Phylum Chordata: Vertebrata

The Vertebrates

Characteristics of Vertebrates

• The following major traits characterize the vertebrate body plan:

– A rigid internal skeleton, with a jointed vertebral column which replaced the notochord and serves as the anchor that provides support and mobility

– Two pairs of appendages attached to the vertebral column

– An anterior skull with a large brain

– Internal organs suspended in a large coelom

– A well-developed circulatory system, driven by contractions of a ventral heart

Vertebrate Phylogeny

Class: Agnatha (Jawless Fish)

• Filter-feeding ancestral vertebrates (lancelets) lacked jaws and gave rise to the jawless fishes.

• The hagfishes, slime eels, and the lampreys are the only jawless fishes (class Agnatha) to survive beyond the Devonian period. They have tough skins instead of external armor. They have an endoskeleton composed of cartilage but they lack jaw structure.

• They lack paired appendages and have a round mouth that acts as a sucking organ by attaching to their prey and rasping at its flesh. They have gill slits for respiration.

Class: AgnathaHagfish Lamprey

Class: Chondrichthyes (Cartilagenous Fish)

• The cartilaginous fishes (class Chondrichthyes) became abundant during the Devonian period.

• They include the sharks, skates and rays, and chimaeras. • They have a skeleton composed entirely of a firm but pliable

material called cartilage.• Their skin is flexible and leathery; the loss of external armor

increased their mobility and their ability to escape from predators.• They lack an internal gas bladder and must swim to maintain

their position in water. If they stop swimming they sink.• Pairs of unjointed appendages called fins control swimming.• These fins include the pectoral, pelvic, and dorsal fins.• Sharks move forward by means of their tail and pelvic fins.• Skates and rays propel themselves by means of undulating

movements of their enlarged pectoral fins.• Nearly all cartilaginous fishes live in the oceans.

Chondrichthyes

Osteoichthyes: Bony Fish• The ray-finned fishes (class Actinopterygii or Osteoichthyes)

have internal skeletons of bone rather than cartilage.• Most species are covered with flat, smooth, thin, lightweight

scales that provide some protection.• Their gills open into a single chamber covered by a hard flap

called an operculum, whose movements improve the flow of water over the gills.

• Gas exchange takes place in the gills.• In most species, lunglike sacs evolved into swim bladders,

which serve as organs of buoyancy.• Ray-finned fishes radiated during the Tertiary into about 24,000

species, in a wide variety of sizes, shapes, and lifestyles.• Some fishes form large aggregations, called schools, in open

waters.• Many species perform complex behaviors.

Osteoichthyes

Transition from Water to Land• The evolution of lunglike sacs in response to the inadequacy of

gills for respiration in oxygen-poor waters set the stage for the invasion of land.

• Some bony fishes were able to supplement their gills with lung sacs when oxygen levels were low.

• This ability allowed them to breathe air and to leave the water temporarily.

• The lobe-finned fishes (class Actinistia) were the first lineage to evolve jointed fins. Coelocanth.

• The lungfishes (class Dipnoi) were important predators in shallow-water habitats in the Devonian, but most lineages died out.

• The three surviving species live in stagnant swamps and muddy waters in the Southern Hemisphere.

• Some descendants of early fishes with jointed fins began to use terrestrial food sources and over time became fully adapted to life on land.

• This lineage is believed to have given rise to the tetrapods: the four-legged amphibians, reptiles, birds, and mammals of today.

Osteoichthyes:

Coelocanth African Lung Fish

Class Amphibia

• The amphibians (class Amphibia) arose during the Devonian period.

• Most amphibian species have small lungs and exchange gases through their skin, and are confined to moist environments.

• There are about 4,500 species of amphibians alive on Earth today.

• The living amphibians are divided into three orders:– Gymnophiona (the wormlike, burrowing caecilians)

– Anura (frogs and toads)

– Urodela (salamanders)• Most species live in water at some time in their lives.

Class Amphibia

Amphibian Life Cycle• In a typical amphibian life

cycle, adults spend part of all of their time on land, but return to fresh water to lay eggs.

• Amphibian eggs can only survive in moist environments.

• Fertilization is external and fertilized eggs develop into larvae that live in water until they undergo metamorphosis to an adult.

• Some amphibians are entirely aquatic, others entirely terrestrial.

The Complete Land Transition• Two morphological changes contributed to the ability of one

vertebrate lineage to control water loss and exploit a wider variety of terrestrial habitats:

– The evolution of an egg with a shell impermeable to water

– A combination of traits that reduced water loss, such as skin that is impermeable to water and kidneys that could excrete concentrated urine

• The vertebrates that evolved these traits are called amniotes.• Amniote eggs have a calcium-impregnated shell that prevents

the evaporation of fluids inside but allows O2 and CO2 to pass through.

• These eggs store large quantities of yolk that allow the embryo to attain a relatively advanced state of development before it hatches.

• Because of an outer shell fertilization must now occur internally before the shell forms around the egg. This leads to the evolution of elaborate and complex reproductive behaviors.

The Amniotic Egg

Class: Reptilia• The reptiles (class Reptilia) are an early amniote lineage that arose

from the tetrapods during the Carboniferous period.• Some species have eggs that do not develop shells and are

retained inside the female’s body until they hatch. Some of these species evolved placentas that nourish the developing embryos.

• Reptiles have skin covered with horny scales that reduce water loss, exchange gas by the lungs, and have a heart divided into 3 chambers that separate oxygenated from unoxygenated blood.

• The turtle lineage (subclass Testudines) has changed very little.• Turtles have a combination of ancestral traits and highly

specialized characteristics that they do not share with any other vertebrate group. Therefore, their phylogenetic relationships are uncertain.

• The subclass Squamata includes lizards, snakes, and the amphisbaenians, a group of legless burrowing animals with greatly reduced eyes.

• The tuataras (subclass Sphenodontida) are a sister group to the lizards. This group was diverse in the Mesozoic, but only two species exist today.

• The crocodilians (subclass Crocodylia) are confined to tropical and warm temperate environments.

Reptiles

Dinosaurs

• The ability to breathe and run simultaneously was a major innovation in the evolution of terrestrial vertebrates.

• In the lineages leading to the mammals, dinosaurs, and birds, the legs assumed a vertical position positioned under the trunk of the body unlike reptiles.

• Muscles that enabled the lungs to be filled and emptied while the limbs moved also evolved. Something reptiles can not do.

• These muscles are present in living birds and mammals, and their existence can be inferred in dinosaurs from their fossils.

Dinosaur fossils recently discovered in China show that in some small predatory dinosaurs, the scales had been highly modified to form feathers.

Class: Aves• Most paleontologists believe that birds evolved from terrestrial bipedal

dinosaurs that used their forelimbs for capturing prey.• These dinosaurs may have initially developed feathers for insulation or

display, and eventually were able to become airborne for short distances.• There are about 9,600 species of birds today, ranging in size from the 2-

gram bee hummingbird to the 150-kilogram ostrich.• As a group, birds eat almost all types of animal and plant material.• They serve as a major agent of seed dispersal by eating the seeds of

plants.• Feathers function not only in flight, but also in thermoregulatory and

display functions.• The bones of birds are modified for flight; they are hollow and have

internal struts for strength.• The breastbone forms a large, vertical keel to which pectoral muscles are

attached. These muscles pull the wings downward during the propulsive movement in flight.

• Flight is metabolically expensive, and thus birds have very high metabolic rates.

• As a consequence, they generate large amounts of heat, whose loss is controlled by feathers that can trap or release warm air.

• Birds have an enlarged cerebellum, the center of sight and muscular coordination.

• Most birds lay eggs in a nest, where the young are usually cared for by the parents after hatching.

Class: Aves

Class: Mammalia• Mammals (class Mammalia) appeared in the early part of the Mesozoic era.• Small mammals coexisted with reptiles and dinosaurs for at least 150 million

years.• Today’s mammals range in size from tiny shrews and bats that weigh only 2

grams to the endangered blue whale, which can measure up to 33 meters long and weigh up to 160,000 kilograms.

• Skeletal modifications accompanied the evolution of the small mammals from their larger reptilian ancestors:

– Bones from the lower jaw were incorporated into the middle ear, leaving a single bone in the lower jaw.

– The number of bones in the skull was decreased.

– The bulk of the limbs and the bony girdles from which they are suspended were reduced.

– Mammals have fewer teeth than reptiles, but mammal teeth are more differentiated.• Soft parts of animals are seldom fossilized; thus, it is difficult to tell when

mammalian features such as mammary glands, sweat glands, hair, and a four-chambered heart evolved.

• The mammals are unique in providing their young with milk secreted by mammary glands.

• Mammalian eggs are fertilized within the female’s body, and prior to birth, embryos undergo a period of development called gestation within a uterus.

Class Mammalia:

• The approximately 4,000 species of living mammals are divided into two major subclasses: Prototheria and Theria (Marsupial and Eutherian or placental mammals).

• The subclass Prototheria contains a single order, the Monotrema, which contains only three species. The monotremes differ from other mammals in that they lack a placenta, lay eggs, and have legs that poke out to the side.

Mammalia: Monotremata

Mammalia: Therians

• Females of the group Marsupialia have a ventral pouch in which they carry and feed their offspring.

• Gestation in marsupials is short; early stages of offspring development take place in the pouch.

• There are about 240 living species of marsupials.

Mammalia: Marsupials

Mammalia: Eutherians

• Most living mammals are eutherians.• Eutherians are more highly developed at

birth than marsupials, and no external pouch houses them after birth. The developing young receive nourishment from their mother through a structure produced by the embryo called a placenta.

• There are about 4,000 living species of eutherians in 16 different groups.

Mammalia: Eutherians