Tetrapods vs. Sarcopterygians Anatomy

Anatomical Differences: Tetrapods vs. Sarcopterygians

• Tetrapods lack median fins, have elongated snouts, and lack opercula.
• Tetrapods possess cervical vertebrae, simple lungs, and a short trachea.
• Ribs in tetrapods support the body, they have zygapophyses, and have one sacral vertebra connecting to the pelvis.
• Tetrapods have stout limbs with digits, a simple hock joint, and specific muscle arrangements: Epaxial muscles connect many vertebrae, Hypaxial muscles are divided into five groups.
• Kidneys in tetrapods extend the length of the trunk, they have urinary bladders, and lateral lines are only present in aquatic species or larvae.
• Adult frogs have an eardrum and stapes.

Tetrapod Hypaxial Muscle Groups

• Subvertebralis: Located immediately ventral to the vertebrae, forming the roof of the thoracic and abdominal cavities.
• Transversus abdominis: The most medial layer of the lateral muscles.
• Internal oblique: The middle layer of the lateral muscles.
• External oblique: The most lateral layer of the lateral muscles.
• Rectus abdominis: The most ventral muscle, forming the floor of the thoracic and abdominal cavities.

Ancestral Tetrapod Locomotor System & Respiration

• Basal tetrapods utilize trunk muscles for lateral body bending locomotion (anguilliform movement).
• This interferes with respiration because lateral bending contracts muscles on only one side, while exhalation requires simultaneous contraction of both sides.

Appendicular Skeleton & Pectoral Girdle

• Appendicular Skeleton: The fins/limbs and their support structures.
• Pectoral Girdle: Supports the front fins/limbs, composed of:
  • Clavicle
  • Coracoid
  • Scapula

Glenoid Fossa & Pelvic Girdle

• Glenoid Fossa: The site of interaction between the pectoral girdle and the humerus, forming the shoulder joint.
• Pelvic Girdle: Supports the back fins/limbs, composed of:
  • Ilium
  • Ischium
  • Pubis

Acetabulum & Obturator Foramen

• Acetabulum: The site of interaction between the femur and the pelvis, forming the hip joint.
• Obturator Foramen: Allows for passage of blood vessels and nerves.

Proximal & Distal Definitions

• Proximal: Closer to the body.
• Distal: Farthest away from the body.

Pectoral & Pelvic Limb Bone Order

• Pectoral Limb (Proximal to Distal):
  • Humerus
  • Radius and Ulna
  • Carpals
  • Metacarpals
  • Phalanges
• Pelvic Limb (Proximal to Distal):
  • Femur
  • Patella
  • Tibia and Fibula
  • Tarsals
  • Metatarsals
  • Phalanges

Olecranon & Calcaneus

• Olecranon: The most proximal part of the ulna, forming the elbow joint with the humerus.
• Calcaneus: A tarsal bone, part of the hock joint, where the tendon for joint extension attaches.

Palmar & Plantar Definitions

• Palmar: Refers to the bottom surface of the front paw.
• Plantar: Refers to the bottom surface of the back paw.

Appendicular Skeleton & Muscle Belly

• Skeletal muscle is most involved with the appendicular skeleton.
• Muscle Belly: The concentrated mass of muscle cells and fibers responsible for contractile force.

Pectoral Girdle Muscle Groups

• Branchiomeric Muscles: Example: Trapezius.
• Axial Muscles: Example: Serratus ventralis.
• Dorsal Muscles: Example: Latissimus dorsi.
• Ventral Muscles: Example: Pectoralis.

Joint Levers: Input & Output Arms

• Joints can act as levers, with the input arm determining force/velocity of the output arm.
• Input Arm: The size of the input arm affects the force and velocity of the output arm.
• Output Arm: A larger input arm generates greater output force, while a smaller input arm allows for greater output velocity.

Tendons & Ligaments

• Tendons: Connect muscles and bones.
• Ligaments: Connect bones and/or cartilages.

Movement Definitions

• Flexion: Bringing bones of a joint closer.
• Extension: Bringing bones of a joint further apart.
• Adduction: Drawing a limb closer to the body's center.
• Abduction: Drawing a limb away from the body's center.

Reptiliomorpha

• The clade containing amniotes and extinct organisms more closely related to amniotes than amphibians.

Amniote Egg Extraembryonic Membranes

• Amnion: Protects the embryo from mechanical damage.
• Chorion: Allows gas exchange with the allantois.
• Allantois: Stores nitrogenous wastes, assists in gas exchange with the chorion.
• Yolk Sac: Stores and transports nutrients to the embryo.

Amniote Egg's Adaptations for Land

• The amniote egg protects the embryo from dehydration, mechanical injury, and infections.

Amniocentesis & Extraembryonic Membranes

• Amniocentesis involves withdrawing amniotic fluid, avoiding fetus, placenta, and umbilical cord.
• The needle passes through the amnion and chorion.

Anatomical Differences: Amniotes vs. Other Tetrapods

• Amniotes have elongated necks, longer tracheas with cartilage rings, lungs divided into alveoli, and longer ribs aiding in ventilation.
• They possess well-developed zygapophyses, two or more sacral vertebrae, slender limbs, mesotarsal joints, and specialized epaxial and hypaxial muscles aiding in ventilation.
• Amniote kidneys are compacted, lateral lines are absent, and eardrums and stapes are always present.

Simple vs. Mesotarsal Hock Joint

• Simple Hock Joint: No distinct motion between tarsal bones.
• Mesotarsal Joint: Tarsal bone fusion creates astragalus and calcaneus, allowing greater movement and stronger hindlimb force generation.

Internal Fertilization in Amniotes

• The protective membranes form within the mother, requiring fertilization before their development.
• External fertilization is not effective as sperm cannot penetrate the protective membranes.

Function of the Stapes in Amniotes

• The stapes transmits vibrations from the tympanum to the inner ear.

Amniote Trachea

• Amniotes have tracheas supported by cartilage rings, unlike basal tetrapods and amphibians.

Fenestrae

• A small hole or opening in the skull.

Cranial Fenestra Types

• Anapsid: No openings in the skull, solid covering over the roof and sides.
• Synapsid: One pair of holes (lateral fenestrae) in the skull.
• Diapsid: Two pairs of holes (lateral fenestrae) in the skull.
• Sauropsid: A diapsid subgroup with modified fenestrae.

Ventilation

• Active process of moving water or gas across the respiratory surface.

Muscles Involved in Respiration

• Intercostal muscles (between ribs)
• Diaphragm (where present)

Costal Ventilation & Aspiration Pump

• Costal Ventilation: Intercostal muscles move ribs, creating negative pressure in the thorax and expanding lungs.
• Aspiration Pump: Intercostal muscles and the diaphragm enlarge the thoracic cavity, expanding the lungs and pulling in air. The diaphragm is essential for this pump.
  • Exhalation occurs through compression of the thoracic cavity by intercostal, diaphragm, and sometimes abdominal muscles.

Sauropsid & Synapsid Lung Structure

• Sauropsids (Faveolar Lungs): Rigid, with tube-like faveolae lining the airways. Air moves unidirectionally over the faveolae.
• Synapsids (Alveolar Lungs): Compress and expand, with gas exchange in alveoli at the end of branching airways. Air moves in and out of the alveoli through the same passage.

Functions of the Circulatory System

• Enables internal respiration
• Distributes nutrients and hormones
• Distributes immune cells and antibodies
• Temperature regulation

Single vs. Dual Circulatory Systems

• Single: Blood passes through the heart once per complete circuit.
• Dual: Blood passes through the heart twice per complete circuit.

Heartbeat Impulse Origin in Amniotes

• Sinoatrial node (SA node)

Blood Flow Paths: Heart Chambers

• Two Chambers:

  • Atrium → Ventricle → Body → Gills → Atrium

• Three Chambers:

  • Right Atrium → Ventricle → Body → Right Atrium
  • Left Atrium → Lung → Left Atrium

• Four Chambers:

  • Right Atrium → Right Ventricle → Lung → Left Atrium → Left Ventricle → Body → Right Atrium

Turtle Heart: Not Inferior to Humans

• Turtles have a partially divided ventricle, leading to some mixing of oxygenated and deoxygenated blood.
• However, turtles can shift blood between the systemic and pulmonary circuits to meet immediate needs (e.g., warming or diving), allowing for separation of oxygenated and deoxygenated blood.

Explore the anatomical differences between tetrapods and sarcopterygians with this quiz. Discover unique features such as limb structure, muscle groups, and respiratory systems, as well as the specific adaptations of tetrapods. Test your knowledge on these fascinating vertebrate classifications.