Skeletal - 5c

• Axial Skeleton:

  • phylogenetically oldest: notochord (fibrous tube but hollow and full of water)
  • functionally replaced and regionally displaced by vertebral column
  • protect nerve cord
  • resist axial compression
  • muscle attachment (myosepta muscle connection)
  • suspend body (on land)
  • protect thoracic cavity (protect lungs)
  • aid in lung ventilation (breathing)
• Vertebrae

  • neural arches phylogenetically first to appear, protect never chord
  • hemal arches in gnathostomes, protect blood vessels
  • centra in most gnathostomes displace notochord
  • two per body segment (one often lost or fused)

  
• Vertebral articulation:

  • Amphicoelous(dish on both sides/both ways) - centra concave on both ends, fishes, some lizards
  • acoelous: centra flat ended mammals
  • procoelous/ opisthocoelous: centra form ball and socket joints (lissamphibia, reptiles)
  • heterocoelous: saddle-shaped joints between centra turtles, birds (thumb joint can be moved up/down and left/right)

  
• Vertebrae parts

  • vertebrae offset with respect to myomeres
  • ribs and spines in mysepta, centra & arches bridge
  • ventral ribs (hemal arches) in fishes line peritoneum
  • dorsal ribs in transverse septum (epaxial/ hypaxial)

  
• Vertebral Parts

  • arches and centra may be seperate elements or fused
  • Apidospondly - primitive fishes and tetrapods
  • Holospondly (hollow+solid): most extant vertebrates
  • most loose inter neural

  
• Vertebral Parts

  • development of intercentrum and pleurocentrum has been influential in early tetrapods phylogeny
  • pleurocentrum most developed in amniotes
  • lissamphibia difficult to place

  
• Vertebral embryology

  • mesenchymal cells of the primary sclerotome migreate to notochord ->
  • condesne between primary somites as secondary sclerotome
  • thought that each primary somite contributes to both anterior and posterior secondary sclerotome

  
• Vertebral embryology

  • in primitive fish embryology, the sclerotome condenses into four parts pairs of cartilages, and this woud account for the four elements per vertebrae seen across many taxa

  
• Vertebral Embryology

  • not so clear in teleosts, where centra directly replace the notochord
  • first the notochord sheath is replaced by the chondrocytes then replacement bone
  • then further direct ossification is added

  
• Vertebral Embryology

  • not so clear in tetrapods where schlerentome surrounds the notochord to form a continuous though constricted tube
  • the tube becomes segmented
  • arches grow from the tube

  
• Vertebral Differentiation

  • trunk and caudal vertebrae with little differentiation
  • except tail of teleost where vertebral elements become highly modified

  
• Vertebral Differentiation

  • Atlas: nodding at occipital condyles
  • Axis: rotation with the atlas
  • cervical : ribs in sauropsids foramina for cervical ganglia
  • trunk: with true ribs, false ribs (doesnt connect to stirnum), or floating ribs, may be divided into throacic and lumbar
• Vertebral Differentiation

  • Atlas: nodding at occipital condyles
  • Axis: rotation with the atlas
  • cervical : ribs in sauropsids foramina for cervical ganglia
  • trunk: with true ribs, false ribs (does not connect to sternum), or floating ribs, may be divided into thoracic and lumbar
  • sacral: fused to pelvic girdle - not fish
  • caudals - last

  
• Appendicular Skeleton

  • pectoral and pelvic girdles - fish
  • medial fins and supports - fish
  • pectoral and pelvic girdles - mammals
  • limbs (chyridia)
• Paired Fins

  • girdle in body wall, made of endochondral (replacment bones) and dermal elements
  • supports an endochondral fin base made of basal and radial pterygiophores
  • that supports dermal fin rays and fin webs - modified scales on top

  
• Paired fins

  • Archipterygial (wings/fins)- with radial pterygiophores on either side of the basals
  • Metapterygial - with radial peterygiophores on anterior side of basals (later specialization)

  
• Paired fins Origin

  • an early theory posited that the paired appendicular skeleton is derived from the gill arches - this doesnt explain the pelvic girdle - must be another theory
  • and the basals from gill rays
  • archipterygial fin would be primitive (gill arch theory - fins are modified gill arches)

  
• paired fin origin

  • fin fold alternative hypothesis tat the paired fins are out grwoths of the body wall
  • metapterygial fin is primitive

  
• Paired fin Origin

  • gene expression findings support fin fold hypothesis
  • TBX gene in amphioxsisexpressed ventro-latterally
  • two TBX genes in sharks + vertebrates (exactly where pelvic and pectoral girdles are
  • Sonic Hedgehog (shh) gene expressed in sarcopterygii extends length of limbs (expressed for complexity)

  
• paired fin origin

  • Hox (found along the chromosome, anterior/posterior) gene expression shows that digits in tetrapods are derived from pterygiophores posterior to the basal pterygiophores axis

  
• Pectoral Girdle

  • mainly dermal bone in actinopterygii and primitive sarcopterygii
  • attached to head, no mobile head allowed
  • incorporates some dermal bones in bony fish

  
• Pectoral Girdle

  • reduction of dermal bony elements in tetrapods
  • often only clavicle or interclavicle remains of both completely lost

  
• Pelvic Girdle

  • NEVER dermal elements
  • single element in fishes
  • does not attach to vertebral column
  • three ossifications in tetrapods - illium - connects with sacrum, ischium and pubis

  
• Pelvic Girdle

  • ischium and oubis elongated in archosaurs
  • bipedalism
  • two patterns - lizard hips/ bird hips

  
• Heterotopic Bones (occur in unusual places)

  • not part of the regular skeleton
  • Sesamoid Bones - aid tendons sliding over joints (knee cap)
  • Heart bone in deer and bovids
  • Eyelid bone in some archosaurs
  • Os penis (baculum) and Os clittoris in manu mammals including primates but lost in humans often used for identification