BI221 Exam 2

• Locomotion, skeletal contribution, posture, and limb element length influence speed, power, and energy conservation.

Muscle Cells

• Slow twitch cells contract slowly with less force, use aerobic respiration, resist fatigue, contain myoglobin, and are well-supplied with blood vessels.
• Fast twitch cells contract rapidly with great force, rely on anaerobic glycolysis, produce lactic acid leading to fatigue, and lack high myoglobin levels, appearing paler.
• Mammals, birds, and fishes exhibit both slow and fast twitch cells more than amphibians and reptiles.

Evolution of Limbs

• The gill arch hypothesis suggests limbs evolved from gill filaments but fails to explain pelvic girdle position and dermal bone in pectoral girdles.
• The fin-fold hypothesis suggests paired fins arose from ventrolateral folds.
• Ventrolateral fin-folds are supported by developmental studies of sharks and fossils of ancient ostracoderms.

Axial Skeleton Adaptations

• The notochord's role reduced, leading to more robust vertebrae.
• A stronger axial skeleton resists torsion.
• Aspidospondylous vertebrae transition to holospondylous vertebrae.
• The stronger axial skeleton reduces flexibility.
• Terrestrial species tend to have fewer vertebrae.
• The long tail became less important for locomotion but serves as balance.
• Stronger articulations exist between vertebrae.
• The stronger axial skeleton supports abdominal muscles that hold viscera.
• Zygapophyses lock adjacent vertebrae preventing excessive twisting.
• The sacral connection evolution helps support hindlimbs, transferring thrust to the axial column.
• Separation of the pectoral girdle allows head movement independent of the axial skeleton.
• Forelimbs gain greater range of motion.

Axial Skeleton Regionalization

• Axial skeleton regionalization increases from trunk/caudal to cervical/thoracic/lumbar/sacral/caudal.
• New muscle attachments facilitate new limb motions.

Middle Ear Evolution in Mammals

• Middle ear ossicles consist of the stapes, incus, and malleus.
• The stapes derives from the second branchial arch, initially bracing the jaw before transmitting sound.
• The incus originates from the first branchial arch, evolving from the upper jaw (palatoquadrate) and shifting to transmit vibrations.
• The malleus also originates from the first branchial arch as part of the lower jaw (mandible), shifting into the middle ear.

Internal Nares Evolution

• Internal nares evolved from the posterior external nares in fishes.
• The posterior nares shifted laterally and crossed the tooth row to position in the mouth roof.
• Kenichthys shows an intermediate form, indicating the transition.
• The shift to internal nares may be linked to low oxygen levels, facilitating air breathing.

Bone Development

• Osteoblasts form bone matrix, becoming osteocytes once embedded.
• Osteoclasts erode bone.
• Intramembranous bones ossify directly without a cartilaginous precursor.
• Endochondral bones form from a cartilaginous model.
• Bone remodeling occurs during development with osteoclasts eroding bone.

The Vertebrate Skull Components

• The vertebrate skull consists of the splanchnocranium, chondrocranium, and dermatocranium.
• The splanchnocranium originates from pharyngeal bars and early jaws/hyoid.
• The chondrocranium supports the brain and sensory structures.
• The dermatocranium originates from dermal bone plates fusing to other cranial elements.

Jaw Evolution

• Autostyly is where the upper jaw was fixed to the cranium.
• Autostyly jaws are effective for biting but limited mobility.
• Amphistylic jaws attach the palatoquadrate to the cranium directly and via the hyomandibula.
• Hyostylic jaws primarily link via the hyomandibula.
• Hyostyly jaws appear independently in actinopterygians and chondrichthyans.
• Actinopterygian hyostyly facilitates lip protrusion and suction.
• Hyostyly sharks aid in streamlining and jaw positioning.
• Tetrapods return to autostyly (metautostyly) for robust gripping jaws.