Vertebrates 6 - Cranial Skeleton Flashcards
Three parts of cranium
Dermatocranium (hard), chondrocranium (around brain), splanchnocranium (gills and mouth).
Roles of the cranium
Protect brain and sensory structures, air/water entry, feeding
Dermatocranium (specifically in fish)
Outer bones of skull. Dermal roof bones, lower jaw bones, dermal opercular bones (over gills)
Chondrocranium
Made from cartilage; protects the brain and allows entry of nerves in fish; in birds and mammals it protects brain as embryo, mostly replaced by skull, forms base of skull, some of the turbinate bones and contains inner ear
Splanchnocranium
mandibular arch, hyoid arch, other visceral arches b/w and around pharyngeal pouches
Chondrichthyes and agnathans
No dermatocranium. Just the other two
Evolution of jaws
First jawed fish: silurian - 420mya. Bony arches became the jaws
Placoderms
First fish to develop jaws. Needed to break through shells of invertebrates. They don’t have some of the bones that modern fish have (not an ancestor).
Changes in splanchnocranium
Agnathans have more gill slits and arches than jawed fish. Gill arches have hinges. Aganthan mouth has no jaw. Arches may have changed in size and orientation, form canal. Muscles evolve to attach.
What did the gill arches become?
Palatoquadrate cartilage (top) and mandibular cartilage (bottom), supported by hyoid arch, remaining gill arches still present.
What else contributed to formation of jaws?
Molecular evidence. A few key genes in jaws. KO do not form jaws.
Evolution in changes in skulls of vertebrates
Dermal roof, lower jaw, and opercular bones.
Herring vs bowfin and extinct fish
Herring has larger and more spaces in dermal roof, lighter, saves energy (schooling for protection - doesn’t need it all). Bowfin is example of “ancient” cranium, more complete dermal roof.
Tetrapod skull
No opercular bones anymore because it has no gills/gill arches
Anapsid
No fenestra (hole) in the back of the skull. Early condition. Modern amphibians and turtles.
Temporal fenestra
Opening in skull. Early anapsids didn’t have room for much mandibular muscle, so the fenestra make more room for muscles and increase bite force
Diapsid
Two temporal fenestrae. Early diapsids include lizards, snake, crocodiles. Late diapsids include dinos and birds.
Synapsid
Have one temporal fenestrae (usually a bit lower and very large). Mammals.
Evolution towards mammals
Seymouria (anapsid), dimetrodon (early synapsid - not a dino!), therapsids (early reptile-mammal intermediate, late synapsid), mammals
Mammalian skull
Large temporal fenestra, differentiated (heterodont) teeth, double occipital condyle, complete secondary palate
Smilodon
Saber-tooth cat. Teeth so big, not much room with open mouth. Kill with a bite through the neck, puncturing the carotids and jugular. Modern big cats have a sleeper hold on the neck, but don’t pierce it.
Evolution of secondary palate (to first mammals)
When eating, the choana would be blocked, so it could still breath. Also made larger nasal cavity - better smell, helps filter air and warm it up (important for endotherm),
Modern mammals palate
Complete secondary palate, separate air and food passages, suckling (hard to breath while suckling)
Changes in lower jaw of vertebrates - fish to derived synapsids
Fish and reptiles have homodont teeth, articular bone. Early synapsid, still pretty similar. Derived synapsid: mostly dentary bone, still have articular bone.
