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Chapter 26

Amniote Origins and

Reptilian Groups

Amniotic Amniotic Amniotic “““PondPondPond”””

Any animal with a shell-less egg remains tied to water.The development of a shelled egg freed the reptilian groups to exploit land.Extraembryonic membranes from previous evolutionary aquatic stages are maintained.Surrounding the whole young organism is a porous, parchment-like or leathery shell.

AmniotesAmniotesAmniotes

The Amniota is a single, monophyletic lineage of Paleozoic tetrapods with this developmental pattern.Before the end of the Paleozoic Era, amniotes diverged into reptilian groups, birds and mammals.

DiversityDiversityDiversity

Members of the paraphyletic class Reptiliainclude the first truly terrestrial vertebrates.Nearly 7000 species are described; about 300 occur in the United States and Canada.The Age of Reptiles lasted over 165 million years and included the dinosaurs.A mass extinction occurred at the end of the Mesozoic; modern reptiles represent surviving lineages.

HistoryHistoryHistoryAmniotes arose from amphibian-like tetrapods, the anthracosaurs, during the Carboniferous.By the late Carboniferous, amniotes had separated into three lineages.– Anapsids have a skull with no temporal opening behind the orbits; modern

turtles are anaspids.

– Diapsids gave rise to all other reptilian groups and to the birds.• The diapsid skull has two temporal openings; one pair below the cheeks

and another above.• Lepidosaurs include ichthyosaurs and modern reptiles except for turtles

and crocodilians. • More derived archosaurs included dinosaurs, living crocodilians, and birds.• Sauropterygians included extinct aquatic groups including the long-necked

plesiosaurs.– Synapsids are mammal-like reptiles with a single pair of temporal openings.

Fig. 26.2

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Changes in Traditional Classification of Reptilian GroupsChanges in Traditional Classification of Reptilian GroupsChanges in Traditional Classification of Reptilian Groups

Cladistic methodology insists on hierarchical arrangement of monophyletic groups.This disqualifies the traditional class Reptilia as a valid taxon because it is not monophyletic.Class Reptilia excludes birds, which descend from the most recent common ancestor of reptiles.This makes the class Reptilia a paraphyleticgroup because it does not include all descendants and their most recent common ancestor.

ClassificationClassificationClassification

Reptiles are therefore identified as amniotes that are not birds. Based solely on shared derived characteristics, crocodilians andbirds are sister groups belonging to a monophyletic group apart from other reptiles, the Archosauria. Some taxonomists defined Reptilia as the Archosauria plus the lepidosaurs, thus including birds.Evolutionary taxonomists argue that birds represent a novel adaptive zone and grade of organization; they maintain the classAves based on the morphological and ecological novelty of birds.Reptilian group” refers to members of four monophyletic groups formerly considered class Reptilia

Are Birds living Dinosaurs?Are Birds living Dinosaurs?Are Birds living Dinosaurs?

Reptiles and birds share several derived characters, including several skull characteristics and a largely aglandular skin with a special type of harder keratin called Beta keratin, which unites them as a monophyletic group.

Distinguishing Reptiles from AmphibiansDistinguishing Reptiles from AmphibiansDistinguishing Reptiles from Amphibians

SkinReptiles have a tough, dry, scaly skin that offers protection against desiccation and injury.The thin epidermis is shed periodically.A thicker, well-developed dermis underneath has chromatophores that provide color.

ScalesScalesScalesReptile scales are primarily of keratin, formed from

epidermis, and not homologous with fish scales.Scales may grow gradually to replace wear, as in alligators.Snakes and lizards replace old scales with new and “shed their skins.”

Other Skin AdaptationsOther Skin AdaptationsOther Skin Adaptations

Turtles add new layers underneath old layers of platelike scutes.Crocodiles and many lizards have bony plates called osteoderms underneath the keratinized scale.Resistance to desiccation is provided by hydrophobic lipids in the epidermis, as well as the presence of Beta keratin.

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Amniotes and the Amniotic Egg Amniotes and the Amniotic Egg Amniotes and the Amniotic Egg The amniotic egg contains four extraembryonic membranes including the amnion.– Embryos develop within the amnion, a membranous sac that

provides an aquatic environment and protects it from shock. – The yolk sac is the first membrane and pre-dates the amniotes

by millions of years– The allantois is a sac that grows out of the hindgut and holds

metabolic wastes during development.– The chorion lies beneath the eggshell and encloses the rest of

the embryo.

With growth, the allantois and chorion fuse to form the chorioallantoic membrane, a provisional “lung.”

Shelled Egg of ReptilesShelled Egg of ReptilesShelled Egg of Reptiles

A shelled egg contains food and protective membranes to support embryonic development on land.Reptiles lay eggs on land; young hatch as lung-breathing juveniles, not as aquatic larvae.The shelled egg widened the division between evolving amphibians and reptiles. More than any other adaptation, this contributed to the evolutionary establishment of reptiles.

Fig. 26.4

Internal FertilizationInternal FertilizationInternal Fertilization

A shelled egg requires internal fertilization; the sperm must reach the egg before it is enclosed.Paired testes produce sperm that are carried by vasa deferentia to the copulatory organ.The female has paired ovaries and oviducts.Glandular walls of the oviducts secrete albumin and shells for the relatively large eggs.

Circulatory System Modifications Circulatory System Modifications Circulatory System Modifications

Reptiles have an efficient circulatory system and higher blood pressure than amphibians.The right atrium that receives unoxygenated blood is completely partitioned from the left atrium.Crocodilians have separated ventricles dividing pulmonary and systemic circulation.Other reptiles have an incompletely separated ventricle, but little

mixture of blood occurs and the effect is two functionally separate circulations.The incomplete separation between the right and left sides of the heart in some groups provides an added benefit of permitting blood to bypass the lungs when pulmonary respiration is not occurring (e.g., during diving or aestivation).

LungsLungsLungs

Reptile lungs are better developed than those of amphibians.Reptiles depend almost exclusively on lungs, supplemented by pharyngeal membranes.Reptiles pull air into the lungs by enlarging the thoracic cavity. Snakes and lizards use expansion of the rib cage; turtles and crocodilians use movement of internal organs to create negative pressure to inhale.

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FeedingFeedingFeeding Reptilian Jaws– The jaws of fish

allowed fast jaw closure to seize food but little force for chewing.

– In reptiles, jaw muscles became larger and arranged for the mechanics of chewing.

Water ConservationWater ConservationWater Conservation

All amniotes have a metanephric kidney drained by the ureter.Nephrons of the reptilian metanephros lack the loop of Henle that allows concentration of solutes. Many reptiles have salt glands near the nose or eyes to secrete salty fluid hyperosmotic to body fluids.Nitrogenous wastes are excreted as uric acid rather than urea or ammonia.The amniotic egg of reptiles permits rapid development of large young in relatively dry environments.

Support of Limbs for LocomotionSupport of Limbs for LocomotionSupport of Limbs for Locomotion

Except for limbless members, all reptiles have better body support than amphibians.Many modern reptiles still walk with legs splayed outward and the belly close to the ground.Most dinosaurs and some modern lizards have more efficient legs directed beneath the body.

Fig. 26.5

Nervous SystemNervous SystemNervous System

The reptilian nervous system is considerably more complex than the amphibian.The reptile brain is still small but the cerebrum is relatively larger.Crocodilians have the first true cerebral cortex; it allows complex behaviors not seen in amphibians.Sense organs are generally well developed, except for hearing.The Jacobson’s organ is highly developed in lizards and snakes to detect odors carried by the tongue

Natural History of Reptilian OrdersNatural History of Reptilian OrdersNatural History of Reptilian Orders

Anapsid Reptiles: Subclass AnapsidaOrder Testudines includes the turtles.Turtles descended from one of the earliest anapsid lineages, probably in the late Permian. Fossils appear in the Upper Triassic, 200 million years ago, and they have occurred ever since.Turtles lack teeth and have tough, horny plates for gripping food.

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ShellsShellsShellsThe shells consist of the dorsal carapace and the ventral plastron.The shell has an outer horny layer of keratin and an inner layer of bone.The bony layer is a fusion of ribs, vertebrae, and dermally-ossified elements. Unique among vertebrates, the limbs and limb girdles are located deep to the ribs.

BreathingBreathingBreathingOne consequence of having a rigid shell is that the turtle cannot expand its chest to breathe.Turtles solve this problem by using abdominal and pectoral muscles as a “diaphragm.”Air is drawn in by contraction of the limb flank muscles, increasing abdominal cavity volume.Exhalation is accomplished by drawing back the shoulder girdle to compress the viscera.These actions are visible as bellows-like movements at the turtle’s “limb pockets.”Movement of limbs during walking also helps ventilate the lungs.Some aquatic turtles gain sufficient oxygen by pumping water in and out of the mouth cavity.

Nervous System and SensesNervous System and SensesNervous System and Senses

The turtle brain is small, less than one percent of its body weight.The cerebrum is larger than that of amphibians; some turtles can learn a path through a maze. Turtles have a middle and an inner ear but sound perception is poor; they make few sounds aside from those made during mating.Turtles have a good sense of smell, acute vision, and color perception about equal to humans.

Reproduction and DevelopmenReproduction and DevelopmenReproduction and Developmenttt

Turtles are oviparous; fertilization is internal and all turtles bury their eggs in the ground.Once the female lays her eggs in a nest, she deserts them. In some turtle families, as in crocodilians and some lizards, nest temperature determines the sex of hatchlings; low temperatures produce males and high temperatures produce females.

Order Order Order SquamataSquamataSquamata: Lizards, Snakes and Worm Lizards: Lizards, Snakes and Worm Lizards: Lizards, Snakes and Worm Lizards

Squamates are the most recent and diverse of diaspids; they make up 95% of living reptiles.Lizards appeared in the fossil record in the Permian but did not radiate until the Cretaceous.Snakes appeared in the late Cretaceous from a group whose descendants include monitor lizards.Snakes gained specializations for losing their legs and therefore for engulfing large prey.

Skull Skull Skull ModificationsModificationsModifications

Diaspid skulls have lost dermal bone ventral and posterior to the lower temporal opening. This allowed evolution in lizards of a mobile skull with movable joints, a kinetic skull.The quadrate, fused to the skull in other reptiles, has a joint at the dorsal end and articulates with the lower jaw.Joints in the palate and across the roof of the skull allow the snout to be tilted up. This allows squamates to seize and manipulate prey, and effectively close the jaw with force.Exceptional skull mobility of snakes is considered a major factor in their diversification.

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ViviparityViviparityViviparity

Viviparity in reptiles is limited to squamates.It has evolved at least 100 separate times.It is associated with cold climates.Viviparity involves increasing the length of time eggs are kept in the oviduct.Developing young respire through extraembryonic membranes.Young obtain nutrition from yolk sacs or via the mother, or a combination of both.

Suborder Suborder Suborder SauriaSauriaSauria: Lizards: Lizards: LizardsDiverse group with terrestrial, burrowing, aquatic, arboreal, and some aerial members.Some have degenerate limbs; the glass lizards are nearly limbless.Have movable eyelids whereas snakes have a transparent covering.Nocturnal geckos only rods; day-active lizards have both rods and cones.Have an external ear that snakes lack.Some survive well in hot and dry regions.– Conserve water by producing semisolid urine with high content of crystalline uric acid.– Some can store fat in their tails to provide energy and metabolic water during drought.

The Gila monster and beaded lizard are only lizards capable of avenomous bite.Lizards keep their body temperature relatively constant by behavioral thermoregulation, although they are ectotherms.Ectothermy is a successful strategy in ecosystems with low productivity and warm climates, such as tropical deserts and grasslands.

Suborder Suborder Suborder SerpentesSerpentesSerpentes: Snakes: Snakes: Snakes

Snakes are limbless and have lost the pectoral and pelvic girdles (except in pythons).The many vertebrae are shorter and wider than in other tetrapods, allowing undulation.Elevation of the neural spine gives the musculature more leverage.The feeding apparatus allows them to eat prey several times their own diameter.The two halves of the lower jaw are loosely joined, allowing them to spread apart.The skull bones also are loosely articulated so the mouth can accommodate a large prey.To keep breathing during the slow process of swallowing, the tracheal opening is extended.

Fig. 26.18a

Snake FeaturesSnake FeaturesSnake FeaturesEyeballs have reduced mobility and a permanent corneal membrane for protection.Most snakes have poor vision, but arboreal snakes in tropical forests have highly developed vision. Snakes lack external ears and do not respond to most aerial sounds.Snakes can feel vibrations at low frequencies, especially vibrations carried in the ground.Chemical senses rather than vision or hearing are the main senses snakes use to hunt prey.Jacobson’s organs are a pair of pits in the roof of the mouth; they are lined with olfactory epithelium and the forked tongue picks up scent particles and conveys them past this organ.A snake’s skin is infolded between scales; when stretched by a big meal, the skin is unfolded.

Snake LocomotionSnake LocomotionSnake Locomotion

Lateral undulation is the S-shaped movement that pushes against rough ground and water.

Concertina movement is extension of S-shaped loops to strike or to climb trees.

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Snake LocomotionSnake LocomotionSnake Locomotion

Rectilinear movement is a straight movement using minute lifting of consecutive ribs.Sidewinding is a sideways looping by desert vipers that “walks” them across loose sand.

Pit vipersPit vipersPit vipers

Have “pits” with nerve endings sensitive to heat emitted by warm-bodied birds and mammals.Viper fangs are hollow and hinged to inject venom when the snake strikes. Of an average of 8000 snake bites each year in the U.S., only about 12 result in death.Nonvenomous snakes kill prey by constriction or by biting and swallowing.

Snakebite and ToxicitySnakebite and ToxicitySnakebite and Toxicity

Saliva of harmless snakes contains limited toxins; this provided a basis for natural selection of venom.Most snake venoms are a complex combination of venom types.Neurotoxins act on the nervous system, causing blindness or stopping respiration. Hemorrhagin type venoms broke down blood vessels; much blood is leaked into tissue spaces.Toxicity is measured by the median lethal dose on laboratory animals, called the LD50.Sea snakes and the Australian tiger snake have the most deadly venom per unit.

Source

Snakebite and Snakebite and Snakebite and ToxicityToxicityToxicity

Large venomous snakes deliver more venom; a king cobra may be the most dangerous.In India and Burma, dense populations and poor footwear contribute to 200,000 bites per year.There are about 8,000 snakebites per year in the United States, resulting in only about 12 deaths.The world total for deaths from snakebite is about 50,000 to 60,000 each year, with most deaths occurring in India, Pakistan, Myanmar, and nearby countries where poorly shod people frequently come into contact with venomous snakes or do not get immediate medical attention once bit.Less than 20 percent of all snakes are venomous, although venomous species outnumber nonvenomous species by 4 to 1 in Australia. Source

ReproductionReproductionReproductionMost snakes are oviparous and lay shelled eggs under logs or rocks or in holes in the ground.Others, including pit vipers, are ovoviviparous.A few snakes are viviparous, having a primitive placenta to exchange nutrients with the young.Snakes can store sperm and lay several clutches of fertile eggs long after a single mating.

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Order Order Order CrocodiliaCrocodiliaCrocodilia: Crocodiles and Alligators: Crocodiles and Alligators: Crocodiles and Alligators

Modern crocodilians are the only surviving reptiles of the archosaurian lineage.This lineage gave rise to the Mesozoic radiation of dinosaurs and to birds.Modern crocodilians differ little from primitive crocodilians of the early Mesozoic.All have a long, well-reinforced skull and jaw musculature for a powerful bite.A type of dentition called thecodont, where teeth are set in sockets, was typical of archosaurs.They have a complete secondary palate, a feature only shared with mammals.

CrocodiliansCrocodiliansCrocodilians

Source

Source

ReproductionReproductionReproduction

Alligators and crocodiles are oviparous; usually 20-50 eggs are laid in a mass of vegetation.Nests left unguarded are easily discovered and raided by predators.High nest temperatures produce males; low temperatures produce females and can result in females outnumbering males five to one.

The End