Reptiles: Nonavian Diapsid
Amniotes
Learning Outcomes:
• Justify the statement that “the
amniotic egg provided solutions
that made development apart
from external watery
environments possible.”
• Compare amniote taxonomy
before and after the application of
cladistic methods.
Evolutionary Perspective
• Amniota
– Monophyletic lineage including
reptiles, birds, and mammals
– Amniotic eggs
• Extraembryonic membranes protect
embryo from desiccation; cushion
embryo; promote gas transfer, and store
waste.
– Impervious skin, nails, waterconserving kidneys, large lungs
Figure 20.2 The amniotic egg.
Cladistic Interpretation of
the Amniotic Lineage
• Mammals
– Most closely related to ancestral
amniotes
– Reptilian lineage
• Birds
• Dinosaurs
• Other reptiles
– Traditional class “Reptilia” is
paraphyletic.
Early Amniote Evolution and
Skull Structure
• Amniote lineages
– Synapsida
• Mammals
• Single opening (fenestra) in temporal region
of skull
– Anapsida
• Skull without fenestra
• Entirely extinct
– Diapsida
• Two fenestrae
• All living reptiles (including birds) and
numerous extinct lineages
Figure 20.4
Amniote kull
characteristics.
Survey of the Reptiles
• Order Testudines
– Turtles
– Bony shell, limbs articulate internally
to ribs, keratinized beak rather than
teeth
– Shell
• Carapace (dorsal) and plastron (ventral)
– oviparous
Figure 20.5 Skeleton of a
turtle.
Survey of the Reptiles
• Order Crocodylia
– Archosaur lineage
– Alligators, crocodiles, gavials,
caimans
– Openings in front of eyes, triangular
eye orbits, laterally compressed
teeth
– Secondary palate
– Oviparous with parental care
Survey of the Reptiles
• Order
Sphenodontida
– Tuataras
– Two rows of teeth
on upper jaw and
single row of
teeth in lower jaw
– New Zealand
– Oviparous
Figure 20.7 Order Sphenodontia
(Sphenodon punctatus).
Survey of the Reptiles
• Order Squamata
– Kinetic skull
• Moveable quadrate bones and other skull
modifications
• Increases skull flexibility
– Suborder Sauria—lizards
•
•
•
•
Usually two pairs of legs
Jaws unite anteriorly.
Oviparous, ovoviviparous, or viviparous
Includes legless amphisbaenias
– Suborder Serpentes—snakes
• Legless
• Skull adaptions for swallowing large prey
• Most oviparous
Figure 20.8 Order
Squamata (Heloderma
suspectum).
Figure 20.9 Order
Squamata, an
amphisbaenian “worm
lizard” (Amphisbaenia
alba).
Evolutionary Pressures
• External structure and locomotion
– Skin
• Thick, dry, and keratinized
• Scales may be modified.
• Epidermal layers shed through ecdysis.
– Support and movement
• Skeleton highly ossified
• Skull with secondary palate
• Additional cervical vertebrae (including atlas
and axis)
• Two or more sacral vertebrae
Figure 20.11 The secondary palate.
Evolutionary Pressures
• Nutrition and the digestive system
– Most carnivores
• Turtles (carnivores, herbivores, or
omnivores)
– Tongues
• Nonprotrusible
– Turtles and crocodilians
• Protrusibile
– Lizards and snakes
Figure 20.12 Order Squamata. A chameleon (Chameleo
chameleon) using its tonge to capture prey.
Evolutionary Pressures
• Nutrition and the digestive system
– Feeding adaptations of snakes
• Bones of upper jaw are moveable on
skull.
• Ligaments loosely join the halves of
upper and lower jaws anteriorly.
• Posterior pointing teeth
• Glottis opens forward in mouth.
• Hinged maxillary bone in vipers
• Venom glands in some
– Modified salivary glands
Figure 20.13 Feeding
adaptations of snakes.
(a) A copperhead
(Askistrodon) ingesting
prey. (b) The skull of a
viper. (c) Note the hinged
maxillary bone into which
the fang is embedded
swings forward when the
mouth opens.
Evolutionary Pressures
• Circulation, gas exchange, and
temperature regulation
– Heart
• Sinus venosus reduced to pacemaker
(except turtles)
• Two atria and incompletely divided ventricle
(except crocodilians)
– Permits shunting blood away from pulmonary
circuit during intermittent breathing
• Conus arteriosus and ventral aorta divided
embryologically into three major arteries
leaving heart
Figure 20.14 Heart and major arteries of a lizard.
Evolutionary Pressures
• Gas exchange
– Internal respiratory surfaces
• Lungs spongelike
– Large surface area for gas exchange
– Negative pressure ventilation
• Posterior movement of ribs and body
wall expands body cavity.
– Modified in turtles
• Decreases body cavity pressure and
draws air into lungs
Evolutionary Pressures
• Temperature Regulation
– Most ectotherms
• Exception brooding Indian pythons
• Behavioral regulation
– Orientation to sun’s rays
– Warming by conduction from warm surfaces
– Cooling by seeking shade or burrows, assuming erect
posture, and nocturnal habits
• Physiological regulation
– Panting
– Diverting blood to skin while basking (marine iguana)
• Torpor
– Temperate climates
– Enter hibernacula
» Not hibernation—body temperature is not
regulated.
Evolutionary Pressures
• Nervous and Sensory Functions
– Brain similar to other vertebrates
• Cerebral hemispheres, optic lobes, and
cerebellum enlarged
– Sensory functions
• Vision
– Dominant sense
– Accommodation by lens movement (snakes) or
changing lens shape (other reptiles)
– Upper and lower eyelids and nictitating
membrane
• Median (parietal eye)
– Outgrowth of roof of forebrain
• Hearing
– Airborne and substrate borne vibrations
Evolutionary Pressures
• Nervous and Sensory Functions
– Sensory functions
• Olfaction
– Jacobson’s (vomeronasal) organs
» Diapsids, especially squamates
• Temperature sense
– Pit organs
» Pit vipers
• Magnetic sense
– Sea turtles
Evolutionary Pressures
• Excretion and osmoregulation
– Metanephric kidneys
– Uric acid
– Internal respiratory surfaces reduce
water loss.
– Water conserving behaviors
• Nocturnal habits
• Avoidance by burrowing
– Water storage
• Lymphatic spaces
• Urinary bladder
Evolutionary Pressures
• Reproduction and development
– Internal fertilization and amniotic egg
makes development apart from external
water sources possible.
– Intromittent organ
• Hemipenes
– Parthenogesis
• A few lizards and snakes
– Courtship behaviors common
– Eggs usually develop unattended by a
parent.
• American alligator and a few others are
exceptions.
Figure 20.15 Reptile eggs and young. The
Madagascar day gecko (Phelsuma madagascariensis).
Figure 20.16 Parental care in reptiles is not
common. The American alligator (Alligator
mississippiensis) is an exception.
Further Phylogenetic
Considerations
• Archosaur branch
– Diverged 200 mya
– Branches
• Pterosaurs
• Dinosaur lineages including birds
• Synapsid branch
– Diverged 320 mya
– Mammals