Vertebral Evolution Across Fishes and Tetrapods

Hagfish and Lampreys

• Overview: Early, simple vertebrate groups often discussed with respect to axial skeleton evolution. Includes hagfish and lampreys; these are contrasted with jawed fishes and later tetrapods.
• Notochord as a persistent axial feature:
  • In these groups, the notochord is a persistent, unconstricted (fluid) rod that extends from the base of the skull to the tail and lies just beneath the spinal cord.
  • The notochord induces formation of the spinal cord during development.
• Hagfish (Myxini):
  • Habitat: can be freshwater or marine; described as parasitic or free-living.
  • Skeletal elements: post-embryonic evidence suggests absolutely no vertebral elements form (no vertebrae) in hagfish.
  • Vertebral derivation: cells that would form vertebrae or neural arches regress and die; this is interpreted as a secondary loss of vertebral elements from a common ancestor that possessed them.
  • Taxonomic status: hagfish are considered vertebrates despite the lack of vertebral elements because the loss is secondary.
  • Notable feature: extreme flexibility and knot-tying capability (the animal can knot itself), which demonstrates the functional significance of the persistent notochord in the absence of vertebrae.
• Lampreys (Petromyzontiformes):
  • Notochord: also has a persistent notochord but differs from hagfish in having neural arches present in some form (though not regular, regularized alignment).
  • Vertebral elements: neural arches or components are present but irregularly aligned along the body; not a uniform set of arches for each body segment.
  • Joints and movement: the presence of neural arches allows a ball-and-socket type joint mechanism that permits movement along the body while still surrounding the notochord.
  • Mobility: the fish-like serpentine movement is facilitated by the combination of a persistent notochord and neural arches, albeit with unusual articulation compared to higher vertebrates.
• Key takeaway about these jawless fishes:
  • Both hagfish and lampreys retain a persistent notochord but differ in the presence and arrangement of vertebral arches.
  • Hagfish show a secondary loss of vertebral elements; lampreys show some neural arches but not in the regular, segmented fashion seen in vertebrates with true vertebrae.
  • These features illuminate early trends in vertebrate axial skeleton evolution: persistence of the notochord, partial or variable development of neural arches, and the evolution of more rigid yet flexible vertebral columns.

Sharks and Cartilaginous Fishes

• Vertebral plan in cartilaginous fishes:
  • The vertebrae are formed from cartilage rather than bone.
  • In larger sharks and some older individuals, calcification can occur within cartilage by depositing calcium salts, strengthening the cartilaginous vertebrae without bone formation.
• Precaudal vs caudal vertebrae:
  • Precaudal vertebrae are those located anterior to the tail (in the trunk region).
  • Caudal vertebrae extend into the tail and typically bear hemal arches (also called chemore arches in older texts).
  • Hemal arches are present in caudal vertebrae but are lacking in precaudal vertebrae, reflecting a regional specialization of the axial skeleton.
• Neural arches:
  • Both precaudal and caudal vertebrae can have neural arches, but the pattern and extent vary along the body.
• Summary of bone/cartilage status:
  • Vertebral elements remain cartilaginous; some calcification may occur with growth, but there is no true bone formation as in osteichthyans.

Cartilaginous Fishes: Calcification and Structure

• Calcification in cartilage:
  • In larger sharks and older individuals, calcium salts may be deposited within cartilage to increase rigidity, without replacing cartilage with bone.
  • This results in sections of the vertebral column that are stiffer yet still primarily cartilaginous.
• Vertebral architecture:
  • Precaudal vertebrae lack hemal arches.
  • Caudal vertebrae have hemal arches; both caudal and precaudal vertebrae can possess neural arches.

Bony Fishes (Osteichthyes) and the Notochord

• Notochord persistence:
  • In many bony fishes (especially more ancient lineages within Osteichthyes), the notochord persists as an unconstricted and prominent axial element in the skeleton.
  • This reflects a stage in which the axial skeleton is dominated by a robust notochord rather than a fully ossified vertebral column.
• Vertebral elements:
  • Vertebrae largely take the form of neural arches along the dorsal side and hemal arches along the tail, but the notochord remains a prominent structure in the axial skeleton for some lineages.

Amphibians and Early Tetrapods

• Transition to land:
  • Amphibians were the first vertebrates to colonize land; they are tetrapods with clearly developed centra (true vertebral bodies).
• Presacral and sacral regions:
  • Presacral region includes vertebrae anterior to the pelvis, with different functional subregions.
  • Sacral vertebrae are those in contact with the pelvic girdle; the vertebrae directly linking to the pelvis.
• Cervical region and skull support:
  • The cervical region in amphibians is present but not as prominent as in reptiles and mammals; movement of the head on the neck has some degree of motion.
  • The atlas (the first cervical vertebra, C1) is the key element linking the skull to the vertebral column; amphibians typically have limited cervical motion compared to mammals.
• Thoracic and lumbar regions:
  • Thoracic vertebrae: associated with ribs; vertebrae with thoracic features form the trunk segment near the chest.
  • Lumbar vertebrae: the trunk region closer to the abdomen; typically rib-less.
• Zygapophyses and vertebral articulations:
  • Amphibians develop zygapophyses (facets) that provide articulation between adjacent vertebrae, contributing to a more stable vertebral column in land locomotion.
• Fishes vs amphibians in vertebral connections:
  • Fishes generally lack well-developed prezygapophyses and postzygapophyses; their vertebrae are more separated.
  • Amphibians introduce connections that stabilize the spine as they support body weight on land.
• Frog vs salamander differences (skull and neck support):
  • Salamanders typically have more developed trunk vertebrae with ribs and can show a series of interconnected vertebrae; frogs have a different skull-support strategy and more limited cervical mobility.
• Neck elongation and body length strategies:
  • In amphibians, body length often increases by adding more trunk vertebrae, enabling longer bodies without changing the size of individual vertebrae.
  • This trend of increasing vertebral count to lengthen the body is seen in other tetrapods as well (e.g., snakes).

Amphibian Trunk Vertebrae: Details in Salamanders

• Tuberculum intergyoidium:
  • A distinctive process on the anterior end of the cervical vertebrae in salamanders that extends toward the skull.
  • It projects toward the region just anterior to the atlas and interacts with the foramen magnum, influencing skull-vertebra mobility.
• Foramen magnum and occipital region:
  • The foramen magnum is the opening at the base of the skull through which the brainstem and spinal cord pass.
  • In salamanders, the occipital region surrounding the foramen magnum interacts with the atlas via the tuberculum intergyoidium, allowing some degree of rotation.
• Skull support compared to frogs:
  • Salamanders possess different skull-support mechanics than frogs, with the atlas/occipital interactions contributing to mobility and stability.
  • Frogs lack the same tuberculum intergyoidium arrangement and display different upper-body kinematics.
• Atlas and axis in tetrapods:
  • Discussion contrasts salamanders with mammals: mammals typically possess both atlas (C1) and axis (C2), providing a robust range of head motion; salamanders may have a different cervical configuration with less complete axis development.

Birds: Neck Elongation and Vertebral Count

• Neck elongation through vertebral addition:
  • Birds increase neck length primarily by increasing the number of cervical vertebrae rather than by enlarging vertebrae themselves.
• Examples of cervical counts:
  • A Swan (a large waterfowl) may have as many as $24$ cervical vertebrae.
• Comparison with mammals:
  • In most mammals, neck length is more often achieved by increasing vertebral size rather than vertebral count (though exceptions exist).
  • Most mammals have $7$ cervical vertebrae, a relatively conserved feature across diverse taxa (with noted exceptions such as some sloths and manatees).
• Cervical vertebrae and skull support:
  • Birds, like other tetrapods, possess an atlas and axis; however, their cervical anatomy is adapted for extreme flexibility required for their head and neck movements during flight and foraging.

Mammals: Cervical Vertebrae, Atlas, and Axis

• Cervical count in mammals:
  • The typical mammalian neck has $7$ cervical vertebrae, a highly conserved feature across most mammals (exceptions exist: noting that some species like certain sloths and manatees are exceptions to this rule).
• Atlas and axis:
  • Mammals possess a two-vertebra system in the upper neck: the atlas (C1) and the axis (C2).
  • This joint arrangement allows a wide range of head movement, including rotation and nodding, facilitated by articulation at the skull-base where the atlas interfaces with the occipital bones.
• Skull support and occipital region:
  • The occipital region surrounds the foramen magnum, through which the brainstem and spinal cord exit the skull.
  • In humans and many mammals, the atlas rests on the occipital condyles, enabling head articulation with the vertebral column.
• Comparison with amphibians:
  • Amphibians typically have a more limited upper-neck articulation (often a single cervical element like the atlas) compared to mammals, which have a separate axis (C2) contributing to rotational freedom.
• General notes on vertebral articulation:
  • Prezygapophyses and postzygapophyses provide articulation surfaces between adjacent vertebrae; mammals and many amniotes have well-developed zygapophyses that stabilize the spine while allowing flexion and extension along the neck.
• Central nervous system and skull connection:
  • The brain is a highly modified portion of the neural tube; it is encased by bone and connected to the spinal cord via the spinal column through the foramen magnum.

Cross-cutting Concepts and Evolutionary Context

• Notochord vs vertebral elements:
  • Across the groups described, many lineages begin with a prominent notochord and later evolve vertebral elements (neural arches and hemal/hemal arches) that progressively replace or supplement the notochord as the primary axial support.
• Hemal arches vs neural arches:
  • Hemal arches (caudal) protect blood vessels in the tail; neural arches protect the spinal cord along the dorsal aspect of the vertebrae.
  • The presence and pattern of these arches vary by lineage and segment (precaudal vs caudal) and reflect functional demands (swimming, land locomotion, tail movement).
• Precaudal vs caudal distinctions:
  • Precaudal vertebrae lie anterior to the tail and often lack hemal arches; caudal vertebrae in the tail typically feature hemal arches and may show specialized adaptations.
• Locomotor and structural implications:
  • In fishes, vertebrae are configured to support aquatic locomotion; in amphibians and other tetrapods, vertebral fusion and intervertebral articulations increase rigidity for weight-bearing on land while still permitting necessary flexibility.
• Ontogeny and phylogeny:
  • The described patterns illustrate how development (embryology: notochord, neural tubes) and phylogeny (lineage transitions from jawless to jawed fishes to tetrapods) interact to shape the axial skeleton.
• Practical and real-world relevance:
  • Understanding these patterns informs comparative anatomy, paleontology (fossil interpretation of vertebral structures), and evolutionary biology.
• Ethical/philosophical notes:
  • The transcript does not discuss ethics or philosophy directly; the study of vertebral evolution is primarily a scientific inquiry into how anatomical structures arise and function across taxa.

Quick Reference: Key Terms and Concepts

• Notochord: a flexible, rod-like structure along the body axis that provides support and cues spinal development; persists in some lineages and is replaced by vertebral elements in others.
• Neural arches: dorsal components of vertebrae protecting the spinal cord.
• Hemal arches: ventral components (in the tail) protecting blood vessels; more common in caudal vertebrae.
• Precaudal vertebrae: vertebrae anterior to the tail.
• Caudal vertebrae: vertebrae within the tail, often bearing hemal arches.
• Atlas (C1) and Axis (C2): the first two cervical vertebrae in many tetrapods; enable head movement and skull articulation.
• Tuberculum intergyoidium: a vertebral process in salamanders extending toward the atlas/foramen magnum region, influencing skull joint mobility.
• Foramen magnum: the large opening at the base of the skull through which the brainstem and spinal cord pass.
• Sacral vertebra: the vertebra contacting the pelvic girdle.
• Presacral vertebrae: vertebrae in front of the sacral region (cervical, thoracic, lumbar).
• Central nervous system (CNS) development: the brain is derived from the neural tube and is protected by bone in the skull; the CNS continues as the spinal cord through the vertebral column.
• Calcification in cartilage: deposition of calcium salts within cartilage to increase rigidity without forming true bone.

Note: This set of notes follows the content and terminology as presented in the transcript, including some terminology and phrasing that reflect the original lecture style. Some nuances of current comparative anatomy may differ in modern texts; the core ideas about the notochord, vertebral development, and regional vertebral characteristics across hagfish, lampreys, cartilaginous fishes, bony fishes, amphibians, birds, and mammals are preserved here.