Elements of appendicular skeleton: f li b Elements of appendicular ...

Elements of appendicular skeleton:
• Pectoral girdle:
forelimb
• Forelimb:

Mammals
Anterior
Posterior

Limbs and associated muscles are
derived from somatic hypomere:

Hox gene expression in:
Bony fishes
Tetrapods

Pectoral girdle
Modifications of
the basic fish
pattern in
vertebrates
Anterior
Dermal
elements
Endochondral
elements

Shark

Medial view
Lateral view

Tiktaalik
rosae
• Sarcopterygian.
• Shallow-water fish, but with:
– Robust pectoral limbs, functional wrist
– Well-supported ribs
– Functional neck

Pentadactyl forelimb structure:
Hallux

Number of digits in
early amphibians
initially varied
from 4-13
Digits reduced in
living amphibians
Theropods

Convergent evolution
of external forms
using different
combinations of
skeletal elements

Historical constraints on adaptation
• Ex: Giant panda’s thumb:
– Pandas related to bears and raccoons
(Carnivora).
– Bears /raccoons are the most omnivorous
carnivores.
• Use front paws to manipulate food.
– Giant panda are more dexterous than bears
and raccoons.
• Diet restricted to bamboo.
• Strip leaves between flexible “thumb” and
other digits.
– Problems:
• Primates have opposable thumbs, not
carnivores.
• 5 (not 4) remaining digits.

Constraints on adaptation
• Panda’s thumb:
– Enlarged carpal bone:
• Bears: slightly enlarged radial
sesamoid bone.
• Carnivores: two 1 st -digit abductor
muscles.
• In bears, one muscle attaches
instead to radial sesamoid.
• Abductor and adductor muscles are
modified 1 st -digit abductors.
– Panda’s thumb is modification of
carpal structures.

Limb
configurations
Amphibian and
reptilian
condition
(2 effective
limb segments)
Mammalian and
archosaur
condition
(3 effective
limb segments)

“Muscular sling”
of mammals

Elements of appendicular skeleton:
hindlimb
• Pelvic girdle
• Hindlimb

Mammals
Anterior
Posterior

Anterior
Posterior

• Acanthostega :
– One of first amphibians with well-defined digits (8)
on forelimbs, hindlimbs.
– Limbs extended laterally, not ventrally.
– Joints (elbow, wrist, knee, ankle) present but with
little mobility.

Sue’s foot
(Tyrannosaurus rex)
Theropod
feet (4 toes)
Utahraptor
Vulture
Hallux

Human pelvis

Human pelvis

Knee
Wrist
Elbow
Ankle

Models of limb evolution
• Account for two major transitions
in chordate limbs:
– Aquatic fin folds to fins.
• “Fin-fold theory”.
– Fins to tetrapod limbs.
• “Axial” and “basal” models.

Cephalochordate
condition
Fin-fold
theory
Fish condition

Fin-fold model

Models of limb evolution

Hagfishess
Chondrichthyes
Selachii
Lampreys
Sharks, rays
Chimaeras
Sarcopterygii
Crossopterygii
Dipnoi
Lungfishes
Coelacanths
Tetrapods
Bichirs
Sturgeons, paddlefishes
Gars
Bowfin
Bonytongues, knifefishess
Tarpons, eels
Herrings,
anchovies
Minnows,
, characins, catf
fishes
Salmons,
trouts
Dragonfishes
Lizardfishes
Lanternfi
ishes
Spiny-rayed fishes
Silversides, killifishes
Cods, anglerfishes
Vertebrata
Gnathostomata
Osteichthyes
Teleostei

Basal model
Axial model

Axial model
• Lungfish condition is plesiomorphic.
• Axial bones homologous with
posterior long bones + carpals +
metacarpals + phalanges.
• Evidence: embryonic and gene
expression patterns of vertebrate
limb development.

inductive sequence

Basal model
• Coelacanth (crossopterygian)
condition is plesiomorphic.
• Long bones are homologous with basal
fin elements (heterochronic change).
• Radials are homologous with carpals +
metacarpals + phalanges.
• Supported by phylogenetic evidence
for monophyly y of the sarcopterygians.

Context of evolution of tetrapod limb
• Late Silurian – Devonian (410-400 mybp):
– Diversification of plants into semi-aquatic and terrestrial
habitats.
– Formation of wetlands and rivers with abundant plant
life.
– Appearance of terrestrial insects.
• Transition from fin to limb preceded that from aquatic
to terrestrial habitats.
– Early limbs sufficient for shallow, muddy waters.
– Multiple, clearly defined digits.
– Weak girdles and limbs, weak wrists.
• Tetrapods: reduction of dermal bone, elaboration of
endochondral elements.

Structure of the vertebrate skeleton
• Two major sections:
– Axial skeleton
– Appendicular skeleton
• Heterotopic elements
• Articulations

Some heterotopic elements
Penis bacula

Structure of the vertebrate skeleton
• Two major sections:
– Axial skeleton
– Appendicular skeleton
• Heterotopic elements
• Articulations (arthroses):

Synarthroses

Amphiarthroses

(forming
meniscus
layers)
Diarthroses

Femoral ball-
and-socket
joint

Trends in the skeletal system
• Internalization: from external dermal skeleton to
internal endochondral skeleton.
• Differentiation of regions of the skull and axial
skeleton.
• Cephalization: i independence d of head from
postcranial skeleton.
• Development and specialization of pentadactyl
limbs.
• Relocation of limbs from lateral to ventral.
• Interlocking of vertebrae via zygapophyses.
• Reduction (loss and fusion) of skeletal elements.