Reptilian Amniotes

Animal Form and Function
Lizards, Snakes, Crocodiles, Turtles
Class Reptilia - Reptilian Amniota
Review Question: The Class Reptilia is not monophyletic. Why not?
Thought Question: Can we find apomorphies within the Amniota that unite
and distinguish ‘reptiles’ from mammals and birds? Why or why not?
Amniotes (including mammals and birds) united by several unique
characters (apomorphies within vertebrate clade)
1. Amniotic Egg
Thought Question: Amniote egg shells have pores for gas
exchange. Are they completely resistant to desiccation? How
could this problem be minimized?
Thought Question: The amnion is filled with nonnutritive fluid (amnionic fluid). What is the advantage of
suspending the embryo in fluid?
Thought Question: Most mammals (Eutherians) do not lay eggs.
Why are they placed in the Amniota?
2. Internal Fertilization
Thought Question: Males of most aquatic animals externally
fertilize eggs. Why is external fertilization impossible for
Thought Question: What morphological feature do male amniotes
possess that non-amniote vertebrates lack?
Thought Question: With the exception of the Anseriformes (what
were these again?), male birds also lack this feature. Do birds
internally fertilize? How???
2. Loss of Post-Embryonic Gills & Aquatic Larval Stage
- young emerge from eggs breathing with lungs
- young are morphologically similar to adults
Thought Question: What does this imply about the diet and
habitat of adults and young?
Review Question: Remember that pharyngeal pouches (a key
component in the development of gills) are a pleisiomorphy in the
Chordata. Are amniotes chordates? Why or why not?
3. Complete Terrestrial Life-Cycle = Dry Skin
- thin epidermis = scales (made of keratin )
Review Question: Birds have homologous scales on the feet,
but most mammals have no scales. What structures do
mammals and birds have that are homologous to scales? (hint:
must be derived from epidermis and composed of keratin)
Thought Question: The structures in the previous question
appear morphologically homologous. How can we demonstrate
that they truly are derived from a common ancestor? (hint: we
have only been able to do this for about 20-30 years)
Thought Question: Some mammals still have scales on part of
the body. Can you think of any?
- thick dermis
Thought Question: It is the thick dermis of amniotes that
protects them from desiccation. How have humans used other
amniote animals to take advantage of this adaptation? Why
don’t we use non-amniotes like fish or amphibians?
3. Metanephric Post-Embryonic Kidneys
Review Question: How does excretion of uric acid minimize
water loss in reptiles? What was the primary benefit of uric
acid excretion in birds?
4. 12 Cranial Nerves
5. Axis
Thought Question: Can a frog turn its head without moving its body?
6. Inhale by ‘sucking’
- snakes/lizards/birds = ?
- crocodiles/turtles = ?
- mammals = ?
7. High Blood Pressure + Separate Pulmonary/Systemic Circulation
Thought Question - Why would organisms that walk on land require
relatively high pressure in the circulatory system? (hint: gravity
and relative density are important here)
Reptiles have a heart that is functionally four-chambered.
Thought Question: Why is a four-chambered system more efficient
than a three chambered system? Why would reptiles need a more
efficient circulatory system than non-amniote vertebrates? (hint:
the answer is NOT that reptiles are endothermic homeotherms most are poikilothermic and at least partially ectothermic).
Thought Question: Turtles often spend long periods of time under
water. During these periods, special muscles constrict the
pulmonary arteries and veins. Because the heart is threechambered, blood is pumped almost exclusively to the systemic
circuit. How is this adaptive?
Thought Question: Can whales do this? Why or why not?
Fossil Record
Anthracosaurs - 350 mya
Anthracosaurs give rise to three lineages (300 mya)
1. Anapsida
2. Synapsida
3. Diapsida
a. Lepidosauria - snakes/lizards
b. Archosauria - crocodiles, birds
Reptilian Diversity
Thought Question: We have only discussed reptilian characters that are
pleisiomorphic for the amniote clade. Are reptiles ‘not as evolved’ as
mammals and reptiles?
Subclass Anapsida
Order Testudines
Turtles are united by apomorphies within the amniote clade
Thought Question: Many species with temperature dependent sex
determination. Why?
Thought Question: Turtles have unusually long life spans (some
reach more than 100 years). Why does this make them vulnerable to
Subclass Diapsida
Order Squamata
- kinetic skull
+quadrate bone
Suborder Sauria - lizards (geckos, iguanas, skinks, chameleons)
chameleon tongue - biological rifle?
Suborder Serpentes - Snakes
Thought Question: Some snakes have vestigial pelvic girdles. What
does this indicate about their ancestry?
Thought Question: How could a loss of limbs be adaptive?
Thought Question: Being a limbless predator presents certain
problems. What are they? How have snakes solved these problems?
Thought Question: Male lizards and snakes have two intromittent
structures (hemipenes). Why? (hint: mammals and lizards walk
Order Rhynchocephalia - one extant species - Tuatara of New Zealand
Thought Question: Many other reptiles have ‘parietal eyes’ that do
not have a lens or retina (hence cannot form an image). What could
such structures be used for?
Order Crocodilia - crocodiles, alligators, caimans
Thought Question: Crocodiles often swallow rocks. Why?
- archosaurian secondary palate (homologous to avian 2nd palate)