Vertebrate Zoology Exam 2 - Amniotes

Yolk

• Energy supply

• In yolk sac (extraembryonic membrane, outside of embryo’s body)

• Yolk sac is made of mesoderm and endoderm

Amnion

• Forms folds of ectoderm + mesoderm

• Formed by mesoderm on the outside and ectoderm on the inside

• Amniotic cavity, surrounds to protect embryo from mechanical shocks, similar to mammals’ amniotic fluid (cushions embryo)

Chorion

• Forms folds of ectoderm + mesoderm

• Chorion has mesoderm on inside (pink layer)

• Encloses other extraembryonic membranes + contacts inside surface of eggshell

• Also works with allantois to form membrane for gas exchange (chorioallantoic membrane)

Chorioallantoic Membrane

• Chorion + allantois work together to make a surface for gas exchange

Allantois

• Inside = endoderm, outside = mesoderm

2 Functions

1. Stores nitrogenous wastes

2. Expands to contact the chorion to form + vascularize the chorioallantoic membrane

What was the ancestral amniote egg likely like?

Flexible, leathery eggshell

The Composition of Waterproof Skin

• Multilayered keratinous epidermis

• Thick dermis

• Hydrophobic lipids in skin limit water loss

Keratin

Forms scales, hair, feathers, nails, beaks, and horns

(*Scales, hair, and feathers are homologous structures)

Alpha Keratin

Contained in keratinized skin of extant amniotes

Beta Keratin

Second, harder type of keratin not found in mammals but found in extant sauropsids

Metanephric Kidney

• Derived character of amniotes

• Drained by a duct called the ureter

Responsibilities:

1. Maintains homeostasis of body fluids

2. Regulates extracellular fluid volume, osmolality, ion concentrations, and pH of blood

3. Gets rid of nitrogenous wastes

Costal Ventilation

• Moving air into and out of lungs using movements of the ribs

• A character of amniotes based on phylogenetic inference and anatomy of fossils

Derived Features of Amniotes + Functional Significance

1. Lateral flange of the pterygoid bone - jaw adductor muscle division for diff. modes of processing food

2. Second cervical vertebra modified into an axis - greater mobility of head on neck, allow schanges in feeding + social behavior

3. Astragalus bone and mesotarsal joint - change in ankle allows more limb-based (rather than axial-based) locomotion

4. More than one sacral vertebra (base of spine) - reflects increasing use of hindlimbs in limb-based locomotion

5. Amniotic egg - no need to return to water for reproduction

6. Waterproof skin - limits water loss and greater independence from water

7. Penis - internal fertilization, essential for amniote eggs

8. Metanephric kidney - improved water retention

9. Costal aspiration - higher metabolic rates and loss of CO₂ via lungs rather than skin

10. Trachea with cartilaginous rings - longer neck, changing feeding + social behavior

Function of: Lateral flange of the pterygoid bone

• Lateral flange is the origin for the division of jaw adductor muscles into adductor mandibulae and pterygoideus muscle (jaw-closing muscle)

• Plays many roles in the feeding systems of amniotes

Function of: Second cervical vertebra modified into an axis

Greater mobility of head on neck

• Allows changes in feeding and social behavior

Function of: Astragalus bone and mesotarsal joint

• 3 small proximal tarsal bones fuse to form the astragalus

• Mesotarsal joint is the plane of bending

• Allows foot to be used like lever than holdfast

• Indicates more limb-based locomotion (role of the hindlimbs)

Function of: More than one sacral vertebra (base of spine)

• Reflects increasing use of hindlimbs in limb-based locomotion

Function of: Amniotic egg

• Eggshell and four extraembryonic membranes (yolk, amnion, chorion, & allantois) allows independence from water for reproduction

Function of: Waterproof skin

• Limits water loss by evaporation

• Allows greater independence from water

Function of: Penis

• Used in internal fertilization, which is essential for amniote eggs

Function of: Metanephric kidney

• Contributes to improved water retention

Function of: Costal aspiration

Breathing with ribs

• Higher metabolic rates

• Allows loss/release of CO₂ via lungs rather than skin

Function of: Trachea with cartilaginous rings

• Longer trachea = longer neck

• Changes in feeding and social behavior

What did the ankle of basal tetrapods look like? How did it function?

• No distinct ankle joint (mesotarsal joint)

• Axial muscles power locomotion

• Feet functioning mainly as pivot points around which the hindlimb rotates

What is the ancestral conflict between locomotion and respiration? (i.e. what 2 things are “required” if both must occur simultaneously?)

1. Locomotion - bending the trunk unilaterally from side to side for locomotion

2. Respiration - compressing the rib cage bilaterally to ventilate the lungs

Can’t run and ventilate lungs at same time

How do Synapsids overcome the conflict between locomotion and respiration?

Spine moves up-down (dorsoventral flexion), avoids conflict with lung ventilation and actually assists lung movement

Why are some lizards (basal amniotes) limited to short bursts of activity?

They retain ancestral modes of locomotion and ventilation, can only rely on short dashes

• As ATP and creatine phosphate are used up, muscles switch to anaerobic metabolism

• If dash is too long, can no longer use phosphates OR anaerobic metabolism, they need oxygen in order to be replenished

Adductor

Adds/in/TOWARD body

Abductor

Away from body

What type of airflow do Synapsids have?

Bidirectional/tidal airflow (in-and-out)

Diaphragm

Muscularization of the postpulmonary septum

• In extant mammals

• Helps move lungs without conflicting with locomotion

Postpulmonary Septum

Sheet of connective tissue in the coelom that separates lungs from digestive organs and forms separate pleural (lungs) and peritoneal (abdominal) cavities

Synapsid VS. Diapsid Temporal Fenestrae

Synapsid - ONE temporal fenestra on each side of head (lower only)

Diapsid - TWO temporal fenestra pairs (lower and upper)

Skull hole (temporal fenestrae) functions

• Stronger bite forces

• Compared to reptiliomorph, simple jaw-closing muscles beneath skull roof

Synapsid VS. Sauropsid Lungs

Synapsid - alveolar, tidal ventilation (compliant)

Sauropsid - faveolar, undirectional ventilation (rigid, noncompliant)

Gastralia

“Free-floating” bones in the ventral abdominal wall (basal amniotes)

Mobile gastralia

Muscle on pelvis and inserts on the gastralia

• Contracts, pulling gastralia, increasing volume of abdominal and pulmonary cavities and produces inhalation

Air sacs

Located in bones anterior and posterior to lungs

• Retained in birds (and some crocodylians)

Extended Sternum

Replaces gastralia in birds as a method of ventilating the lungs

• Moves, pivots, and helps drive air into air sacs

Immobile lungs

• In some derived sauropsids (like pterosaurs)

• Only ventral portions are compliant

Uncinate processes

Muscles attached to ribs to facilitate movement of ribs and sternum for respiration

Gular Pumping

Monitor Lizards

• Ventilation by drawing air into gular region through the nares (expands and contrats entire throat)

(Image: Asian Water Monitor)

Muscular Sling

Turtles

• Can’t move ribs or ventral surfaces of bodies (bc of shell) to change volume of lungs, instead uses a sling created by abdominal muscles

Pelvic Ventilation

Crocodylomorphs

• Derived character

• Rotate pubic bones, increasing volume of abdominal cavity for lungs to expand

Hepatic piston

Extant crocodylians

• Moves liver to help ventilate lungs

Describe avian respiration.

• “Two- breath” model - always at least one breath in each reservoir

• Parabronchial lungs

• Unidirectional flow, cross-current exchange system (air and blood pass in opposite directions)

• Thin capillaries for easier oxygen diffusion

Air Capillaries

Gas-exchange structures of bird lungs, millions of interconnected small tubules radiating from the parabronchial lungs

Downside of alveolar lung:

On exhale, there is a moment where we are not getting oxygen

Why is high blood pressure bad?

Can damage the delicate, thin lung tissue

Why did amniotes evolve separate pressure systems for circulation? What are those 2 systems?

Prevent high blood pressure causing damage to the delicate lung

• Systemic (body) pressure is ALWAYS higher than pulmonary (lung) pressure because of lungs’ thin tissue

What direction do arteries flow blood?

Away from heart

What direction do veins flow blood?

Toward heart

What 2 lineages independently reduced to a single systemic arch? Which arch is retained in each lineage?

Arch 4 develops into the systemic aorta

1. Mammals - retains left arch

2. Birds - retains right arch

Which 2 lineages have a ventricular septum in their hearts?

Mammals and birds

Ventricular septum

A permanent septum separating the ventricle into systemic and pulmonary sides

• Allows different blood pressures in each circuit

What is the order of blood flow in hearts with a permanent septum?

(Mammals and bird hearts)

STARTS in the Right (pulmonary), deoxygenated blood

1. Inferior vena cava

2. Right atrium

3. Right ventricle

4. To pulmonary artery, going to the lungs

Now to Left (systemic), oxygenated blood

5. From lung, to left atrium

6. Left ventricle

7. To rest of body through the systemic aorta

What is the order of blood flow in hearts without a ventricular septum?

(Turtles and lepidosaurs)

Right (Pulmonary), deoxygenated

1. Starts in right atrium

2. Flows into cavum venosum

3. Over the muscular ridge to cavum pulmonale

4. To pulmonary artery

Left (Systemic), oxygenated

5. Left atrium

6. Cavum arteriosum

7. Leaves through aortas

What groups can perform shunting of blood?

Turtles, Lepidosaurs, & Crocodylians

How does a right to left shunt benefit lizards, turtles, and crocodylians?

A pulmonary to systemic shunt allows a higher rate of blood flow and allows for quicker warming of the body

Foramen of Panizza

Hole allowing flow to both aortas from the left side of Crocodylian heart