Axial skeleton Notochord, vertebral column, median fins, ribs, sternum Where do these tissues come from in the embryo?
dermamyotome myotome sclerotome Vertebral column, ribs
NOTOCHORD SPINAL CORD SOMITES VERTEBRAE MUSCLES NERVES DERMIS RIBS
Neural arch, intervertebral foramen, neural spine, hemal arch, hemal spines, centrum, sometimes 2 centra per segment esp in tails
Shape of centra varies according to weight bearing. Fish – both concave Early terrestrial e.g. amphibians, ball & socket
Both surfaces flat with intervertebral disc – birds & mammals Terrestrial verts – zygapophyses increase strength and resistance to twisting.
Ribs & Sternum Develop in myosepta lateral to vertebrae. Strengthen myosepta and body wall, provide attachments for trunk and tail muscles, protect visceral organs. Fish – intermuscular ribs (dorsal ribs) subperitoneal ribs (ventral ribs)
Tetrapods – support weight of body when lying prone, prevent undue pressure on lungs & viscera, crucial in breathing. Ribs attach more dorsally than in fish, 2 points of attachment tuberculum tuberculum capitulum
Costal cartilages – flexible to allow breathing Sternum – not in turtles and snakes
Evolution of axial skeleton Structure depends on environment & locomotion – these determine the forces acting.
Fish Very little differences in vertebrae Caudal fins – propulsive power –attached to vertebral elements
Tetrapods • resist bending in vertical plane • resist torsion • formation of neck – • skull no longer united with • pectoral girdle Cervical vertebrae Fish – Amphibia - Reptiles – Birds – Mammals - 0 1 6-10 11-25 1st = atlas 7
formation of sacrum Sacral vertebrae Fish – 0 Amphibians – 1 Reptiles – 2-3 Birds – 10-23 Mammals - 3 • birds and mammals – thoracic vs lumbar distinction
Reptiles • 2nd cervical vertebra = axis • ribs v. important for breathing in reptiles • abdominal ribs or gastralia • stronger sacrum • autotomy plane through middle • of 1 or 2 caudal vertebrae
Birds Flying Bipedal locomotion Very flexible neck, 11-25 cervical vertebrae
First appearance of thoracic (partly fused) & lumbar (fused) vertebrae • short cervical ribs • Ancestral birds had long tail, modern birds fused to pygostyle, supports tail feathers. More proximal caudal vertebrae can move to alter tail feathers in flight. • thoracic vertebrae unite with sternal ribs (ossified costal cartilages), huge sternum with keel for origins of flight muscles synsacrum
Mammals • 7 cervical vertebrae even in giraffe (atlas & axis allow good movement). Fusions stiffen neck – jumping rodents, kangaroos. • thoracic vertebrae have ribs, costal cartilages to sternum • short lumbar ribs = pleurapophyses • strong sacrum with 3+ vertebrae and ribs, more for bipedalism (5 in humans)
Don’t want a neck in water! • same 7 cervical vertebrae fused in marine mammals • no zygapophyses, large centrum • ribs not strong enough for land, suffocate on land