THE STRUCTURE OF THE SKULL IN VERTEBRATES
STATION 1: COMPOSITION OF THE SKULL
As discussed in lectures there are three elements which contribute to skull formation in the vertebrates:
A) The Neurocranium (Chondrocranium)
The original cartilaginous box (or Chondrocranium) of the Elasmobranchs (sharks and rays), which has now been replaced by Bone (Endochondral Bone). The neurocranium is a specialised portion of the splanchnocranium and comes from neural crest cells.
Identify the bones of the neurocranium in the wolf skull provided, using the lab drawings, and table, and the painted skulls on demonstration. Note that these bones are endochondral (or replacement or nondermal) bones. They cannot be distinguished from dermal bone, since they differ only in embryonic origin. The neurocranium bones are painted black on the demonstration wolf and cat skulls. These bones are grouped as the occipitals, sphenoids and ethmoids.
The original cartilaginous neurocranium formed a box housing the brain. You will cut into this box when looking at the dogfish brain. The cranial nerves punch through the cartilaginous sides of the box. Now the skull is bone.
Can you see the holes where the cranial nerves exit?
What group of bones now form the back of the box?
What group of bones form the bottom and sides of the box? The front of the box is the perpendicular bone dividing the nostrils. What is it called?
On either side, in the nostrils are a complex of thin bones looking like swirls of flaky pastry. What are they called?
What happens with the top of the box?
Draw a stylized (rectangular) brain box and label the parts of the neurocranium.
B) The Splanchnocranium
The contributions of the original gill arches to the skull. The original branchial skeleton of cartilage came from neural crest cells
The ossifications of the Splanchnocranium of the teleost fishes cannot be readily identified on the wolf skull since they are either the framework for subsequent dermal tooth-bearing bones, or have moved from their association with the jaws to form the ear bones: quadrate (incus), articular (malleus) and hyomandibular (columella or stapes). Look at the alligator and identify the quadrate and articular bones.
C) The Dermatocranium
The original dermal scales (or armour) of Ostracoderms sink down and have attach to the neurocranium and are ossified as the Dermal Bones. These are from Dermatome (Epimere mesoderm). See dermal armour of Amia, the bowfin which sits on the cartilaginous neurocranium. In other animal groups the cartilage disappears and when you look down on the skull you are looking at the bones of the dermatocranium.
The dermatocranium contributions to the mammal skull that we wish to learn are all those bones that are labeled on the drawing of the wolf skull, plus the dentary bone of the lower jaw. Identify all of these dermal bones on the wolf skull and learn them. These bones are grouped as the facial, vault, orbital, temporal, palatal and mandibular series.
Loss of Dermal Bone
There are skulls of Amia, Alligator and Wolf on display. Using the drawings and the skulls provided, identify the dermal bones in each skull. Count the total number of dermal bones (paired and unpaired) in each species. Please use only the drawings in this lab guide for the count. Note that one pair of bones on one of the drawings is from the splanchnocranium and should not be counted. If any bone is not listed in the Table, it is dermal in origin.
These drawings should provide an understanding of the trend towards loss of dermal bone (WillistonÕs Law).
STATION 2: MODIFICATIONS TO THE SKULL: Fossae
Modifications for more powerful jaws.
Study the Turtle skull on demonstration. See the otic (or temporal) notch in the dorsal posterior region on both sides of the midline. This is an adaption for muscle attachment that is nccessary because of increased jaw musculature, and to offset the interference of the dermal bone contributions. The turtle skull, like the fish skull has no fossa (hole) and is anapsid.
For the same purpose as the Otic Notch, namely, jaw musch attachment, there evolved in other groups of reptiles a pair of openings on either side of the skull in the temporal region, called the temporal fossa. Study the location of the supratemporal fossa, and the infratemporal fossa on the skull of the Alligator. The presence of two temporal fossae as in the alligator is the diapsid condition. This condition is found in some reptiles and birds. Some fossil reptiles lost the lower (Infratemporal) fossa - this is the parapsid condition (now extinct). The loss of the supratemporal fossa and presence only of the infratemporal is the synapsid condition. It occurred in some extinct reptiles, and is represented now by the mamnals. Study examples of the above on demonstration. Note the eye orbit may be separate from the fossa (cat, horse, human) or confluent (wolf, rat).
Notice on the wolf skull the median longitudinal sagittal crest to attach temporal muscles and a transverse nuchal crest for muscles supporting the head.
STATION 3: PALATE
Modifications for breathing air. Secondary Palate:
There have been three stages in the evolution of the secondary palate. The fishes and amphibia have a complete roof to the mouth which is the primary unmodified palate. It is the floor of the neurocranium. This was inconvenient when breathing while eating.
How do amphibians get away with using this primitive system?
Reptiles show a trend in the evolution of a Secondary Palate. The Turtle on demonstration shows a development of the maxilla, premaxilla and palatine bones which provided a partial secondary palate.
The Alligator is a further stage and shows a complete bony secondary palate. Mammals (Wolf, Ox) have a functional complete secondary palate, though not the complete bony palate of Alligator, the posterior portion being the fleshy soft palate, with the hard palate in the anterior. In order to save weight, birds have a totally fleshy secondary palate.
STATION 4: JAW ATTACHMENT TO THE SKULL
The Splanchnocranium of the mandibular arch (2nd segment) formed the upper and lower jaws of cartilage called the palatoquadrate (upper) and MeckelÕs cartilage (lower). The hyoid arch (3rd segment) surrounded the spiracle opening.
The Ostracoderms, with one fused head plate of dermal bone (extinct) and modern Agnathans (e.g. Lamprey), lack jaws and display the paleostylic condition, in which none of the gill arches are directly associated with the skull. The hyoid arch is a functioning gill arch.
The Placoderms, with several plates of dermal bones in the head (extinct) and primitive fish, Holocephali (e.g. the Rat Fish-Chimaera) display the condition in which the palatoquadrate articulates, or is fused to, the chondrocranium with no supporting function from the hyoid arch. This is the autostylic condition of jaw suspension.
Early sharks and bony fish (now almost all extinct) had the palatoquadrate attached by ligaments to the chondrocranium. In addition, the hyoid arch specialized to form the Hyomandibular which helped to stabilize the posterior end of the jaws. This double type of suspension is referred to as amphistylic jaw suspension.
In modern bony fish and modern day sharks (e.g. dogfish) the Hyomandibular of the Hyoid arch forms a bridge attaching the jaws to the skull. The jaws, free from the skull, can be swung forward a little. Connective tissue and devices like optic pegs help position the jaws. This type of suspension is known as hylostyic. In bony fish, the quadrate and articular bones replaced the cartilage and several dermal bones covered the jaw cartilages.
What is the advantage in having a swinging jaw?
In tetrapods, the upper jaw alone suspends the lower jaw. This condition is metautostylic. This frees the hyomandibular of the hyoid arch from jaw suspension and it is incorporated into the ear. The number of upper and lower jaw bones progressively becomes reduced. Mammals have only one paired bone, the dentary, in the lower jaw. The articular and quadrate bones are jaw joints in most vertebrates but are moved to the ear in mammals.
What are the bones of the upper and lower jaws in mammals?
What does the hyomandibular do in fishes?
What does the hyomandibular become in amphibians and reptiles?
What does the hyomandibular become in mammals?
What does the articular bone become in mammals?
What does the quadrate bone become in mammals?
Lab updated September/03 by Sandra Millen