SKELETAL ELEMENTS:

Dermal Skeletono Ostracoderms – armored fisheso Tetrapods – retain dermal

elements in skull, jaws, and pectoral girdle

Endoskeleton - Develops from sclerotome

(mesenchyme in skeletogenous region.o Visceral skeleton – assoc., w/

gills and skeletal elements derived from them (jaw cartillages)

o Somatic skeleton – axial skeleton (vertebral column ribs, sternum, skull) and appendicular skeleton

- Myotomes, divided by myosepta, and divided into epaxial and hypaxial halves by horizontal septum.

- Septum is divided into two ventrolateral septa by the coelom

- Sclerotomes of adjacent segments is divided into loose anterior half and denser posterior half

- The Posterior halves become anterior part of vertebra and the Anterior halves become the posterior part of vertebra.

Agnathans:- The vertebral elements or

ARCUALIA, are only the basidorsals and interdorsals.

Gnathosthomes- Additional vertebral elements, the

basiventrals and interventras. Mesenchyme from sclerotomes

accumulate and form Perichordal mesenchyme.

The notochord and sheat calcify into centra

Basidorsal and Basiventral (Posterior halves of sclerotome) – form the neural and hemal arches

Interdorsal and Interventral (Anterior halves of sclerotome) – gives rise to the intercalary arch

Primitive vertebrae – composed of separate pieces

Primitive amniotes – 2 pieces : pluerocentrum (interdorsal arcualia) and hypocentrum (basiventrals)

Pleurocentrum – forms the main mass of the centrum.

Hypocentrum – remains as hemal arches (chevron) contributes to the atlas and axis.

Calcified notochord and sheat + arcualia = centrum and arches of vertebra.

Vertebral Column- Vertebrae - consist of a centrum, 1

or 2 arches, and various processes Acelous - flat ended (mammals) Amphicelous – concave on both ends

(most fishes, salamanders, and caecillians)

Procelous – concave and convex (anurans, and reptiles)

Opisthocoelous – convex and concave (salamanders)

Heterocelous – saddle-shaped centrum at both ends (birds)

Vertebral Processes- Projections from arches and

centr8a- Give articulate w/ ribs and serves as

site of muscle attatchement Transverse processes – most common

type of processes, separate epaxial and hypaxial muscle.

Diapophyses & Parapophyses – articulate w/ ribs.

Prezygapophyses (cranial) & Postzygapophyses (caudal) – limit flexion and torsion of the vertebral column.

Dorsal Region- Name given to the vertebrae

between cervical and sacral. Crocodilians, lizards, amphibians,

mammals – ribs are confined to anterior region

o Thoracic: 12-15 vertebrae w/ ribs

o Lumbar: vertebrae w/out ribs Cervical region Atlas – provides cradle in skull;

hypocentrum & pleurocentrum is odontoid process of axis.

Axis – has two pleurocentra. Sacral vertebrae Short transverse processes that brace

pelvic girdle and hindlimbs Sacrum – single bony complex

composed of fused sacral vertebrae. Caudal region Primitive tetrapods: more than 50

caudal vertebrae Present-day tetrapods: shorter caudal

vertebrae with cyclindrical centra. PHYLOGENY OF VERTEBRAL

COLUMNS Rachitomous vertebra – in

crossopterygian vertebrae w/c consist of hypocentrum and 2 pleurocentrum.

Pleurocentra = increase in size Hypocentrum = decrease in size

o Agnathans- Only skeletal elements assoc., with

notochord are paired (lateral neural cartilages)

o Cartilaginous fishes- Vertebrae includes neural arches

and dorsal intercalary plates.o Teleost

- Regional differentiation: trunk and caudal regions

- Well-ossified amphicelous vertebrae

- Centra from perichordal mesenchyme and sheat elements

o Chondrosteans (sturgeons & paddlefish) & modern lungfishes- Incomplete centra- non constricted notochord- Basidorsal and basiventral only- Diplospondyly – has two centra and

two sets of arches / body segment(sharks)

o Amphibians- Regional differentiation:

o Cervical – 1 vertebrao Trunk – ribso Sacral – 1 vertebrao Caudal – absent of

zygapophyses- Basidorsal appear; basiventral

reduced- Centra is made of perichordal

mesenchyme represented by hypocentrum

- Pleurocentrum form Intervertebral disk

- Cranial joint w/ 1-2 occipital condyles

- Urostyle – fused post-sacral vertebra

o TETRAPODS- Regional differentiation: cervical,

thoracic, lumbar, sacral, caudal- Centrum, descendant of

pleurocentrum- Vertebra compounded of arches

and perichordal componentso REPTILES

- Basidorsal, interdorsal, basiventral- Pluerocentrum is phylogenetic

ancestral centrum

- Procelous centrum replaces notochord

- Cervical: eight pieces- -increased number of cervical,

increased flexibility of head- -craniovertebral joint: ball and

socket type of joint- Single occipital condyle- Atlas and axis is odontoid for

rotation- Proatlas in crocodiles is anterior to

atlas- Dorsals – trunk- Sacral – two for support- Cudal – Chevron (hemal arch),

Autonomyo BIRDS

- All pairs of arcualia present with supradorsals and complete series of hypocentra

- Heterocelous centrum from perichordal and arch components

- Numerous cervical ( 25 in swans)- Single occipital condyle- Thoracic tend to fuse- Synsacrum: last thoracic, lumbar,

two sacral, some caudal fused with pelvic girdle

o Provides support for bipedal locomotion

- Pygostle: last 4 or 5 caudal that supports the tail feathers

o MAMMALS- Basidorsal form neural arch- Acelous centrum from

pleurocentrum- Hypocentrum intervertebral disk- Craniovertebral joint: two occipital

condyle- Cervical: seven - Thoracic: twelve to fifteen- Lumbar: vary- Sacrum: 3 to 5- Caudal: chevron bone

- Tail bone/ coccygeal – last 3 to 5 caudal vertebrae

RIBS- Amniotes: reduced or absent

except in thoracic and sacral regions.

- Two kinds:o Dorsal/intermascular ribs

and ventral, pleural/subperitoneal ribs

o Majority of vertebrates are ventral tyoe

- May be long or short, cartilaginous or bony

- Articulate medially w/ vertebrae and extend into the body wal

o Agnathans: no ribso Sharks: dorsalo Teleost: ventral, some have two pair of

ribso Tetrapods: bicipital ribso Reptile: ribs are short in neck and long

in trunko Turtles – fused to carapaceo Snakes – long and curvedo Crocodilians – gastralia ( not a rib)

- Sternal rib: maybe ossified (birds), or remain cartilaginous (mammals); articulate w/ sternum

- Uncinate processes: provide rib cage w/ additional support (birds)

o Thoracic ( 12 in human and 13 in cat); true rib, false rib, floating rib

STERNUM- An amniote structure- For stronger and mobile forelimb- Function: protection, strengthens

body wall, muscle attachment, rib attachment

- Endochondral bone

- Arisen from the fusion of anterior members of true abdominal ribs (parasternal)

o Amphibians – no sternum in early amphibian; only anurans have

o Amniotes – plate of cartilage and replacement bone; articulates w/ pectoral girdle w/ variable number of ribs.

o