Vertebrae and Axial Skeleton

Axial Skeleton

• Includes vertebrae, ribs, sternum, and gastralia.
• Non-comparative anatomists include the skull and jaws.
• Not all vertebrates have centra

Primitive Vertebrae

• Aspondylous: No centrum present, seen in Lamprey.
• Aspondylous and Aspidospondylous: No centrum, parts remain separate, primitive fishes.

Vertebral Parts

• Aspidospondylous: separate parts, Sarcopterygian fishes, Primitive tetrapods, Acipenseriformes.
• Holospondylous: fused parts, neopterygians, bichirs, Chondrichthyes, Extant tetrapods.

Aspidospondylous Vertebrae Types

• Rachitomous: unequal arch bases, Osterolepimorphs, Primitive tetrapods.
• Embolomerous: equal arch bases, Some Late, Amphibians.
• Stereospondylous: only Intercentrum, Some Early, Amphibians.

Holospondylous Vertebrae Types

• Monospondylous with Intercentrum: Modern Amphibians, Lepospondylous.
• Monospondylous with Pleurocentrum: Amniotes.

Vertebral Types

• Amphicoelous: biconcave.
• Generally fairly rigid due to presence of notochord.
• Allows bending only in one plane - side to side, Modern Chondrichthyes, Neopterygii (except gar), Amphibia, Tuataras.
• Acoelous: flat.
• Limited bending, Dinosaurs, pterosaurs, and mammals, Nucleus Pulposus (notochord), Interverebral Disk, Intercentrum.
• Opisthocoelous and Procoelous: concave on one side.
• Bend without stretching nerve cord, Intervertebral disc fused to centrum (the knob), Opistho: ungulate necks, gar, Pro: Squamata, Crocodilia.
• Acoelous vs. Pro- & Opisthocoelous: Acoelous - not very flexible
• Bends away from middle of centrum.
• Nerve cord stretches.
• Opisthocoelous and Procoelous - vertebrae in contact over entire range of motion.
• Nerve cord isn't stretched.
• Greater range of motion.
• Heterocoelous: saddle-shaped.
• More motion, Bird and turtle.

Ribs and Connections (apophyses)

• Amniotes – intercentrum is lost or incorporated.
• Capitulum to pleurocentrum in reptiles, Capitulum between centra in mammals

Rib Functions in Tetrapods

• Attachment for locomotory & respiratory muscles.
• Uncinate Processes – act as levers to flare rib cage - birds, tuataras, & early labyrinthodonts, resist compression in birds.
• Ventral ribs are fish ribs.
• Cervical vertebrae do not have ribs

Rib Modifications

• Cervical Ribs – Dinosaurs, Pleurapophyses ribs fused to vertebrae.

Sternum

• Midventral, endochondral structure.
• Multiple origins/loss.
• Origin of chest muscles, Not in fishes, early tetrapods, turtles, snakes, many limbless lizards.
• Generally small in modern Amphibians, single bone in Reptilia.
• Large carina (keel) in most birds.
• Mammals - chain of elements - sternebrae.
• Humans have sternebrae

Gastralia or Abdominal Ribs

• Abdominal ribs, Dermal bone, Some lizards, sauropterygians, crocs, tuataras, & some dinos.
• Sites for ventral muscle attachment, Homologous to ventral dermal scales in labyrinthodonts and osteolepimorphs.

Arcualia (Arch Centers)

• In Chondrichthyes and early bony fishes, cells from sclerotomes form up to four blocks, no blocks in later groups or tetrapods.
• Centra evolved many times, Chondrichthyes, Neopterygii, Amphibia, Reptilia + Synapsida.
• Sclerotomal cells migrate to coat notochord.

Teleosts

• Chordal Center - sheath becomes cartilage, intervertebral ligament.
• Perichordal Center - Cells from sclerotome ossify intramembranously.
• Intervertebral Ligament - remnants of notochordal sheath.
• Arch anlagen form Centers and arches fuse.
• Additional layers added to perichordal centers.

Tetrapod Development

• Cells in primary sclerotome split.
• Migrate to notochord forming, Perichrodal Rings form and eventually connect to form Perichordal Tube.
• Cells grow upwards - Neural Arches, spines, Chondrification then ossification, Intervertbral disks differentiate.
• Vertebrae from 2 sclerotomes

Fish Vertebrae

• Many similar malformations to human vertebral malformations.
• Can eliminate gravity and the appendicular skeleton from the equation.

Fish Vertebrae (The Start)

• Hagfish have nothing.
• Lampreys have small cartilages anteriorly.
• Other agnathans are probably the same.
• Primitive Gnathostome Column: Placoderms, acanthodians, early Chondrichthyes, Acipenseriformes, most sarcopterygian fishes - neural and hemal arches ride on notochord.

Fish Vertebrae (Phylogenetic Patterns)

• Modern Chondrichthyes; Neopterygii, bichirs have fully formed centra.
• Generally amphicoelous except gar (opisthocoelous).
• Lateral flexure could cause column to buckle.
• Centra prevent compression.
• Intervertebral ligament returns stiffness.
• Caudal fins: Heterocercal(Primitive), Diphycercal (Lungfishes, Coelacanths, Bichirs), Homocercal(Teleosts).
• Shark Heterocercal provides lift.
• Sturgeon Heterocercal holds.
• Homocercal provides some lift, not in line with classical view.
• Diphycercal: Australian Lungfish, Coelacanth, Bichir.

Weberian Apparatus

• Modifications of anterior 4-5 vert.
• Found In Otophysi (minnows, tetras, catfish, S. Am. Knifefishes).
• Connects swim bladder to inner ear.

Patterns In Osteolepimorphs

• Intercentrum dominates - forms ring around notochord.
• Pleurocentrum is small.
• Ribs are short and thick.

Tiktaalik

• Adds Atlas.
• Neck.
• Dorsoventral motion.

Patterns In Ichthyostega & Acanthostega (Traditional)

• Rachitomous - Pleurocentrum (PC) is behind intercentrum (IC).
• CT – Reverse Rachitomous - pleurocentra in front of and fused to intercentra.
• Zygapophyses reduce twisting.
• Sacrum attaches to pelvic girdle.

Amniotes

• Axis - Side to side movement.
• Added in Anthracosaurs - stem amniotes.
• Odontoid process or dens is the pleurocentrum of the atlas.
• Additional cervical vertebrae also added.
• Variable in sauropsids.
• Most mammals have 7 vertabrae.
• Manatees and two-toed sloths have 6 vertabrae.
• Three-toed sloths have 9 vertabrae.
• Like anthracosaurs, monospondylous with pleurocentrum.
• Os Odontoideum: Occasionally the odontoid process does not fuse.
• Fails to restrict atlantoaxial movement.
• Can lead to impingement of the upper spinal cord or vertebral artery.
• Turtles: Vertebrae incorporated into shell; Heterocoelous cervical vert.
• Snakes and Some Lizards: Zygospehenes (anterior); Zygantra (posterior); Like zygapophyses; May have evolved multiple times.
• Zygophenes and zygantra ensure flexion of vertebral column in one plane.

Birds

• Flexibility in flight decreases strength of wings.
• Wasted energy bending vert. column.
• Yet, need to move neck a lot.
• Neck very flexible - heterocoelous.
• Rest mostly fused.
• Fusion of anterior thoracics.
• 1 free thoracic.
• Synsacrum - Rest of thoracics, lumbars, sacrals pelvic girdle and 1 caudal fused.
• Pygostyle supports tail feathers.
• Fusion results in less weight.

Function in Cursorial Animals

• Legs move in parasagittal direction.
• Dinos, mammals.
• Legs act as pillars (abutments).
• Body suspended between pillars.
• Compressive forces - solid structures (vertebrae).
• Tensile forces - cables (ligaments, muscles).
• Nodal – point where supporting pier changes.
• Anticlinal vertebra indicates nodal.
• Compression – a pushing force that shortens objects.
• Tension a pulling force that stretches objects.
• Cantilever: Balance Weight.
• Tail of Iguanodon keeps weight over hips.

Arch Suspension

• Arch Suspension bridge.
• Must maintain shape to suspend weight; Cervical - ligaments and muscles like violin bow.
• Abdominal muscles and sternum like archery bow.

Lever Arms

• Input lever arm (li) is where force enters the system.
• Output lever arm (lo) is where force leaves.
• Fulcrum is the point of rotation.
• There are three classes of levers; depends on where the fulcrum is and where work is performed.

Lever Classes

• Class 1 - load and force on opposite sides; Skull and atlas; Can increase strength or speed.
• Class 2 - load is between fulcrum and force; lo is always less than li; Strength - less effort moves larger weight; Achilles tendon pushing or pulling foot.
• Class 3 - force is between fulcrum and load; li is always less than lo; Speed - a larger force is actually needed to move a smaller weight; Most muscles act like this.
• Class 4 - continuous lever; Screwdriver - li is the diameter of the handle and lo is the diameter of the shaft; lo is less than li here.

Length of neural spines

• Indicate mechanical advantage.
• Longer spines are stronger, meaning longer input lever arm.
• Optimized for strength.
• Needed if head moves.

Regionalization

• Increased through each lineage.
• Thoracolumbar splits in several lineages.
• In synapsids, associated with diaphragm.

Aquatic Animals

• Secondarily aquatic animals reduce or lose zygapophyses.

Human Vertebrae

• Human's have a unique posture.
• The vertebral column is sigmoidal, but largely straight up and down.
• Weight held over hips for stability.
• Easy to topple, but add in pregnancy; Center of mass increases forward about 5 cm and in front of hips during pregnancy.
• Lordosis - curvature of lumbar vertebrae is always present.
• 3 vert. used in curve in women (2 in men).
• Lumbar vertebrae are more wedge-shaped in women than men forming a larger curve of support.
• Hip joint also flares out further and is wider.
• L3 is not wedge-shaped in men, but it is in women.
• L4 is more of a wedge in women, and L5 is equal.
• Needed for bipedalism - found in Austrolopithecus afarensis.