Lesson 17: Amniotes

Amniota

• 5 toes

• evolved 312 million years ago via anthracosaurs

  • either to synapsids (mammals) or sauropsides (to reptiles/birds)

• synapomorphies 

  • amniotic egg

  • thicker and more waterproof skin 

  • rib ventilation of the lungs 

Early Split

• synapsids (mammals) and sauropsids (birds/reptiles)

• diverged very early in Amniote evolution

  • 312-306 million years ago

Amnion

• extraembryonic membrane that encloses a fluid-filled cavity 

  • i.e. the pond 

• where embryonic development occurs 

Amniotic Egg

• most amniota developed a shelled egg that did not depend on water

  • protective and semipermeable

  • allows gasses, limits water loss

• all amniotic eggs have 4 extraembryonic membranes 

  • chorion

  • allantois 

  • amnion 

  • yolk sac

Thicker and More Waterproof Skin

• mostly gas-proof, therefore cannot breathe through the skin

• keratinized scales, hair, feathers, claws

Rib Ventilation of the Lungs 

• anamniote

  • mouth breather

  • mouth and throat muscular contraction and expansion

  • pushing air into the lungs

  • positive pressure ventilation

• amniote

  • aspiration

  • rib and muscular contraction and expansion

  • sucking air into lungs

  • negative pressure ventilation

Jaws

• jaws efficiently designed for applying crushing or gripping force to prey 

• move from fast closure and suction feeding to large muscles to grip and chew 

• skulls provide holes for jaw musculature 

Anapsids 

• ancestral turtles 

• have a skull with no temporal opening behind the orbits 

Diapsids

• skull has two pairs of temporal openings

  • one pair below the cheeks and another above 

• gave rise to all other traditional reptiles and birds 

  • turtles appear anapsid, but secondarily lost diapsid condition 

Synapsids

• a single pair of temporal openings 

• mammal and mammal like reptiles 

4-chambered heart

• differs from the amphibian 3-chambered heart

• efficient and versatile for the circulatory system 

• incomplete ventricle separation in some reptiles is handy for hibernation in the winter or estivation in the summer 

Water Conservation Via Urea

• kidneys and urinary bladder are adapted for life on land

  • water is mostly reabsorbed 

  • mammals have the most efficient kidneys of the amniotes, as their urine is made of urea

  • reptiles and birds void their urine as a semi-solid paste of uric acid 

• some reptiles need to excrete salt via salt glands 

Reptile Salt Glands

1. nasal glands 

   • lizards and marine birds

   • under nasal passage or above orbit 

2. salivary gland

   • sea snakes

   • under tongue 

3. lacrimal gland

   • sea turtles 

   • tear ducts

4. lingual gland 

   • crocodiles

   • on tongue 

Reptile Nervous System

• ‘lizard brain’ allows for a bigger cerebrum and cerebellum

• better integration of sensory information and control of muscles

• some lizards and snakes can ‘see’ UV and infrared lights 

• lizards and snakes have a Jacobsen’s organ 

  • olfactory in the roof of mouth transmitted from the tongue 

Non-Avian Reptiles

• Diapsida

  • Lepidosauria

    • squamata

    • sphenodota

  • testudines

  • archosauria 

    • crocodilia

    • dinosaurs