axial

Notochord

long, continuous rod of fibrous connective tissue

origin of Vertebral column

Originated as block-like elements in early vertebrates

What is the basic structure of a vertebral segment?

Each segment can include up to 2 dorsal arches, 2 ventral arches, and 2 centra.

How is the vertebral column related to body segmentation?

It aligns with segmented muscle blocks (myomeres) and is separated by myosepta.

What are dorsal arches and their types?

Arches on the top of the spinal cord; types are:

Neural arches

Interneural arches

What are ventral arches and their types?

Arches below the notochord; types are:

Hemal arches

Interhemal arches

What are centra in the vertebral column?

They are the enlarged bases of the ventral arches at the notochord.

What is the intercentrum and what does it support?

It's the base of the hemal arch and dominates in temnospondyls (anamniotes).

What is the pleurocentrum and what does it support?

It's the base of the interhemal arch and dominates in amniotes.

What is the origin of ribs?

Ribs develop from cartilage in embryonic myosepta (connective tissue between muscle blocks).

How do fish and tetrapod ribs differ?

Fish: Long, thin ribs that don't form a closed ribcage

Tetrapods: Shorter ribs that may form a ribcage (especially in amniotes), often articulate with sternum

What is the sternum and its function?

A midline bone (or cartilage) that connects ribs ventrally

Helps form the ribcage and supports respiration in tetrapods

What are gastralia and their function?

Dermal bones in the abdominal region (not attached to vertebrae or ribs)

Support the belly and aid in breathing (found in some reptiles and fossil tetrapods)

Agnathans (Jawless Fishes)

What is the main axial support in agnathans?

large, prominent notochord; vertebral elements are mostly absent.

Chondrichthyes (Cartilaginous Fishes)What is the vertebral column like in modern Chondrichthyes?

Prominent vertebrae

Notochord is reduced but still present within the column

Amphicoelous joints between vertebrae

Osteichthyes (Bony Fishes)

What is the axial skeleton like in primitive Osteichthyes (e.g., sturgeon)?

Dominant vertebral column but secondarily unossified.

What is the axial skeleton like in advanced Osteichthyes (e.g., teleosts)?

Ossified, well-developed vertebral column.

Sarcopterygians (Lobe-Finned Fishes)

How does vertebral ossification differ in sarcopterygians?

Ancient forms (e.g., Eusthenopteron): more ossified

Modern forms (e.g., Coelacanth): less ossified

Early Tetrapods

What vertebral centrum dominates in temnospondyls?

Intercentrum - ancestral to modern amphibians.

Early Tetrapods

What vertebral centrum dominates in anthracosaurs?

Pleurocentrum - gave rise to amniotes.

Early Tetrapods

What are characteristics of Ichthyostegids?

Late Devonian (~365–360 Ma)

Pelvic girdle fused to vertebral column

Digits replaced fin rays—definitive limbs

Modern Amphibians

What is the vertebral column like in urodeles (salamanders)?

traight and fish-like with a single midventral sternal plate.

Modern Amphibians

What is the vertebral column like in anurans (frogs)?

Arched, with a xiphisternum sometimes containing cartilage.

What allows head-neck articulation in sauropsids?

The atlas and axis (first two cervical vertebrae).

What are axial adaptations in snakes, turtles, and birds?

snakes: Extra zygapophyses to resist twisting

Turtles: Vertebrae fused with shell

Birds:

Uncinate processes for muscle attachment

Long, flexible cervical vertebrae

How many cervical vertebrae do most mammals have?

7

What is the innominate bone?

A fused bone made of the ilium, ischium, and pubis.

zygapophyses

Interlocking processes between vertebrae that prevent twisting

cervical vertebrae

Allow independent head movement,

What is the synsacrum in birds?

A fusion of lumbar, sacral, and some caudal vertebrae—provides support for flight and bipedal stance.

What is the carina in birds and what does it do?

A keel-like structure on the sternum that anchors powerful flight muscles.

What are heterocoelous cervical vertebrae, and where are they found?

Saddle-shaped vertebrae allowing great neck flexibility—seen in birds and turtles.