Vertebral Column

NOTOCHORD

Primitive Axial Skeleton — Replaced by vertebral column

Unsegmented, composed of dense, fibrous connective tissue

First skeletal element in embryo of chordates

VERTEBRAL COLUMN

Segmented (more/less flexible), arched rod flanked by axial musculature

Function — Suspends the trunk, Protects the spinal chord

CENTRUM

Ossification of notochord

Function — Anchor + support to arches

Elements — Intercentrum (cranial), Pleurocentrum (caudal)

Structures of the Centrum

Types of Centra Based on Shape

AMPHICOELUS	Concave (both ends) Limited motion (permitted in any direction)	Fishes, Apoda, Urodela
OPISTHOCOELUS	Convex (Front), Concave (Back) Permit motion (any direction)	Salamanders, Parrots, Ungulates
PROCOELUS	Concave (Front), Convex (Back) Permit motion (any direction)	Anurans, Modern Reptiles
ACOELUS/ AMPHIPLATYAN	Can withstand compression, limits motion	Mammals
HETEROCOELUS	Saddle-Shaped Allows vertical & lateral flexion Prevents rotation around axis of spine	Birds

ARCHES

PROCESSES (APOPHYSES)

PHYLOGENY OF VERTEBRA

RHACHITOMOUS	Present (earliest land vertebrates) Centra retains geometric relationships, but robust Articulates with one another, restricting the notochord
STEREOSPONDYLOUS	Pleurocentra — Reduced/lost Intercentra — Spool-shaped
EMBOLOMEROUS	Pleuro/Intercentra — Similar Gives each neural arch two spool-shaped centra
GASTROCENTROUS	Present (amniotes & relatives) Pleurocentra — Enlarged, fused Intercentra — Reduced (small wedges) or lost
HOLOSPONDYLOUS	Neural Arches, Centra — Ossify Origin of centrum is unknown (pleurocentrum, intercentrum, or both?) Notochord — Intervertebral discs

AGNATHANS

Notochord — Persists

Lateral Neural Cartilages (skeletal elements; limited to tail region in hagfishes)

Dorsolateral Cartilaginous Plate — Single, longitudinal; perforated by foramina for spinal nerves

CHONDRICHTHYANS

Notochord — Present (adult vertebral column)

Centra — Chordal & perichordal cartilage; amphicoelus

Two types of vertebra

• Trunk

• Caudal (haemal arch & haemal spine)

Squalus — Have foramina for dorsal and ventral roots

Fibroelastic ligament — Connects neural spines along vertebral column

ACTINOPTERYGIANS

Vertebrae — Well-ossified

Centrum — Dumb-bell shaped vacuole

Porous cartilage-like material — Space between successive vertebrae

Supraneural bones — Neural spines; very tall, sometimes surmounted

SARCOPTERYGIANS

NO CENTRA — Dipnoans, chondrostei, coelocanth

NOTOCHORD — Present, unconstricted; contains little cartilage/bone (Basidorsal, Basiventral, Interdorsal, Intervertebral Cartilage)

EVOLUTION OF TETRAPOD VERTEBRAE

Rhipidistian + early labyrinthodants — Consists of several bones per segment

Rhachitomi — Large intercentrum, small pleurocentra

Stereospondyli — Retains intercentrum

Embolomeri — Increased size of pleurocentrum to size of intercentrum

Amniotes — Enlarged pleurocentrum to near/complete exclusion of intercentrum

REGIONAL SPECIALIZATION IN TETRAPOD COLUMNS

TYPES OF CERVICAL VERTEBRA

SACRAL VERTEBRAE

Stout transverse process

SYNSACRUM

Fusion of thoracic, lumbars, sacral, few caudals with pelvic girdle (little flexibility)

Armadillos — 13 fused sacrals & caudals

CAUDAL VERTEBRAE

Early Tetrapods — 50 or more

Reduced to tail in modern mammals

Processes & Arches — Shorter, rudimentary

UROSTYLE

Anurans — Continuous elongated perichordal cartilage (base of tail); post-sacral vertebrae

AUTOTOMY

Regeneration of tail (lizards, salamader, tuatara); sheds part of tail to escape

PYGOSTYLE

Fusion of last caudals (skeleton of bird; tail)

COCCYX

Rudimentary — With transverse processes, lacks arches

Diminished in size, fused in apes & humans

VERTEBRAL FORMULA IN MAMMALS

Ribs

Articulate with vertebrae, extends into body wall; formed intersegmentally (scleroblasts) from two mesodermal somites (centra)

Gastralia (Abdominal Ribs) — Not ribs (reptiles), remnants of dermal exoskeleton

FISHES

Basal Actinopterygians, Telosts — Two sets of ribs (each vertebra)

Skates, Chimaeras, Teleosts, Agnathans — No ribs

TETRAPODS

Ribs — Associated with vertebrae, from atlas to base of tail; restricted in number, short ones ankylosed (stiffens); long ribs (anterior thoracic region) acquired anchorage (sternum)

Tetrapod ribs are homologous and bicipital—dorsal tuberculum (transverse process), ventral capitulum (hypocentrum)

AMPHIBIANS

Very short (anurans, urodeles)

Ankylosed (transverse process) in anurans

Ribs (burrow-dwelling apodans) are long

Anurans — Not bicipital

NONAVIAN REPTILES

Lizards & Crocodilians — Long ribs in trunk vertebrae, short ribs in neck

Sphenodon — Long ribs in trunk, ribs continue to tail

Geckos (ribs with every cervical vertebra)

*More specialized lizards lack atlas & axis

BIRDS

First two pairs of ribs — articulate with last two cervical vertebrae at base of neck with carinates (generally movable, lack sternal segment)

Thoracic Ribs — Next 5 pairs are thin, flat, broad, and have uncinate processes

Caudal Ribs — Ankylosed to synsacrum

MAMMALS

Confined in thorax

If more than 10 ribs are present, remaining are floating/not connected with sternum

Transverse Foramen — Fusion of transverse process and centra

Tetrapod Sternum

Tetrapod-only structure; predominantly found in amniotes (endochondral in origin)

Presence, size, anatomical relationship — Correlated with extent to forelimbs are employed in locomotion

Functions

• Base against pectoral girdles and ribs are braced

• Anatomical origin of ventral muscles of forelimb

Largest in flying animals, reduced/absent in tetrapods lacking forelimbs

Paired Mesenchymal — Unites, undergoes chondrogenesis

AMPHIBIANS

Well-differentiated (anurans)

Necturus — More/less isolated nodules of cartilage associaed with linea alba caudal to coracoid cartilages of pectoral girdle; sternal element in other salamanders at same location

Apodans — No trace of sternum

NONAVIAN REPTILES

Lizards — Agile climbers & rubbers have large shield-shaped sternum/cartilage (replacement bone)

Crocodilians — Simple, cartilaginous plate attached to precoracoid of pectoral girdle, continues caudally in mineralized fibrous membrane incorporating the gastralia

Turtles — No sternum

BIRDS

Sterna — Power flight birds developed enormous keel/carina where flight muscles are attached

MAMMALS

Composed of series of bony segments (strenebrae)

Sternebrae/xiphisternum — Cartilaginous/bony xiphoid process

Marine mammals — Only few ribs reach it, individual sternebrae tend to fuse