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skeletal system: the skull
Understanding head structure becomes increasingly complex throughout evolution.
In many forms heads increase in relative size and complexity.
Brain and sensory structures increase in size and importance.
Supporting structures increase in size.
Such structures arise from
: visceral skeleton, cartilaginous brain case, dermal head skeleton.
oldest supporting elements in head
arise from neural crest
maintain place of pharyngeal tissues for breathing and filter feeding
supports pharyngeal gill slits.
In vertebrates unifies and distributes forces from gills
later distributes forces from jaw muscles
cartilage appears later
ossification occurs later and these elements become bony
May consist of as many as five articulated elements on a side
: pharyngobranchial, epibranchial, ceratobranchial, hypobranchial, basibranchial
cartilaginous brain case
Neurocranium- associated sensory structure capsules included
forms around brain and major sense organs
Useful to know where spaces between early elements are - allows identification of nerves and blood vessels passing between them.
In sharks the structure is a chondrocranium because it is cartilage
Structures that serve this purpose are ossified in other forms
These structures develop best where dermal structures are absent
Appear in several sections.
Structures arise from head mesenchyme condensations - form elongated cartilages next to the notochord.
Trabecular cartilages - anterior
Parachordal cartilages - posterior
Occipital cartilages - further posteriorly
Sensory structures have supporting cartilaginous capsules
: nasal optic otic
Contributes to neurocranium formationprotects and supports deep head structures, but does not usually completely enclose the brain.
Dermal bone provides most of the protection.
Dermal shield - pattern consistent although elements often fuse.
Pelagic forms often show dermal shield reduction.
Dermal bones retained in sedentary swimmers and bottom dwellers.
Light because it is mostly spongy bone but strong.
May be superficial as in placoderms, forming an exoskeleton.
Deper in teleosts, holosteans and tetrapods.
In these animals it becomes associated with the neurocranium.
Crossopterygian pattern is similar to tetrapod descendants.
Number of bony plates is reduced in tetrapods from ancestral and related fishes.
complete skull is assembly of splanchnocrainium, neurocranium, dermatocranium
Some structures arise from neural crest cells and mesenchyme from mesoderm.
During skull development these elements fuse.
Ethmoid plate forms between nasal capsules and anterior trabeculae.
Parachordals fuse to form basal plate.
Occipitals grow dorsally around the spinal cord to form the occipital arch.
Origin of jaws
jaws arise from branchial arches that support the mouth.
Allow animal to eat larger foods.
Appeared first in acanthodians
Jawless vertebrate pharynx supported by fused complex (=branchial basket)
Cartilaginous bars (=pharyngeal bars)
Arise from anterior gill arches by several theories.
Theories for the origin of jaws
Splanchonocranium of jawed forms is simplest because of fewer pharyngeal bars.
Pharyngeal arches between gill slits are supported by internal bars or separate four jointed sections.
Sections operate as levers pulled by pharyngeal muscles.
Jaws develop as one or more cartilaginous pharyngeal bars in wall of the mouth.
Upper jaw element=palatoquadrate cartilage
Lower jaw element is Meckel's cartilage (as in adult sharks)
2nd pharyngeal element supports jaws.
Autostylic jaw suspension
Autostylic jaw suspension
Ancestral jawed fishes had palatoquadrate stabilized against the chondrocranium so only lower jaw could move.
Later jawed fishes use hyoid arch also to support jaw.
Ancestors of both bony and cartilaginous fishes have epibranchial of hyoid arch become hyomandibula.
Hyomandibula stabilizes the posterior aspect of jaws.
Joins the anterior palatoquadrate process.
Amphistylic jaw suspension.
Later fish modify jaw suspension to hyostyly, often with a symplectic bone.
Mammals show craniostyly with less movement capability.
Over view of skull evolution
Chondrocranium is support for brain and assisted by some splanchnocranial elements.
Epipterygoid comes form splanchnocranium.
Splanchnocranium gives rise to hyomandibula, articular, quadrate and hyoid apparatus.
Dermatocranium encases the chondrocranium (and associated splachnocranial elements).
more common than akinesis.
Allows greater mobility of different skull elements and more complex movement patterns.
Mammals skulls are akinetic.
Is composite structure made of three elements.
Osteostracans had a single dermal head shield.
Dorsal, close set eyes separated by a single pineal opening.
Single anterior nostril.
Lateral line sensory fields laterally on head shield.
Low profile, bottom (benthic) dwellers.
Branchial arches head by head shield, supported by paired gill lamellae or interbranchial septae.
Cartilage plates and muscles actions drew water into mouth to pass over gills.
Suspended particles extracted before water was expelled.
had more active lifestyles.
Different body forms.
Deeper bodies, small scales on heads not head shields.
May have had marginal pointed scales around mouth.
Suction feeding arose with water column intake.
May have had predacious tendancies, but lacked strong jaws.
Heterostracan feeding habits
water entered mouth.
Flowed over gills suspended in branchial pouches.
Into common chamber and exiting barnchial pore.
Bony plates form head shield.
Small plates toward tail allowed for movement.
Cyclostome feeding patterns as seen in lab
lack bony skeleton
Gnathostomes - Placoderms specifically
dermal head shields
ossified braincases with attached jaws
joint between head and first vertebra
predators 1-6 meters long.
earliest jawed fishes
small, stremlined, active swimmers.
Small, non-overlapping scales but plates on head.
Gills protected by an operculum
lateral cranial fissure gap that partially divided posterior braincase.
Gap provided exit for cranial nerve X
Mandubular arch similar to that in sharks and bony fishes
Hyoid arch with five successive banchial arches present.
little, if any bone
Dermal ossicles only
Chondrocranium forms braincase
Splanchnocranium and six gill arches present primitively.
Palatoquadrate supported by hyomandibula
Modern sharks have spiracle and detached upper jaw.
Modern sharks can protrude upper jaw for feeding.
Opercular and extrascapular bones
Long jaws for feeding
Diversity of body and head forms in modern Teleosts.
Jaw opening in primitive actinopterygian:
Suspensorium allows jaw rotation around it.
Suspensorium articulates with opercular bones.
Pectoral girdle relatively fixed but neurocranium can rotate on it to lift the head.
Diversity in fish skull form
All work similarly but different proportions allow animals to do different things.
Early lung fishes
Upper jaws fused to braincases
Fed on hard foods
Coelacanths have cranial kinesis
Sharp, pointed labyrinthodont teeth.
Draincases jointed, not as in modern lungfishes.
Persistent notochord that extended into the head for more support.
Nasal sac opens into the mouth as choanae.
Choanae are as in tetrapods.
Modern amphibians have reduced splanchnocrania
Skull of frog
skull elements reduced and fused.
Light skulls for hopping forms.
Shift in jaw muscle attachment to the skull.
Anapsids with no temporal openings - all muscles attach deep to dermatocranial elements. Muscles - neurocranium to lower jaw.
Therapsids with temporal openings - jaw muscles shift position to edges of openings.
Forms initial zygomatic arch.
Jaw muscles move onto dermatocranial surface in therapsids and modern mammals.
Squamate cranial kinesis
3 kinds, depending on where the joint is and what parts move.
1. Metakinesis, joint at back of skull
2. Mesokinesis, joint behind eye
3. Prokinesis, joint in front of eye.
solve the muscle position and space problem another way.
Posterior skull bones=emarginated for muscle origin.
Trend increases through time.
diapsids with lower border of lower opening missing, increasing cranial kinesis.
Joints allow the snout to lift or bend downward.
Changes angle at which teeth engage prey to prevent its squirting out of mouth from teeth at wrong angle to grasp it.
use the tongue to capture prey.
Supported by large hyoid apparatus (which branchial arch derivative?)
Snake skull biomechanics
: water moccasin
mechanical model of machine that allows kinesis in the animal.
Kineic-inertial feeding mechanism instead
have kinesis also because they are light and bones bend, even though adult skulls are fused.
Cranial kinesis in a crow
Nasofrontal hinge, allows bill to flex upward.
Range of movement and gape has increased by cranial kinesis.
Additional flexibility is allowed by the tips of the beak themselves.
Radiation of therapsids
They continue the synapsid line and have great body form diversity.
How are numerous bones in ancestral form reduced in descendants?
These bones=composites, arising from more than one center of ossification.
Occipital, sphenoid, temporal
Before this was understood, they were given different names.
Importance of a secondary palate
it allows you to chew and breath at the same time.
interception of food particles in water.
Can regulate particle size because larger ones cannot enter the small sieve space.
Terrestrial salamander catches a tasty worm by extending and shaping tongue to surround and scoop up the prey. This is called lingual feeding.