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The vertebrates comprise a large
group of chordates, and are subdivided into seven classes (3 classes of fish,
amphibians, reptiles, birds, and mammals).
Vertebrates
have an internal skeleton of cartilage or bone, with vertebrae surrounding the
dorsal nerve cord.
The subphylum Vertebrata
consists of about 43,700 species of animals with backbones. Vertebrates exhibit
all three of the chordate characteristics at some point during their lives. The
embryonic notochord is replaced by a vertebral column in the adult. The
vertebral column is made of individual hard segments (vertebrae) surrounding the
dorsal hollow nerve cord. The nerve cord is the one chordate feature present in
the adult phase of all vertebrates. The vertebral column, part of a flexible but
strong endoskeleton, is evidence that vertebrates are segmented. The vertebrate
skeleton is living tissue (either cartilage or bone) that grows as the animal
grows.
The endoskeleton and muscles
form an organ system that permits
rapid and efficient movement. The pectoral and pelvic fins of fishes evolved
into jointed appendages that allowed vertebrates to move onto land. The skull,
the most anterior component of the main axis of the vertebrate endoskeleton,
encases the brain. The high degree of cephalization in vertebrates is
accompanied by complex sense organs concentrated in the head region. Eyes
developed as outgrowths of the brain. Ears were equilibrium devices in aquatic
vertebrates that function as sound-wave receivers in land vertebrates.
Vertebrates have a complete digestive system and a large coelom. Their
circulatory system is closed, with respiratory pigments contained within blood
vessels. Gas exchange is efficiently accomplished by gills, lungs, and in a few
cases, moist skin. Kidneys are efficient in excretion of nitrogenous waste and
regulation of water. Reproduction is usually sexual with separate sexes.
The first vertebrates were
fishlike. Fishes are aquatic, gill-breathing vertebrates that usually have fins
and skin covered with scales. The larval form of a modern-day lamprey, which
looks like a lancelet, may resemble the first vertebrates: it has the three
chordate characteristics (like the tunicate larva), as well as a two-chambered
heart, a three-part brain, and other internal organs that are like those of
vertebrates.
Small, jawless, and finless
ostracoderms were the earliest vertebrates. They were filter feeders, but
probably were also able to move water through their gills by muscular action.
Ostracoderms have been found as fossils from the Cambrian through Devonian
periods, when the group finally went extinct. Although extant jawless fishes
lack protection, many early jawless fishes had large defensive head shields.
These long, eel-like, jawless
fish are free-swimming predators on other fish. Lampreys hatch in freshwater and
many live their lives entirely in freshwater. Some lampreys migrate to the sea,
but must return to freshwater to reproduce. Lampreys have a sucker-like mouth
that lacks a jaw.
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Sea lamprey mouth
Members
of the class Myxini have a partial cranium (skull), but no vertebrae. Their
skeleton is made of of cartilage, as is that of sharks. Hagfish lack jaws, and
for this reason used to be classified with the lampreys in a group called the
Agnatha ("no jaws") or the Cyclostomata ("round mouth").
Hagfish
The fish first appeared during
the Cambrian Period. Whether fish first evolved in fresh or salt water is
unclear from the fossil record.
The
jawless fish are the most primitive group, although they were a very important
group during the Silurian and Devonian periods. Hagfish
and lampreys are the only living members of this class today. They have long, cylindrical
bodies with cartilage skeletons and no paired fins.
The first jawed fish were the
Placoderms, an extinct group of Devonian-aged jawed fishes. Placoderms were
armored with heavy plates and had strong jaws and paired pectoral and pelvic
fins. Paired fins allow fish to balance and to maneuver well in water, which
facilitate both predation and escape.
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The
fossil is a cast of the placoderm, Bothriolepis
.
The evolution of jaws is an
example of evolutionary modification of existing structures to perform new
functions. Jaws are modified gill arches,
and allowed the exploitation of new roles in the habitats: predators with
powerful jaws. There
are two classes of jawed fish: the cartilaginous fish and the bony fish.
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The class Chondrichthyes
contains approximately 850 species
of
skates, rays, and sharks. They
have jaws, lots of teeth, paired fins, and a cartilage endoskeleton. Cartilaginous fish first
appeared during the Devonian Period and expanded in diversity during the
Carboniferous and Permian before nearly disappearing during the great extinction
that occurred near the end of the Permian. A
large group of cartilagenous fish still survives today and is an important part
of the marine fauna.
These fish have five to seven
gill slits on both sides of the pharynx, and lack the gill covers found in bony
fish. The chondrichthyian body is covered epidermal placoid (or toothlike)
scales. Developmental studies show the teeth of sharks are enlarged scales.
The largest sharks are filter
feeders, not the predators of Hollywood movies. Basking and whale sharks eat
tons of crustaceans (small krills, etc.) filtered from the water. Most sharks
are fast-swimming, open-sea predators. The great white shark feeds on dolphins,
sea lions and seals (and people sometimes). In other words, anything is WANTS
to!
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Shark
and Ray
Rays and skates live on the
ocean floor; their pectoral fins are enlarged into winglike fins; they swim
slowly. Stingrays have a venomous spine. The electric ray family can feed on
fish that have been stunned with electric shock of over 300 volts. Sawfish rays
have a large anterior "saw" that they use to slash through schools of
fish.
There are about 20,000 species
of bony fish, found both in marine and freshwater, comprising the class
Osteichthyes. This class is divided into two
groups: the lobe-finned (Sarcopterygii) and ray-finned fish (Actinopterygii).
The bony fish have a bony skeleton. Most species in this class are ray-finned
with thin, bony rays supporting the fins. A few fishes are lobe-finned and are
thought to be related to the ancestors of amphibians.
Cross
section of a fish
Ray-finned Fish
(Actinopterygii)
The ray-finned fish include
familiar species such as tuna, bass, perch, and trout.
Ray-finned fish are the most successful and diverse
of the vertebrates (more than half of all vertebrate species belong to this
group). Thin, bony supports with radiating bones (hence the term ray-finned)
hold the fins away from the body. Ray-finned fish obtain their food by filter
feeding and by preying on insects and other animals. Their skin is covered by
scales formed of bone. These scales are homologous to our own hair (and the
feathers of birds), being derived from the same embryonic tissues. The gills in
this group of fish do not open separately and are covered by an operculum.
Ray-finned fish have a swim bladder, a gas-filled sac, that regulates buoyancy
and depth. Sharks lack this feature, which enables fish to "sleep"
without sinking. The swim bladder acts much the way a ballast tank does on a
submarine to control buoyancy.
Salmon, trout, and eels can
migrate from fresh water to salt water, but must adjust kidney and gill function
to the tonicity of their environments. In freshwater, the fish is hypotonic
relative to its aqueous (watery) environment. Water is constantly flooding into
the fish, and must be removed by the fish's excretory system. In seawater, the
fish is now hypertonic or isotonic relative to the seawater, requiring
conservation of body water.
Bony fishes depend on color
vision to detect both rivals and mates. Sperm and eggs are released into the
water, with not much parental care for the newborn. Most fish have fertilization
and embryonic development taking place outside the female's body.
This group includes six species
of lungfishes and one species of coelacanth that has muscular fins with large,
jointed bones attaching the fins to the body. Lobe-finned fish have fleshy fins
supported by central bones, homologous to the bones in your arms and legs.
These fins underwent
modification, becoming the limbs of amphibians and their evolutionary
descendants such as lizards, canaries, dinosaurs, and humans.
The lungfish are a small group
found mostly in freshwater stagnant water or ponds that dry up in Africa, South
America, and Australia.
Australian
lungfish
Coelacanths live in deep oceans.
They were once considered extinct, although more than 200 have been captured
since 1938. Mitochondrial DNA analysis supports the hypothesis that lungfish are
probably the closest living relatives of amphibians.
Coelacanth,
a living fossil.
The crossopterygian fish
(represented by the marine extant deep-living coelacanth and extinct freshwater
forms) are regarded as ancestors of early amphibians.
Extinct
crossopterygians had strong fins, lungs, and a streamlined body capable of
swimming as well as traveling short distances out of water.
Comparison
of the skeletons of a crossopterygian lobe-finned fish and an early amphibian.
The term "tetrapod" (meaning four-limbed or four-footed) has historically been applied to the land vertebrates (amphibians, reptiles, dinosaurs, birds, and mammals). All other animals from this point have four limbs and are called tetrapods.
Most zoologists would accept
that the Devonian lobe-finned fishes were ancestral to the amphibians. Animals
(both vertebrate as well as many invertebrates such as insects) that live on
land use limbs to support the body, especially since air is less buoyant than
water. Lobe-finned fishes and early amphibians also had lungs and internal nares
to respire air.
Two hypotheses have been
proposed to explain the evolution of amphibians from lobe-finned fishes.
The first amphibians diversified
during Carboniferous Period (commonly known as the Age of Amphibians).
Class
Amphibia: Animals Move Ashore
This class includes 4000 species
of animals that spend their larval/juvenile stages in water, and their adult
life on land. Amphibians
must return to water to mate and lay eggs. Most adults have moist skin that
functions in helping their small, inefficient lungs with gas exchange. Frogs, toads, newts,
salamanders, and mud puppies are in this transitional group between water and
land.
Amphibian features not seen in
bony fish include:
Reproduction involves a return
to the water. Ther term "amphibian" refers to two life styles, one in
water, the other on land. Amphibians shed eggs into the water where external
fertilization occurs, as it does in fish. Generally, amphibian eggs are
protected by a coat of jelly but not by a shell. The young hatch into aquatic
larvae with gills (tadpoles). Aquatic larvae usually undergo metamorphosis to
develop into a terrestrial adult.
Amphibians, like fish, are
ectothermic; they depend upon external heat to regulate body temperatures. If
the environmental temperature becomes too low, ectotherms become inactive.
Salamanders more likely resemble
earliest amphibians due to their S-shaped movements. Salamanders practice
internal fertilization; males produce a spermatophore that females pick up.
Frogs and toads are tailless as adults, with their hind limbs specialized for
jumping.
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Frogs
This class of 6000 species
includes the snakes, lizards, turtles, alligators, and crocodiles.
Reptiles that lay eggs lay an
egg surrounded by a thick protective shell and a series of internal membranes.
Reptiles have internal
fertilization: their gametes do not need to be released into water for
fertilization to occur.
The amniotic egg is a superb
adaptation to life on land. While amphibians need to lay their eggs in water,
their descendants (reptiles) were not as strongly tied to moist environments and
could truly expand into more arid areas. Reptiles were the first land
vertebrates to practice internal fertilization through copulation and to lay
eggs that are protected by a leathery shell with food and other support for the
growing embryo.
The amniote egg contains
extra-embryonic membranes that are not part of the embryo and are disposed of
after the embryo has developed and hatched. These membranes protect the embryo,
remove nitrogenous wastes, and provide the embryo with oxygen, food, and water.
The amnion, one of these extra-embryonic membranes, creates a sac that fills
with fluid and provides a watery environment in which the embryo develops. The
embryo develops in a "pond within the shell".
Structure
of the amniote egg
Evolutionary
History of Reptiles
Reptiles first evolved during
the Carboniferous time and partly displaced amphibians in many environments. The
first reptiles (often referred to as the stem reptiles) gave rise to several
other lineages, each of which adapted to a different way of life. Reptilian
success was due to their terrestrial (amniotic) egg and internal fertilization,
as well as their tough leathery skin, more efficient teeth and jaws, and in
some, bipedalism (traveling on their hind legs, allowing the forelimbs to grasp
prey or food, or become wings). One group, the Pelycosaurs (fin-backed or sail
lizards) are related to therapsids, mammal-like reptiles ancestral to mammals.
Other groups returned to aquatic environments. Ichthyosaurs were fishlike (or
dolphin-like) free-swimming predators of the Mesozoic seas. The plesiosaurs had
a long neck and a body adapted tp swimming though use of flippers (legs that
evolutionarily reverted to a flipper-like shape). These free-swimmers also
adapted to live birth of their young (since they could not return to the land to
lay eggs). Thecodonts were the reptiles that gave rise to most of the reptiles,
living and extinct. Pterosaurs were flying reptiles that dominated the Mesozoic
skies. They had a keel for attachment of flight muscles and air spaces in bones
to reduce weight.
Dinosaurs (descended from some
thecodonts) and mammal-like reptiles' had their limbs beneath the body providing
increased agility and facilitating gigantic size. Lizards have their elbows out
(like you do when you do a push-up). By having their elbows in, dinosaurs and
mammals place more of the weight of the body on the long bones instead of the
elbows, ankles, and knees.
Relationship
between limbs and body. Note that reptiles have their upper limbs jutting out
from the body, while mammals have their limbs in line with the body, supporting
and more easily raising the body mass off the ground.
Reptiles dominated the earth for
about 170 million years during the Mesozoic Era. The mass extinction of many
reptile groups at the close of the Mesozoic (the Cretaceous Period) has been
well documented and the subject of many hypotheses. The 1980 hypothesis by Luis
and Walter Alvarez and others proposes the impact of a large meteorite at the
end of the Cretaceous period caused a catastrophic environmental collapse that
led to the extinction of nearly 50% of all species of life on Earth. The
survivors, birds and mammals, reaped the spoils and diversified during the
Cenozoic Era. Three groups of reptiles remain: turtles, snakes/lizards, and
crocodiles/alligators.
About 6,000 species of reptiles
comprise the Class Reptilia. Most live in tropics or subtropics. Lizards and
snakes live on land, while turtles and alligators live in water for much of
their lives. Reptiles have a thick, scaly skin that is keratinized and
impermeable to water. This same keratin is a protein found in hair, fingernails,
and feathers. Protective skin prevents water loss but requires several molts a
year. Reptilian lungs are more developed than those of amphibians. Air moves in
and out of the lungs due to the presence of an expandable rib cage in all
reptiles except turtles. Most reptiles have a nearly four-chambered heart. The
crocodile has a completely four-chambered heart that more fully separates
oxygen-rich blood from from deoxygenated or oxygen-poor blood. The
well-developed kidneys excrete uric acid; less water is lost in excretion.
Reptiles are ectothermic; they require a fraction of the food per body weight of
birds and mammals, but are behaviorally adapted to warm their body temperature
by sunbathing.
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Photograph
of a lizard (L) and a gavial (R)
Snakes and lizards live mainly
in the tropics and desert. Lizards have four clawed legs and are carnivorous;
marine iguanas on the Galapagos are adapted to spend time in the sea; frilled
lizards have a collar to scare predators, and blind worm lizards live
underground. Snakes evolved from lizards and lost their legs as an adaptation to
burrowing. Their jaws can readily dislocate to engulf large food. The snake's
tongue collects airborne molecules and transfers them to the Jacobson's organ
for tasting. Some poisonous snakes have special fangs for injecting their venom.
Turtles have a heavy shell fused
to the ribs and thoracic vertebrae; they lack teeth but use a sharp beak; sea
turtles must leave the ocean to lay eggs onshore.
Galapagos
tortoises
Crocodiles and alligators are
largely aquatic, feeding on fishes and other animals. They both have a muscular
tail that acts as a paddle to swim and a weapon. The male crocodile bellows to
attract mates. In some species the male also protects the eggs and young.
Cladistic analyses place the
birds, alligators, and dinosaurs in the same clade, the Archosauria (or
"ruling reptiles"). This group is a major group of diapsids
(vertebrates that have two openings in their skulls) that have single openings
in each side of the skull, in front of the eyes (antorbital fenestrae), among
other characteristics. This helps to lighten the skull, provides more room for
muscles and other tissues, and allows more skull flexibility when eating. Other
typical archosaurian characteristics include another opening in the lower jaw
(the mandibular fenestra), a high narrow skull with a pointed snout, teeth set
in sockets, and a modified ankle joint.
The ancestral archosaurs
probably originated some 250 million years or so ago, during the late Permian
period. Their descendants (such as the dinosaurs) dominated the realm of the
terrestrial vertebrates for a most of the Mesozoic Era. The birds and
crocodilians are the last living groups of archosaurs.
The class Aves (birds) contains
about 9000 species. Birds evolved from either a dinosaurian or other reptilian
group during the Jurassic (or possibly earlier). The earliest bird fossils, such
as the Jurassic Archaeopteryx or Triassic Protavis, display a
mosaic of reptilian and bird features (teeth in the bill, a jointed tail, and
claws on the wing are reptilian; feathers and hollow bones are bird-like).
Archaeopteryx,
once considered the first bird.The fossil is from the Solenhoefen Limestone
(Jurassic) of Germany
The distinguishing feature of
birds is feathers: which provide insulation as well as aid in flight.
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Remember, not all animals that
fly have feathers, but all almost every endothermic animal (warm-blooded) has a
covering of hair or feathers for insulation. The recent (1999) discovery of a
"feathered" dinosaur adds credence to this speculation. The dinosaur
could not fly, so of what use would feather be but insulation (or possibly
mating).
Modern birds appeared during the
early Tertiary, and have adapted to all modes of life: flying (condors, eagles,
hummingbirds), flightless-running (ostriches, emus), and swimming (penguins).
Birds exhibit complex mating
rituals as well as social structure (a pecking order!).
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Class
Mammalia: Got Milk?
Class Mammalia contains around
5000 species placed in 26 orders (usually).
The
three unifying mammalian characteristics are:
Milk is a substance rich in fats
and proteins. Mammary glands usually occur on the ventral surface of females in
rows (when there are more than two glands). Humans and apes have two mammary
glands (one right, one left), while other animals can have a dozen or more. All
mammals have hair at some point during their life. Mammalian hair is composed of
the protein keratin. Hair has several functions: 1) insulation; 2) sensory
function (whiskers of a cat); 3) camouflage, a warning system to predators,
communication of social information, gender, or threats; and 4) protection as an
additional layer or by forming dangerous spines that deter predators.
Modifications of the malleus and incus (bones from the jaw in reptiles) work
with the stapes to allow mammals to hear sounds after they are transmitted from
the outside world to their inner ears by a chain of these three bones.
Mammals first evolved from the
mammal-like reptiles during the Triassic period, about the same time as the
first dinosaurs. However,
mammals were minor players in the world of the Mesozoic, and only diversified
and became prominent after the extinction of dinosaurs at the close of the
Cretaceous period.
Mammals have since occupied all
roles once held by dinosaurs and their relatives (flying: bats; swimming:
whales, dolphins; large predators: tigers, lions; large herbivores: elephants,
rhinos), as well as a new one (thinkers and tool makers: humans).
There are 4500 species of living
mammals.
Subclass
Prototheria: Order
Monotremata: Monotremes (typified by the platypus and echinda) lay eggs that
have similar membranes and structure to reptilian eggs.
Females
burrow in ground and incubates their eggs. Both males and females produce milk
to nourish the young There are two families living today and quite a few known
from the fossil record of Gondwana. Monotremes are today restricted to Australia
and New Guinea. The earliest fossil monotreme is from the early Cretaceous, and
younger fossils hint at a formerly more widespread distribution for the group.
While their fossil record is scarce, zoologists believe that monotremes probably
diverged from other mammals during the Mesozoic. Monotremes have many
differences with other mammals and are often placed in a separate group, the
subclass Prototheria. They retain many characters of their therapsid ancestors,
such as laying eggs, limbs oriented with humerus and femur held lateral to body
(more lizard-like), a cloaca, skulls with an almost birdlike appearance, and a
lack of teeth in adults. This suggests that monotremes are the sister group to
all other mammals. However, monotremes do have all of the mammalian defining
features of the group.
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Subclass
Metatheria: Marsupials (such as the koala, opossum, and kangaroo) are born while
in an embryonic stage and finish development outside the mother's body, often in
a pouch. Marsupial young leave the uterus, crawl to the pouch, and attach to the
nipple of a mammary gland and continue their development. Marsupials were once
widespread, but today are dominant only in Australia, where they underwent
adaptive radiation in the absence of placental mammals. The Metatheria contains
272 species classified in several orders. Metatheres diverged from the lineage
leading to the eutherian (placental) mammals by the middle of the Cretaceous
period in North America. The earliest marsupial fossils resemble North American
opossums. Marsupial fossils are found on other northern hemisphere continents,
although they seem not to have been prominent elements of those faunas. On the
other hand, in South America and Australia, marsupials continued to be dominant
faunal elements. The marsupials of South America began to go extinct in the late
Miocene and Early Pliocene (Cenozoic era) when volcanic islands grew together
and formed the Isthmus of Panama, allowing North American placental mammals to
cross into South America. Australian marsupials remain diverse and dominant
native mammals of the fauna. During the Cenozoic Era many marsupials in South
America and Australia underwent parallel (or convergent) evolution with
placental mammals elsewhere, producing marsupial "wolves",
"lions", and saber-toothed marsupial "cats".
Red Kangaroo with its joey
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Subclass
Eutheria: There are
4000 described species of placental mammals, a group that includes dogs, cats,
and people. The subclass is defined by a true placenta that nourishes and
protects the embryos held within the mother's body for an extended gestation
period (nearly two years for an elephant, and nine very long months for a
human). The eutherian placenta has extraembryonic membranes modified for
internal development within the uterus. The chorion is the fetal portion of
placenta, while the uterine wall grows the maternal portion. The placenta
exchanges nutrients, oxygen, and wastes between fetal and maternal blood.
There are 12 orders of placental
mammals. Classification is based on the mode of locomotion and methods of
obtaining food. Prominent orders include the bats (order Chiroptera), horses
(order Perissodactyla), whales (order Cetacea), mice (order Rodentia), dogs
(order Carnivora), and monkeys/apes/humans (order Primates).
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