The axial skeleton

Axial Skeleton

Midline of the body, including the vertebral column and ribcage, providing support against gravity, protection, points of attachment for muscles, and locomotion. Made up of flat and irregular bone

Notochord

A long rod of fibrous connective tissue that wraps around a fluid core. First evolved for support and locomotion

Vertebral Column

A series of cartilaginous or bony elements that first evolved to protect the spinal cord and dorsal aorta (blood supply). Now its main function is to provide muscle attachments

Dorsal Arches

Structures resting on the notochord that surround the neural tube. Made up of the neural arch and interneural/intercalary arch

ventral arches

associated with/surround the blood supply/dorsal aorta. Sits on the notochord bottom. Made up of the hemal and interhemal arches.

Centra

The central part of vertebrae that replaces the notochord and serves to anchor and support the arches. Ventral arches expand to form this structure where they meet the notochord

Tetrapod Vertebrae Anatomy

Includes neural spine/spinous process, transverse processes, neural canal, pedicel, and laminae, all contributing to spinal structure and function.

neural spine/spinous process

bumps of the spine

transverse processes

point of attachment for ligaments to muscles or adjacent vertebrae

Neural canal

open space where the spinal cord passes through, completely surrounded and protected by bone

Pedicel

attaches the neural arch to centra

Laminae

attach transverse process to the neural spine

Fish regions of the vertebral column

trunk (body) and caudal (tail). Lack differentiation reflects that it is not used in support, just an elastic beam to support undulating motion. Trunk vertebrae have ribs on them

Amphibian regions of the vertebral column

trunk, caudal, cervical (neck), and sacral (hips). Axial skeleton and musculature retain similarities to fish. Point of attachment is similar to fish

Amniote regions of the vertebral column

Caudal, cervical, sacral, thoracic (back), and lumbar (lower back).

Aspondyly

A condition with no centra, seen in very early vertebrates.

Monospondyly

A condition with a single centra, common in most vertebrates.

Dispondyly

A condition with two centra for a single vertebra, represented in the fossil record.

Polyspondyly

A condition with multiple centra for a single spine, found in holocephali and lungfishes

Aspidospondyly

Primitive vertebrae with separate elements, allowing for flexible movement. Found in primitive tetrapods

Holospondyly

Modern vertebrae with fused elements, providing better support for terrestrial life.

Acoelous

Vertebrae that are flat on each end, resisting compressive forces and helps to maintain shapes. Common in most mammals.

Amphicoelous

Vertebrae with an hourglass shape, concave anteriorly and posteriorly. Solid in modern vertebrates, enhances side-to-side motion, found in many bony fishes, some amphibians and lizard

Procoelous

Vertebrae that are concave anteriorly (toward the head), allowing for good movement in any direction, found in frogs and most reptiles.

Opisthocoelous

Vertebrae that are concave posteriorly (towards the butt), providing flexibility but also a stable axial column, found in ungulates.

Heterocoelous

Vertebrae shaped like a saddle, concave posterior, convex anterior, allowing for incredible flexibility, seen in cervical vertebrae of birds and turtles.

Intervertebral Discs

Pads of fibrocartilage that absorb shock and contain the gel-like remnant of the notochord called the pulposus in the middle of it.

Types of ribs in bony fishes

dorsal and ventral ribs. Homologous with the hemal arches of the caudal vertebrae. Point of attachment for muscle of locomotion

True Ribs

Ribs that connect directly to the sternum ventrally. Costal cartilage goes directly to the sternum

False Ribs

Ribs that articulate with each other but not with the sternum. Costal cartilage joins other costal cartilage, not to the sternum

Floating Ribs

Ribs that do not articulate with anything ventrally. No costal cartilage

Sternum

Midventral skeletal structure providing muscle attachment and protection. Fish do not have one, amphibians have varying sizes of one. Some mammals have it segmented

Parts of the sternum in humans

manubrium, sternabody, and xiphoid cartilage

Ribcage

sternum and ribs (ossified or chondrified) together

Gastralia

Abdominal ribs found in some vertebrates, serving as muscle attachment points. Does not articulate with the vertebrae and are restricted to the ventral body wall. Dermal in origin. Crocodilians and some lizards.

Plastron

A bony plate in turtles that forms the floor of the shell. Fused with clavicles and other skeletal elements, and likely the gastralia

Heterocercal Tail

An asymmetric tail found in chondrichthyans and primitive bony fishes, aiding in buoyancy. Vertebral column turns upward, deep into the dorsal lobe. vertebrae extends deep into the tail. Don’t have lung or air bladders.

Diphycercal Tail

A roughly symmetrical tail found in lungfishes and coelacanths. Vertebral column extends straight back (symmetry). Coelacanths have 3 lobes

Homocercal Tail

A symmetrical tail found in teleosts, supporting locomotion. Narrowed vertebral column runs to the base and slants upward to support the dorsal edge. Vertebrae terminates at the base of the tail.

Locomotion transitioning from fish to amphibians

Side-to-side undulation. Fish move in an undulating fashion as a result of side to side flexion. Amphibians have synchronized lateral body swings that both lift the limb and plant the foot (sprawling posture)

Weight bearing stresses adaptions

Terrestrial vertebrates with fused centra for better weight stability, More robust centra to resist compression of the axial skeleton, sacral region with pelvic girdle articulates (joint formation)

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

better able to feed, surveil surrounding, act as predator, protect the skull from the forces resulting from locomotion

Atlas and Axis

The first two cervical vertebrae (C1 and C2) facilitating head rotation and nodding by acting as a pivot point. Found in amniotes

Atlantoaxial joint

where atlas and axis vertebrate meets

Atlas

named for the greek figure, allows for up and down movement. Skull sits on top of it. Occipital condyle sits on the atlas cavities. Doesn’t have a centra

Axis dens

attaches to the atlas, creates side to side motion of the head

Turtles

shell is a composite unit, unique because appendicular skeleton can fit inside axial skeleton. Unclear where the origin of the shell came from

Other reptiles

additional structures prevent torsion (twisting) in snakes but allow for significant lateral bending. Birds have highly mobile vertebrae (heterocoelous) to help with preening and fusion of lower vertebrae to pelvic girdle to make them more aerodynamic.

Mammals

have seven cervical vertebrae (except sloths and manatees), in jumping mammals the cervical vertebrae fuse (ex. armadillos and kangaroo rats), number of thoracic and lumbar vertebrae is variable, 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 endoskeleton for support, buoyancy of water does that for them. Have to deal with drag and orientation in 3D space. Axial skeleton is in line, streamlined profile, fins maintain line of travel

Terrestrial environment

Gravity is the problematic force, land animals are either sprawled out against the ground or the body is suspended between the legs. Vertebral column acts as a bridge and the legs are the supports for it to suspend the body and resist compression

Regionalization of Vertebral Column

Differentiation of vertebral regions in various vertebrates, reflecting adaptations to their environments. Reptiles have a reduction of ribs in front of hindlimbs to assist in locomotion and a shift towards more robust centra. Mammals have complex attachment of the musculature to the vertebral column and big, robust centra that has both rigidity and flexibility.