Reptiles: Nonavian Diapsid Amniotes

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, water-conserving 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.

Figure 20.3 Amniote phylogeny.

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 skull characteristics.

Learning Outcomes: Section 20.2 Describe characteristics of the nonavian reptiles. Compare characteristics of members of the orders Testudines and Crocodylia. Justify the inclusion of superficially different snakes and lizards in a single order, Squamata.

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).

Learning Outcomes: Section 20.3 Describe the functions of the skin of reptiles. Compare the feeding mechanism of snakes to the feeding mechanisms of other reptiles. Compare the reproductive biology of crocodiles to reproduction by other reptiles.

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 tongue 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 Figure 20.15 Reptile eggs and young. The Madagascar day gecko (Phelsuma madagascariensis).

Figure 20. 16 Parental care in reptiles is not common Figure 20.16 Parental care in reptiles is not common. The American alligator (Alligator mississippiensis) is an exception.

Learning Outcomes: Section 20.4 Describe the evolutionary fate of the archosaur branch of the reptilian lineage. Describe the evolutionary fate of the synapsid branch of the amniote lineage.

Further Phylogenetic Considerations Archosaur branch Diverged 200 mya Branches Pterosaurs Dinosaur lineages including birds Synapsid branch Diverged 320 mya Mammals