AP Biology Notes

AP Biology
Notes: Chordata II

Class Amphibia: Salamanders, frogs, and caecilians

Today the amphibians (class Amphibia) are represented by about 4,800 species:

Salamanders (order Urodela, “tailed ones”)

· Some of the 500 species of urodeles are entirely aquatic, but others live on land as
adults or throughout life. Most salamanders walk with a side-to-side bending of the
body that may resemble the swagger of the early terrestrial tetrapods.

Frogs (order Anura, tail-less ones”)

        ·         The 4,200 species of anurans are more specialized than urodeles for moving
        on land. Adult frogs use powerful legs to hop along the terrain. Frogs nab insects
        by flicking out their sticky tongues. Among adaptations that reduce predation,
        anurans may be camouflaged or secrete a distasteful, even poisonous, mucus
        from skin glands. Many poisonous species are also brightly colored, perhaps to
         warn predators who associate the coloration with danger.

Caecilians (order Apoda, “legless ones”)
·         Apodans, the caecilians (about 150 species), are legless and nearly blind. The reduction
        of legs evolved secondarily from a legged ancestor.    Superficially resembling earthworms,
        most species burrow in moist forest soil in the tropics. A few South American species live
        in freshwater ponds and streams.

Amphibian means “two lives,” a reference to the metamorphosis of many frogs from an aquatic
stage, the tadpole, to the terrestrial adult. Tadpoles are usually aquatic herbivores with gills and
a lateral line system, and they swim by undulating their tails. During metamorphosis, the tadpole
develops legs, the lateral line disappears, and gills are replaced by lungs Adult frogs are carnivorous
hunters. Many amphibians do not live a dualistic—aquatic and terrestrial—life

Most amphibians retain close ties with water and are most abundant in damp habitats. Those adapted to
drier habitats spend much of their time in burrows or under moist leaves where the humidity is higher.
Most amphibians rely heavily on their moist skin to carry out gas exchange with the environment. Some
terrestrial species lack lungs entirely and breathe exclusively through their skin and oral cavity.

Amphibian eggs lack a shell and dehydrate quickly in dry air. Most species have external
fertilization, with eggs shed in ponds or swamps or at least in moist environments. Some species
lay vast numbers of eggs in temporary pools where mortality is high. In contrast, others display
various types of parental care and lay relatively few eggs. In some species, males or females
may house eggs on their back, in the mouth, or even in the stomach. Some species are ovoviviparous
or viviparous, retaining the developing eggs in the female reproductive tract until released as juveniles.

For the past 25 years, zoologists have been documenting a rapid and alarming decline in amphibian
populations throughout the world. Several causes that have been proposed include environmental
degradation (especially acid rain) and the spread of a pathogen, a chytrid fungus.

Amniotes

The amniote clade consists of the mammals, the birds, and the vertebrates commonly
called reptiles, including turtles,lizards, snakes, and crocodiles. The evolution of amniotes from an
amphibian ancestor involved many adaptations for terrestrial living including:

The amniotic egg.
Waterproof skin.
Increasing use of the rib cage to ventilate the lungs.

The amniotic eggs enabled terrestrial vertebrates to complete their life cycles entirely on land.

In contrast to the shell-less eggs of amphibians, the amniotic eggs of most
amniotes have a shell that retains water and can be laid in a dry place.
The calcareous shells of bird eggs are inflexible, while the leathery eggs of
many reptiles are flexible.
Most mammals have dispensed with the shell.

· Inside the shell of the amniotic egg are several extraembryonic membranes that function in gas
exchange, waste storage, and the transfer of stored nutrients to the embryo. These develop from
tissues layers that grow out from the embryo.

· An evolutionary radiation of amniotes during the early Mesozoic era gave rise
to three main groups, called synapsids, anapsids, and diapsids. These names
are based on key differences in skull anatomy.

· The synapsids included mammal-like reptiles, the therapsids, from which mammals evolved.

· The anaspid lineage is probably extinct.

· The diapsids include most or all groups of modern reptiles (depending on the placement of turtles), as well as a diversity of extinct swimming, flying, and land-based reptiles.

Reptile

Reptiles have several adaptations for terrestrial life not generally found in amphibians.

Scales containing the protein keratin waterproof the skin, preventing dehydration in dry air.
Reptiles obtain all their oxygen with lungs, not through their dry skin. As an exception,
many turtles can use the moist surfaces of their cloaca for gas exchange.

Most reptiles lay shelled amniotic eggs on land. Fertilization occurs internally, before the
shell is secreted as the egg passes through the female’s reproductive tract. Some species
of lizards and snakes are viviparous, their extra embryonic membranes forming a placenta
that enables the embryo to obtain nutrients from its mother.

Reptiles, sometimes labeled “cold-blooded,” do not use their metabolism extensively to control body temperature. However, many reptiles regulate their body temperature behaviorally by basking in the sun when cool and seeking shade when hot. Because they absorb external heat rather than generating much of their own, reptiles are more appropriately called ectotherms. One advantage of this strategy is that a reptile can survive on less than 10% of the calories required by a mammal of equivalent size.

There are about 6,500 species of extant reptiles, classified into four groups:

1) Testudines (turtles)

· Turtles evolved in the Mesozoic era and have scarcely changed since. The usually hard shell is an
adaptation that protects against predators. Those turtles that returned to water during their evolution
crawl ashore to lay their eggs.

2) Sphenodontia (tuataras)

· The two recognized species of tuatara (Sphenodon punctatus and Sphenodon guntheri) are
found on approximately 30 small, relatively inaccesible, islands off the coast of New Zealand.
The species was once widely distributed throughout New Zealand, but became extinct on the
mainland before the arrival of European settlers.

3) Squamata (lizards and snakes)

· Lizards are the most numerous and diverse reptiles alive today. Most are relatively small. Modern lizards nest
in crevices and decrease their activity during cold periods, a strategy that may have enabled them to survive
the Cretaceous “crunch.”

Snakes are probably descendents of lizards that adapted to a burrowing lifestyle through the loss of
limbs. This limbless condition remains today, even though most snakes live above ground. Vestigial pelvic
and limb bones in primitive snakes such as boas, is evidence that snakes evolved from reptiles with legs.
Snakes are carnivorous and a number of adaptations aid them in hunting and eating prey. Snakes have acute
chemical sensors and are sensitive to ground vibrations. The flicking tongue fans odors toward olfactory
organs on the roof of the mouth Heat-detecting organs of pit vipers, including rattlesnakes, enable
these night hunters to locate warm animals. Some poisonous snakes inject their venom through
a pair of sharp hollow or grooved teeth. Loosely articulated jaws enable most snakes to swallow
prey larger than the diameter of the snake itself.

4) Crocodilia (alligators and crocodiles).

· Crocodiles and alligators (crocodilians) are among the largest living reptiles. They spend most of their
time in water, breathing air through upturned nostrils. Crocodilians are confined to the tropics and
subtropics.

* In the traditional classification, these are orders within the class Reptilia. In one alternative classification,
each is a class.

Birds

In addition to amniotic eggs and scales, modern birds have feathers and other distinctive flight equipment.
Almost every part of a typical bird’s anatomy is modified in some way to enhance flight. One adaptation to
reduce weight is the absence of some organs. For instance, females have only one ovary. Modern birds are
toothless and grind their food in a muscular gizzard near the stomach. The skeletons of birds have several
adaptations that make them light and flexible, but strong. The bones are honeycombed to reduce weight
without sacrificing much strength.

· Flying requires a great expenditure of energy from an active metabolism. Birds are endothermic, using their
own metabolic heat to maintain a constant body temperature. Feathers and, in some species, a layer of fat
provide insulation. Efficient respiratory and circulatory systems with a four-chambered heart keep tissues
well supplied with oxygen and nutrients. The lungs have tiny tubes leading to and from elastic air sacs that
help dissipate heat and reduce body density.

· The large brains of birds (proportionately larger than those of reptiles or amphibians) support very complex behavior. During the breeding season, birds engage in elaborate courtship rituals. This culminates in copulation,
contact between the mates’ vents, the openings to their cloacas. After eggs are laid, the avian embryo is kept
warm through brooding by the mother, father, or both, depending on the species. The most obvious adaptations
for flight are wings. Wings are airfoils that illustrate the same principles of aerodynamics as airplane wings.

· Feathers are among the most remarkable of vertebrate adaptations They are both extremely light and strong.
Feathers are made of the protein keratin, the same material in reptile scales and mammalian hair and nails.
Feathers may have functioned first as insulation during the evolution of endothermy and were later co-opted
as flight equipment. Birds have downy feathers and contour feathers. The downy feathers of birds lack hooks
on barbules, producing a fluffiness that provides excellent insulation because of the trapped air. Contour feathers
are the stiff ones that contribute to the aerodynamic shapes of the wing and body.

There are about 8,600 extant species of birds classified in about 28 orders.

Ratites, which lack both a breastbone and large pectoral muscles. The ratites include
the ostrich, kiwi, and emu.

Carinates because they have a carina, or sternal keel, which anchor the large
pectoral muscles. Carinate birds exhibit a great variety of feather colors,
beak and foot shapes, behaviors, and flying styles. Nearly 60% of living
bird species are in the order passeriformes, or perching birds.

Mammals:

There are about 4,500 extant species of mammals. Vertebrates of the class Mammalia were first defined
by Linnaeus by the presence of mammary glands. All mammalian mothers nourish their babies with milk,
a balanced diet rich in fats, sugars, proteins, minerals, and vitamins, produced in the mammary glands.
All mammals also have hair, made of the keratin Hair and a layer of fat under the skin retain metabolic heat,
contributing to endothermy in mammals. Endothermy is supported by an active metabolism, made possible
by efficient respiration and circulation. Adaptations include a muscular diaphragm and a four-chambered heart.

        Fertilization is internal, and the embryo develops in the mother’s uterus. In eutherian (placental)
        mammals and marsupials the lining of the uterus and extraembryonic membranes collectively form
        a placenta, where nutrients diffuse into the embryo’s blood.

Modern mammals are split into three groups:

1) Monotremes—the playtpuses and the echidnas

    ·            The only living mammals that lay eggs. The reptile-like egg contains enough yolk to nourish
         the developing embryo. Monotremes have hair and females produce milk in specialized glands.
        After hatching, the baby sucks milk from the mother’s fur because they lack nipples.

2) Marsupials include opossums, kangaroos, bandicoots, and koalas.

        ·         A marsupial is born very early in development and in most species completes its embryonic
        development while nursing within a maternal pouch, the marsupium. In most species, the tiny
        offspring climbs from the exit of the female’s reproductive tract to the mother’s pouch. In
        Australia, marsupials have radiated and filled niches occupied by eutherian mammals in
        other parts of the world.

3) eutherian mammals (placentals)

Young eutherians complete their embryonic development within the uterus,
joined to the mother by the placenta

The current hypothesis for the evolutionary relationships among eutherian orders, based on
molecular systematics, clusters them into four main clades.

Clade Afrotheria includes elephants, aardvarks, hyraxes, and manatees.

Clade Edentata is composed of sloths, anteaters, and armadillos, all from South America.

Third clade includes
                 The bats (Chiroptera),
                 The “core insectivores” (such as shrews and moles),
                 The carnivores, artiodactyls (pigs, cows, camels, and hippos) and
                 The perissodactyls (horses and rhinoceroses),
                 The cetaceans ( Whales and dolphins).

Fourth clade (and largest) Include: Lagomorphs and the primates