The axial skeleton

Axial: midline of the body, vertebral column and ribcage. Makes up the long axis of the vertebrate body, providing point of attachment for muscles, support, protection, and locomotion

• Flat and irregular bones

2 structural components:

• Notochord: long, continuous rod of fibrous connective tissue that wraps around a fluid core. First evolving for support and locomotion

• Vertebral column: a repeating series of distinct cartilaginous or bony elements

Original function of the vertebral column appears to be for protection of the spinal cord and dorsal aorta (blood supply)

Later vertebrates, it became important for attachment sites of the musculature. Big in tetrapods. Axial skeleton for support against gravity which requires attachments to muscle

In tetrapods, it also works for suspension of the body and locomotion on land

Basic components: (will see fusion of parts as evolution happens

• Dorsal arches: resting on the notochord surrounding the neural tube

  • Neural arch: elaborate projections

  • Interneural (intercalary) arch: between the neural arches, 

• Ventral arches: associated with/surround the blood supply/dorsal aorta. Sit on notochord bottom

  • Hemal arch: larger

  • Interhemal arch: slightly smaller

• The bases of the ventral arches expanded to form the centra where they meet the notochord

• Centra served to anchor and support the arches

  • Centra replaces notochord

Tetrapod vertebrae anatomy: 

• Neural spine/spinous process are the bumps of the spine

• Transverse processes are a point of attachment for ligaments to muscles or adjacent vertebrate

• Neural canal: open space where the spinal cord passes through, completely surrounded and protected by bone

• Pedicel: attaches the neural arch to the centra

• Laminae: attach transverse process to the neural spine

Notochord was the first to evolve, then the dorsal and ventral arches, ventral arches wrap around the bottom of the notochord and expand to make the centra. Eventually makes the notochord obsolete and go away. Centra’s job is for support

Regions of the vertebral column

• Trunk (body) and caudal (tail): in fishes, the only thing we had for a really long time

• Amphibians now have the addition of cervical (neck) and sacral (hips) (along with trunk and caudal)

• Amniotes gain the thoracic (back) and lumbar (lower back, still above sacral though)  regions but lose the trunk. They still have caudal, cervical, and sacral. 

Centra may be

• Aspondyly: no central, very early vertebrates before expansion of ventral arch

• Monospondyly: single central, most vertebrates fall into that category

• Dispondyly: 2 centra for a single vertebrae, well represented in the fossil record

• Polyspondyly: multiple centras for a single spine, ex) holocephali and lungfishes

Types of centra

• Primitive tetrapods had vertebrae that demonstrate aspidospondyly (all elements are separate)

• In general, today’s vertebrates have vertebrae that show holospondyly (elements are fused into a single vertebra), though there is variation

  • Aspidospondyly would be for kinetic movement, flexible spinal column

  • Holospondyly would be on terrestrial life, better for support

  • Most marine mammals would still have holospondyly but some have shifted back towards aspidospondyly

Shape of centra:

• Linked into a chain of vertebrae, making up the axial column

• Shapes:

  • Acoelous: flat on each end, resists compressive forces, most mammals will have this, helps to maintain shape

  • Amphicoelous: concave anteriorly and posteriorly, hourglass shape, in modern vertebrates they are solid, in the past they weren’t. Found in majority of bony fishes, some amphibians and lizards. Benefit: enhances undulating motion, side to side flexion of the axillating column

  • Procoelous: concave anteriorly (toward the head), found in frogs and most reptiles, advantage is that it allows for pretty good movement in any direction

  • Opisthocoelous: concave posteriorly (toward the butt), found in ungulates, very flexible, provides stable axial column

  • Heterocoelous: looks like a saddle, concave posterior, convex anterior. Incredibly flexible, seen in cervical vertebrae of birds and turtles

• In mammals, intervertebral discs attach to the articular surface of the centra. Used for  shock absorption

  • Contain the remnant of the notochord called the nucleus pulposus

  • Intervertebral discs are a pad of fibrocartilage with a gel-like nucleus pulposus in the middle of it. 

• In other animals, there is a pad of intervertebral cartilage

• Rims of adjacent centra are joined by intervertebral ligaments

Types of ribs: bony fishes

• Dorsal ribs and ventral ribs

• Homologous with the hemal arches of the caudal vertebrae

• Point of attachment for muscles of locomotion

Types of ribs: tetrapods

• True ribs: meet ventrally with the sternum. Connect with the sternum directly

  • Costal cartilage goes directly to the sternum

• False ribs: articulate with each other, not the sternum

  • Costal cartilage joins other costal cartilage, not to the sternum

• Floating ribs: do not ventrally articulate with anything

  • No costal cartilage

• Mammals: our ribs are only thoracic

• In birds: have floating ribs in the neck

Sternum: midventral skeletal structure

• Offers a site of origin for various chest muscles and protection, site for muscle attachment

• Together with the ribs (whether chondrified or ossified) it makes up the rib cage

• Fish don’t have sternum

• Amphibians its very variable, salamanders its just a tiny dot, frogs its more

• Some Mammals have sternebrae (cats), segmented sternum

• Humans have manubrium, sternabody, and xiphoid cartilage 

Gastralia (abdominal ribs) are in some vertebrates and located posterior to the sternum

• Do not articulate with the vertebrae and are restricted to the ventral body wall

• Serves as points of muscle attachment and additional support

• Dermal in origin

• Crocodilians and some other lizards have them

Plastron: a bony plate in turtles that makes up the floor of the shell

• Fused with clavicles and other skeletal elements, and likely the gastralia

Ventral dermal bones are generally absent in birds and mammals

In many fish, there are similar bones of dermal origin in the belly region

Phylogeny

• Fishes

  • Vertebral column is unossified in chondrichthyans and primitive bony fish (sturgeons and paddlefish)

  • Acts as an elastic beam- musculature produces lateral bending motions (undulating from side to side)

  • Caudal Skeleton and fins

    • Heterocercal tail- asymmetric tail; vertebral column turns upward, deep into the dorsal lobe. Vertebrae extends deep into the tail. Found in chondrichthyans and primitive bony fishes

      • ex) thresher sharks that use their tail to stun prey

    • Diphycercal tail- roughly symmetrical tail; vertebral column extends straight back (symmetry). Found in sarcopterygians (lungfishes and coelacanths)

      • Coelacanths have 3 lobes

    • Homocercal tail- symmetrical with equal lobes; narrowed vertebral column runs to the base and slants upward to support the dorsal edge. Vertebrae terminates at the base of the tail. Found in teleosts

      • ex) yellow finned tuna or tangs

    • Diphycercal and homocercal tail evolved from an ancestor with a heterocercal tail

      • These species have lungs or air bladders used for neutral buoyancy

      • Sharks have neither- lift is a consequence of the tail. Heterocercal tail is used to maintain buoyancy. Top fin is for locomotion, bottom fin is stabilization

• Tetrapods

  • Transition to land was a huge selective pressure- animals needed to change from buoyant bodies to ones suspended against gravity

  • The aquatic ancestor was likely aspidospondylous (unfused centra)

  • Gave rise to terrestrial, holospondylous (fused centra) descendants because it is more weight stable and better at counteracting gravity

  • Fossils show that early holospondyls had long, deep tails that would have been good for swimming (similar to modern salamanders). But we only see the beginning and the end point, not the middle

  • Locomotion remained similar transitioning from fish to amphibians (side to side ungulation)

    • Fish move in an undulating fashion as a result of side to side flexion

    • Synchronized lateral body swings both lift the limb and plant the foot (sprawling posture)

  • Terrestrial life meant that weight-bearing stresses on the axial column significantly increased

    • Fewer and more robust central because it is resisting compression of the axial skeleton

  • For secondarily aquatic tetrapods, there was an elongation of the tail and trunk and increase in the number of centra (for increased flexibility)

  • Appearance of the sacral region where the pelvic girdle articulates (joint formation) with the axial column.

    • Having a nice big joint is advantageous for stability and transfer of forces (to support explosive movement like running and jumping)

  • Connection between the skull and pectoral girdle was lost, leading to the cervical region

    • Better able to feed, surveil surrounding, act as a predator, protect the skull from the forces resulting from locomotion. 

• Amniotes (still tetrapods)

  • Atlas (C1) and axis (C2) facilitate head rotation and nodding by acting as a pivot point- forms the atlantoaxial joint. 

    • Atlas and axis are the first 2 vertebra in the cervix

    • The skull sits on top of Atlas (named for the greek figure) and allows for up and down movement. Occipital condyle sits on the atlas craters. Like an asteroid

    • Atlas doesn’t really have a centra because it works the the axis dens to do side to side motion of the head

• Turtles

  • Shell is a composite unit

  • Turtles are unique in that their appendicular skeleton is inside their ribcage

  • Origin of this trait is not revealed by the fossil record- the most primitive turtle already had the shell.

  • Only group that can put their appendicular skeleton into their axial skeleton.

• Other reptiles

  • Additional structures prevent torsion (twisting) in snakes but allow for significant lateral bending. Their axial column is very flexible to allow them to wrap around things but not for twisting.

  • Birds have highly mobile vertebrae (heterocoelous) and fusion of lower vertebrae to pelvic girdle. Aerodynamic

• Mammals

  • Have seven cervical vertebrae (some exceptions: sloths (10) and manatees (6))

  • In some jumping mammals the cervical vertebrae fuse. ex) armadillos and kangaroo rats

  • Number of thoracic and lumbar vertebrae is variable and caudal vertebrae are reduced compared to reptiles

Form and Function: changes in form relate to changes in function

• Fluid environment

  • Aquatic organisms do not rely on the endoskeleton for support- the buoyancy of the surrounding water does that

  • 2 problems:

    • Drag: axial skeleton needs to be in line to counteract drag. Streamlined profile. Not as good at maintaining straight orientation which is why they need fins to maintain that line of travel

    • Orientation in 3D space

• Terrestrial environment

  • Gravity is the problematic force- land animals are either sprawled out against the ground or the body is suspended between the legs

  • The vertebral column acts as a bridge and the legs are the supports for it (like posts) to suspend the body and resist compression

• Regionalization of vertebral column

  • Fishes

    • Have trunk and caudal regions

    • Lack differentiation reflects that it is not used in support, just an elastic beam to support undulating motion

    • Trunk vertebrae have the ribs on them

  • Amphibians

    • Cervical, trunk, sacral, and caudal regions

    • Due to amphibious lifestyle, axial skeleton and musculature retain similarities to fish. POA to muscles is similar to fish

  • Reptiles

    • Cervical, thoracic, lumbar, sacral, and caudal regions

    • Reduction of ribs in front of hindlimbs, short right in from the hindlimbs to assist in locomotion

    • Shift towards more robust centra and some fusion of the regions (sacral)

    • Birds: heterocoelous vertebrae help with preening

  • Mammals

    • Cervical, thoracic, lumbar, sacral, and caudal regions

    • Complex attachment of the musculature to the vertebral column indicates significant demands of locomotion

    • Big, robust centra. Have rigidity and flexibility