadrate became smaller and were eventually renamed
iii. Have a dentally and squamosal jaw articulation
iv. Articular and the quadrate became ear bones, malleus and incus respectively
v. Points iii and iv serve to differentiate mammals from mammal like reptiles
Direct evidence to support the origin of the mammalian ear bones
was found in 2007.
• Fossils of a 125 my old mammal, Yanoconodon allini show fragments of the cartilages that hold ear bones in place.
Reptile groups
i. Anapsid (no temporal fenestrata)
ii. Synapsids (1 temporal fenestrata)
iii. Diapsid (2 temporal fenestrata)
Development of reptiles
Reptilian are named based on the differences in the arrangement of the roof and side walls of the skull to the
muscles of the jaw.
i. In fish, amphibians and stem reptiles, the jaw muscles attach beside the brain case and under the skull’s
superficial roof bones.
ii. Over time, one or two pairs of openings (fenestrata=windows) formed in the temporal region of the skull and
the muscles moved out through them (holes made jaws stronger because it can grip better than surface grip)
