The Skull
Chapter 7
Introduction
• The skeleton gives the body its basic shape,
supports weight, offers levers for movement, and
protects soft parts of the body.
• Because of its make-up bones are often the only
part of animals left behind for fossilization.
• The skeletal system is composed of an
exoskeleton and an endoskeleton.
– Exoskeleton is from integumentary elements,
– The endoskeleton from mesoderm deep within the
body.
• During the course of vertebrate evolution,
most bones of the exoskeleton stay within the
integument and protect surface structures.
• Other bones have sunk inwards, merging with
the deeper bones and cartilage of the
endoskeleton into a composite structure.
– For this reason it is difficult to divide the skeletal
system based on embryonic origin and function.
• Therefore it is divided into two regions, the Skull and
the Post Cranial Skeleton.
• Although merged into a harmonious unit, the
skull (or cranium) is actually a composite
structure formed of 3 distinct parts.
– Each from a separate phylogenetic source.
1. The splanchnocranium, which first arose to
support pharyngeal slits,
2. The chondrocranium, which underlies and
supports the brain and is composed of
endochondral bone, cartilage, or both.
3. The dermatocranium, which forms the outer
casing of the skull and has its origins from dermal
bone.
Chondrocranium
• Elements of the chondrocranium appear to lie in series
with the base of the vertebrae.
• This has led anatomists to hypothesize that the back wall of the
skull may represent several ancient vertebral elements.
• In elasmobranchs, the expanded and enveloping
chondrocranium supports and protects the brain.
– It does not ossify, instead cartilage grows over the brain to
complete the protective walls and roof of the braincase.
• However, in most vertebrates the chondrocranium is
primarily an embryonic structure serving as the
scaffolding for the developing brain and sensory
organs.
– The remaining chondrocranium partially or fully ossifies
Splanchnocranium
• The splanchnocranium an ancient chordate
structure.
• In early chordates it is associated with the
filter-feeding structures.
• Among vertebrates it generally supports the
gills and offers attachment for respiratory
muscles
• In gnathostomes it also contributes to the
jaws and hyoid apparatus.
• Pharyngeal bars in protochordates arise from
the mesoderm and for the unjointed branchial
basket.
• Pharyngeal arches of aquatic vertebrates are
usually associated with their respiratory
structures.
• Because of this they are referred to as
branchial arches, or gill arches.
• Each arch is composed of a series of up to 5
articulated elements per side;
– Pharyngeobranchial, epibranchial,
ceratobranchial, hypobranchial, and basibranchial.
• Branchial arches that support the mouth are
called jaws.
• The first fully functioning arch of the jaw is the
mandibular arch, composed of the
paloquadrate (dorsally) and Meckel’s cartilage
(ventrally)
Origin of Jaws
• Jaws arose from one of the anterior pair of gill
arches.
• Evidence of this comes from several sources:
1. Embryology in sharks suggests that jaws and
branchial aches develop similarly in series and both
arise from the neural crest.
2. Nerves and blood vessels are distributed in patterns
similar to branchial arches and jaws.
3. The musculature of the jaws appears to be
transformed and modified from branchial arch
musculature.
• It is unknown if jaws originated from the a
combination of any of the 1st four branchial
arches.
– The serial theory is the simplest view and holds
that the 1st (or 2nd) arch gave rise exclusively to
the mandibular arch, the next arch exclusively to
the hyoid, and the remainder are the branchial
arches of gnathostomes.
– The composite theory states that 10 branchial
arches existed in primitive species that underwent
a complex series of losses or fusions of between
selective parts of several arches that came
together to produce a single composite mandible.
Serial Theory
Composite Theory
Types of Jaw Attachment
• Evolution of the jaw is often traced by how
the mandible attaches to the skull.
– Agnathans represent the earliest paleostylic stage
in which none of the arches attach to the skull.
– Euautostylic attachment is the earliest
gnathostome condition, in which the mandibular
arch is suspended from the jaw itself.
– Early sharks exhibit amphistylic jaw suspension, in
which the jaw is attached through two primarily
articulations.
• Modern sharks still have a variation of this attachment
type
– Most modern bony fishes, jaw suspension is hyostylic
because the mandibular arch is attached to the
braincase primarily through the hyomandibula.
– Amphibians, reptiles, and birds have metautostylic
attachment of the jaw.
• Jaws attach directly through the quadrate bone.
• The hyomandibula plays no part; instead it gives ride to the
columella (stapes) involved in hearing.
• Other parts of the third arch give rise to the hyoid apparatus
that supports the tongue.
– In mammals, jaw suspension is craniostylic: the entire
upper jaw is incorporated into the braincase.
• The lower jaw, suspended by the squamosal bone, is
composed entirely of dentary bone.
• The paloquadrate and Meckel’s cartilage still develop, but
remain cartilaginous except at the end where the incus and
malleus develop.
Dermatocranium
• Dermal bones that contribute to the skull comprise the
dermatocranium.
• Phylogenetically, these bones arise from the bony armor of
the integument of early fishes and sink inward to become
part of the developing skull.
• Dermal bones first become associated with the skull in
ostracoderms.
• In later groups, additional bones of the overlying
integument also contribute.
• The dermatocranium forms the sides and roof of the skull
to complete the protective bony case around the brain.
• Teeth that arise within the mouth are also of dermal origin.
Parts of the Dermatocranium
• Dermal elements in modern fishes and living
amphibians have tended to be lost or fused so that the
number or bones present in the skull is reduced.
• In amniotes, bones of the dermatocranium
predominate, forming most of the braincase and lower
jaw.
• The dermal skull may contain a series of bones joined
firmly at sutures in order to enclose the brain, these
can be grouped in series and include:
– Facial, Orbital, Temporal, Vault, Palatal, and Mandibular
Phylogeny of the Skull
Agnathans
• The earliest vertebrates, known from soft
tissue impressions only, do not possess any
elements of the skull.
• Ostracoderms possessed a head shield formed
from a single piece of dermal bone.
• Modern Cyclostomes lack bone entirely and
have a braincase composed entirely of
cartilage.
Fishes
• Placoderms
– Up to ½ of the body is covered in bony plates of
dermal bone that enclose the pharynx and
braincase.
– Dermal plates in the head were joined into a
cranial shield.
– The braincase was heavily ossified, and the upper
jaw attached to it.
– In most a well defined joint existed between the
braincase and first vertebra.
• Chondrichthyans
– Cartilagenous fishes possess almost no bone.
– Denticles are present embedded in the integument.
– The dermatocranium is absent, instead the
chondrocranium has been expended upward.
– Modern sharks lack strong, direct attachment
between the paloquadrate and the hyomandibula.
• Instead, it is suspended by the Meckel’s cartilage and a
strong ligament.
– The first gill is reduced, forming a spiracle, or absent.
– During feeding sharks may use suction to capture
prey, more often they attack prey directly.
• Upper and lower jaws articulate with each other, and are
suspended from the hyoid arch.
• This attachment allows the jaws to swing down and forward.
– Jaw protrusion may assist in synchronized meeting of the upper
and lower jaw.
– Retraction of the jaws following feeding restores hydrodynamic
properties.
• Actinopterygians:
– Early members had long jaws extending to the front of
the head, numerous teeth, and an operculum that
covered the gills
• These are lost in tetrapods
– Within actinopterygians an extensive radiation
occurred that continues to the present.
• This makes it difficult to generalize about trends within the
skull.
• If a common trend exists it is for increased liberation of bony
elements to serve a more diverse role in food procurement.
– Most living actinopterygians are suction feeders.
• Negative pressure sucks water and prey into the mouth.
• Compression of the buccal cavity forces excess water out of
the gills.
• Suction feeders possess a well muscularized buccal cavity
and powerful, kinetic jaws.
• Sarcopterygians:
– In early lungfish. The upper jaw was fused to the
ossified braincase.
– This suggests that these fishes fed on hard foods.
– Bone of the dermatocranium resemble those of
actinopterygians, the palatoquadrate articulates
with the nasal cavity and maxilla.
– Unlike actinopterygian fishes, the braincase is
typically ossified as two distinct units.
Nasal Capsules
• Nasal capsules hold the olfactory epithelium in
the form of a paired nasal sac. In
actinopterygians, the nasal sac typically doesn’t
open into the mouth,
– Instead, its incurrent and excurrent openings establish
a one way flow across the epithelium.
• Each nasal sac of tetrapods opens directly into
the mouth via internal naris, or choana.
– Each nasal sac opens to the exterior by way of the
external naris (nostril).
– This helps establish a respiratory flow with the lungs.
Early Tetrapods
• Early tetrapods arose from rhipidistian ancestors and
retained many of their skull features.
• Beginning in tetrapods, the hyomandibula ceases to be
involved in jaw suspension and instead becomes dedicated
to hearing.
• The opercular series of bones is lost.
• The pectoral girdle loses its attachment to the back of the
skull.
• Roofing bones and the chondrocranium become more
tightly associated, reducing the mobility of the snout.
• The stapes aids in hearing, but in early tertapods it is a
robust bone that also acts as a support between the
braincase and palatoquadrate.
• The skull of modern amphibians is much simpler
that their fossil ancestors.
• Many of the dermal bones are lost or fused into composite
bones.
• The splanchnocranium is reduced.
• The hyomandibula plays no role in jaw
suspension.
• Taken over exclusively by the articular and quadrate bones.
• The branchial arches of the hyobranchial
apparatus support external respiratory gills in
larva, but become reduced to the hyoid
apparatus that supports the tongue.
• On land amphibians typically use a sticky tongue to capture
prey.
• It is propelled by muscles, over short distances, and musclehydrostatic fluid projection, at a distance.
Primitive Amniotes
• The skull roof, similar to early tetrapods, is
formed from the dermatocranium.
• The palatoquadrate of the mandibular arch is
reduced into a epipterygoid and sperate
quadrate.
• The hyoid arch becomes the stapes and, in
association with the lower jaw, assists in
sound transmission.
• Temporal Fenestrae, openings in the outer
dermatocranium are present in some amniotes.
– Turtles are anapsid, having no openings
– The diapsid skull, found in crocodilians, has two
temporal fenestrae.
• Birds and modern reptiles have a modified diapsid skull
– Therapsids and mammals have synapsid skulls that
contain a single opening.
Modern Reptiles
• In lizards, loss of the lower temporal bar
produces the modified diapsid skull.
– This serves to partially liberate the posterior part of
the skull from the snout, producing the mesokinetic
part of the skull.
• Without these kinematic linkages, jaw closure would be
scissor –like, and law closing forces on the prey would have a
forward component.
– In the skull of many lizards, rotation of the linkages
permits changes in the geometric configuration of the
skull.
• As a consequence lizards can alter the position of the tooth
row
• In snakes, the frontal and parietal roofing bones
have grown down around the sides of the skull to
form most of the walls of the braincase.
• Snake skulls are prokinetic, there are no bony
cross connections which allows them to rotate
each side of the jaw independently.
• This is especially important during swallowing.
• Crocodilians possess both cranial bars, and no
evidence of kinesis is present.
• Also, modern crocodiles possess a secondary
palate which separates the nasal passages from
the mouth.
Birds
• Birds are also ancestrally diapsid, but show
considerable modification.
• Their braincase is enlarges to compensate
for the larger brain within.
• The palatal bones are varied, but show signs
of reduction.
• Birds are toothless and their jaw is covered
by a keratinized sheath, or beak.
• The skill is prokinetic, and a strong
postorbital ligament extends behind the eye
to the lower jaw.
Mammals
• The skull of mammals represents a highly
modified synapsid pattern.
• Various dermal elements are lost.
• Fusion of separate centers of ossification produce
composite bones in the skull of placental
mammals.
• The occipital bone defines the foramen magnum
and closes the posterior wall of the braincase.
• A ventrally located occipital condyle articulates
with the atlas, the first vertebra to produce the
neck joint.
• On the side of the braincase, behind the orbit,
a large temporal bone is formed by fusion of
all 3 part of the skull.
• The splanchnocranium has been modified to
include the malleus, incus, and stapes of the
inner ear.
• Mammals possess 3 sets of turbinates within
the nasal passage.
• They increase the respiratory epithelial surface and aid
in scent reception and heat loss.
Middle Ear Bones
• Two profound changes in the lower jaw mark
the transition from therapsid to mammal.
1. The loss of the postdentary bone of the
lower jaw.
2. The presence of middle ear bones.
• In early synapsids, the lower jaw includes the tooth-bearing
dentary bones.
• In derived synapsids this set of bones has been lost and the
dentary has enlarged to assume the exclusive role of lower
jaw function.
• Their removal from the jaw joint permits their more
specialized role in transmitting sound.
– This is believed to have occurred to assist in hearing.
– Or, changes in feeding style freed these bones from their
attachment to the jaw and secondarily allowed them to aid in
hearing.
• Either way, changes in the jaw were accompanied by
changes in the method of food preparation prior to
swallowing.
– Mammals are the only group to chew, or masticate, their food.
– Mastication requires changes in the jaw and tooth structure to
prevent damage to either.
Secondary Palate and Akinesis
• In addition to changes in the lower jaw, the
presence of the secondary palate is also related
to mastication.
• The secondary palate includes a hard palate of
bone and a posterior, fleshy, soft palate.
– This effectively separates the food chamber form the
respiratory chamber above.
– Similarly, it completes the form roof of the food
chamber, so the pumping action of the throat of an
infant creates negative pressure within the mouth
without interfering with respiration.
• Mastication has been accompanied by precise tooth
occlusion.
• Precise occlusion requires a firm skull, so mammals
have lost all cranial kinesis, leaving an akinetic skull.
• As a further consequence of precise occlusion the
pattern of tooth replacement differs from most other
mammals.
• In lower vertebrates teeth wear and are replaced
continually (polyphydont), so the tooth row is always
changing.
• To avoid disruption of occlusion, mammalian teeth are
only replaced once (diphydont) with young, “milk
teeth” being replaced with “permanent” teeth in
adults
END