Ch 34 Vertebrates

Coelacanths (Actinistia)

Once thought to be extinct but discovered by fishermen in 1938

Tunicates (Urochordata)

are more closely related to other chordates than are lancelets. The chordate characters of tunicates are most apparent during their larval stage, which may be as brief as a few minutes. In many species, the larva uses its tail muscles and notochord to swim through water in search of a suitable substrate on which it can settle, guided by cues it receives from light- and gravity-sensitive cells.

Rays are closely related to sharks but have a different life style. Most rays are

bottomdwellers that feed by using their jaws to crush molluscs and crustaceans. They have a flattened shape and use their greatly enlarged pectoral fins like water wings to propel themselves through the water. The tail of many rays is whiplike and, in some species, bears venomous barbs that function in defense

Paleontologists have also discovered evidence that some dinosaurs

built nests and brooded their eggs, as birds do today. Some anatomical evidence supports the hypothesis that at least some dinosaurs were endotherms

Snakes are

carnivorous and a number of adaptations aid them in hunting and eating prey. They have acute chemical sensors, and though they lack eardrums, they are sensitive to ground vibrations, which helps them detect the movements of prey. Heat-detecting organs between the eyes and nostrils of pit vipers, including rattlesnakes, are sensitive to minute temperature changes, enabling these night hunters to locate warm animals

The skeleton of lampreys is made of

cartilage. Unlike the cartilage found in most vertebrates, lamprey cartilage contains no collagen. Instead, it is a stiff matrix of other proteins. The notochord of lampreys persists as the main axial skeleton in the adult, as it does in hagfishes. However, lampreys also have a flexible sheath around their rodlike notochord. Along the length of this sheath, pairs of cartilaginous projections related to vertebrae extend dorsally, partially enclosing the nerve cord.

Three surviving lineages of gnathostomes

chondrichthyans, ray-finned fishes, and lobe-fins.

Lobe-fins (Sarcopterygii)

Also originated during the Silurian period. Key derived characteristic is the presence of rod-shaped bones surrounded by a thick layer of muscle in their pectoral and pelvic fins. During the Devonian, many lobe-fins lived in brackish waters, such as in coastal wetlands. There they used their lobed fins to help them move across logs or the muddy bottom (as do some living lobe-fins). Some Devonian lobe-fins were gigantic predators.

Tretrapods

Are gnathostomes that have limbs. The fins of lobe fins evolved into limbs and feet of tetrapods.

All dinosaurs except birds became extinct by the end of the

Cretaceous period (66 million years ago). Their extinction may have been caused at least in part by the asteroid or comet. Some analyses of the fossil record are consistent with this idea in that they show a sudden decline in dinosaur diversity at the end of the Cretaceous. However, other analyses indicate that the number of dinosaur species had begun to decline several million years before the Cretaceous ended

Fertilization is _________ in most amphibians

External. the male grasps the female and spills his sperm over the eggs as the female sheds them. Some amphibian species lay vast numbers of eggs in temporary pools, and egg mortality is high. In contrast, other species lay relatively few eggs and display various types of parental care. Depending on the species, either males or females may house eggs on their back, in their mouth, or even in their stomach. Certain tropical tree frogs stir their egg masses into moist, foamy nests that resist drying

aquatic osteichthyans known informally as

Fishes.

Adult lancelets retain key chordate traits

Following metamorphosis, an adult lancelet swims down to the seafloor and wriggles backward into the sand, leaving only its anterior end exposed. Cilia draw seawater into the lancelet's mouth. A net of mucus secreted across the pharyngeal slits removes tiny food particles as the water passes through the slits, and the trapped food enters the intestine. The pharynx and pharyngeal slits play a minor role in gas exchange, which occurs mainly across the external body surface.

Vertebrates possess two or more sets of

Hox genes. Other important families of genes that produce transcription factors and signaling molecules are also duplicated in vertebrates. The resulting additional genetic complexity may be associated with innovations in the vertebrate nervous system and skeleton, including the development of a skull and a backbone composed of vertebrae. In some vertebrates, the vertebrae are little more than small prongs of cartilage arrayed dorsally along the notochord. In the majority of vertebrates, however, the vertebrae enclose the spinal cord and have taken over the mechanical roles of the notochord.

Haikouella resembled a lancelet

Its mouth structure indicates that, like lancelets, it probably was a suspension feeder. However, Haikouella also had some of the characters of vertebrates. For example, it had a well-formed brain, small eyes, and muscle segments along the body, as do the vertebrate fishes. Unlike the vertebrates, however, Haikouella did not have a skull or ear organs, suggesting that these characters emerged with further innovations to the chordate nervous system. (The earliest "ears" were organs for maintaining balance, a function still performed by the ears of humans and other living vertebrates.)

The other major living lineage of lepidosaurs consists of lizards and snakes, or squamates

Many squamates are small; the Jaragua lizard, discovered recently in the Dominican Republic, is only 16 mm long—small enough to fit comfortably on a dime. In contrast, the Komodo dragon of Indonesia is a lizard that can reach a length of 3 m. It hunts deer and other large prey, delivering venom with its bite.

Lampreys

Marine and freshwater environments. Some are that feed by clamping their round, jawless mouth onto the flank of a live fish, their "host." Parasitic lampreys use their rasping mouth and tongue to penetrate the skin of the fish and ingest the fish's blood and other tissues.

All hagfishes live in ______ environments

Marine. Most are bottom-dwelling scavengers that feed on worms and sick or dead fish. Rows of slime glands on a hagfish's flanks secrete a substance that absorbs water, forming a slime that may repel other scavengers when a hagfish is feeding. When attacked by a predator, a hagfish can produce several liters of slime in less than a minute. The slime coats the gills of the attacking fish, sending it into retreat or even suffocating it. Biologists and engineers are investigating the properties of hagfish slime as a model for developing a space-filling gel that could be used, for instance, to stop bleeding during surgery

Lancelets (Cephalochordata)

Most basal group of living chordates. Bladelike shape.

Early signs of a skull can be seen in

Myllokunmingia. About the same size as Haikouella, Myllokunmingia had ear capsules and eye capsules, parts of the skull that surround these organs. Based on these and other characters, Myllokunmingia is considered the first chordate to have a head. The origin of a head—consisting of a brain at the anterior end of the dorsal nerve cord, eyes and other sensory organs, and a skull—enabled chordates to coordinate more complex movement and feeding behaviors. Although it had a head, Myllokunmingia lacked vertebrae and hence is not classified as a vertebrate.

Ray-finned fishes (Actinopterygii)

Named for the bony rays that support their fins, the ray-finned fishes originated during the Silurian period. The group has diversified greatly since that time, resulting in numerous species and many modifications in body form and fin structure that affect maneuvering, defense, and other functions

Tuataras

One lineage of lepidosaurs. Found only on the island off the coast of New Zealand. When humans arrived in New Zealand, the rats that accompanied them devoured tuatara eggs, eventually eliminating them on the main islands. The tuataras that remain on the outlying islands are about 50 cm long and feed on insects, small lizards, and bird eggs and chicks. They can live to be over 100 years old. Their future survival depends on whether their remaining habitats are kept rat-free

The vast majority of vertebrates belong to the clade of gnathostomes called

Osteichthyes. Unlike chondrichthyans, nearly all living osteichthyans have an ossified (bony) endoskeleton with a hard matrix of calcium phosphate. Includes tetrapods along with bony fishes in the clade Osteichthyes.

Research on lancelets has also revealed important clues about the evolution of the chordate brain.

Rather than a fullfledged brain, lancelets have only a slightly swollen tip on the anterior end of their dorsal nerve cord. But the same Hox genes that organize major regions of the forebrain, midbrain, and hindbrain of vertebrates express themselves in a corresponding pattern in this small cluster of cells in the lancelet's nerve cord. This suggests that the vertebrate brain is an elaboration of an ancestral structure similar to the lancelet's simple nerve cord tip.

Adaptations of sharks

Sharks have sharp vision but cannot distinguish colors. The nostrils of sharks, like those of most aquatic vertebrates, open into dead-end cups. They function only for olfaction (smelling), not for breathing. Like some other vertebrates, sharks have a pair of regions in the skin of their head that can detect electric fields generated by the muscle contractions of nearby animals. Like all nonmammalian aquatic vertebrates, sharks have no eardrums, structures that terrestrial vertebrates use to transmit sound waves in air to the auditory organs. Sound reaches a shark through water, and the animal's entire body transmits the sound to the hearing organs of the inner ear.

Salamander

Some are entirely aquatic but others live on land as adults or throughout life. Most salamanders that live on land walk with a side-to-side bending of the body, a trait also found in early terrestrial tetrapods. Paedomorphosis is common among aquatic salamanders; the axolotl, for instance, retains larval features even when sexually mature

Amniotes

Tetrapods that have a terrestrially adapted egg. Includes reptiles and mammals.

Pharyngeal Slits or Clefts

The digestive tube of chordates extends from the mouth to the anus. The region just posterior to the mouth is the pharynx. In all chordate embryos, a series of arches separated by grooves forms along the outer surface of the pharynx. In most chordates, these grooves (known as pharyngeal clefts) develop into slits that open into the pharynx. These pharyngeal slits allow water entering the mouth to exit the body without passing through the entire digestive tract. Pharyngeal slits function as suspension-feeding devices in many invertebrate chordates

Two groups of chondrichthyan

The largest and most diverse group consists of the sharks, rays, and skates. A second group is composed of a few dozen species of ratfishes, also called chimaeras

Amphibians

The most basal group of tetrapods. Three clades: salamanders (clade Urodela, "tailed ones"), frogs (clade Anura, "tail-less ones"), and caecilians (clade Apoda, "legless ones").

Haikouella

The most primitive of the fossils of early chordates that appear to straddle the transition to vertebrates.

Dorsal, Hollow Nerve Cord

The nerve cord of a chordate embryo develops from a plate of ectoderm that rolls into a neural tube located dorsal to the notochord. The resulting dorsal, hollow nerve cord is unique to chordates. Other animal phyla have solid nerve cords, and in most cases they are ventrally located. The nerve cord of a chordate embryo develops into the central nervous system: the brain and spinal cord.

three clades of archosaurs with living members

The turtles, the crocodilians, and the birds.

Gnathostome success probably resulted from a combination of anatomical features

Their paired fins and tail (which were also found in jawless vertebrates) allowed them to swim efficiently after prey, and their jaws enabled them to grab prey or simply bite off chunks of flesh. Over time, dorsal, ventral, and anal fins stiffened by bony structures called fin rays also evolved in some early gnathostomes. Fin rays provide thrust and steering control when aquatic vertebrates swim after prey or away from predators. Faster swimming was supported by other adaptations, including a more efficient gas exchange system in the gills.

In spite of their name, however, many amphibians do not live a dual— aquatic and terrestrial—life

There are some strictly aquatic or strictly terrestrial frogs, salamanders, and caecilians. Moreover, salamander and caecilian larvae look much like the adults, and typically both the larvae and the adults are carnivorous.

The lancelet Branchiostoma, a cephalochordate

This small invertebrate displays all four main chordate characters. Water enters the mouth and passes through the pharyngeal slits into the atrium, a chamber that vents to the outside via the atriopore; large particles are blocked from entering the mouth by tentacle-like cirri. The serially arranged segmental muscles produce the lancelet's wavelike swimming movements.

Origin of Tetrapods

Those that entered shallow, oxygen-poor water could have used their lungs to breathe air. Some species probably used their stout fins to swim and "walk" underwater across the bottom. Tretrapod body plan was a modification of the lobe fin body plan.

By the end of the Devonian period, lobe-fin diversity was dwindling, and today only

Three lineages survive. Coelacanths (Actinistia), lungfishes (Dipnoi), and the third lineage adapted to life on land and gave rise to vertebrates with limbs and feet, called tetrapods—a lineage that includes humans.

Neural crest

a collection of cells that appears along the edges of the closing neural tube of an embryo. Neural crest cells disperse throughout the embryo, where they give rise to a variety of structures, including teeth, some of the bones and cartilage of the skull, several types of neurons, and the sensory capsules in which eyes and other sense organs develop.

Once a tunicate has settled on a substrate, it undergoes

a radical metamorphosis in which many of its chordate characters disappear. Its tail and notochord are resorbed; its nervous system degenerates; and its remaining organs rotate 90°. As an adult, a tunicate draws in water through an incurrent siphon; the water then passes through the pharyngeal slits into a chamber called the atrium and exits through an excurrent siphon. Food particles are filtered from the water by a mucous net and transported by cilia to the esophagus. The anus empties into the excurrent siphon. Some tunicate species shoot a jet of water through their excurrent siphon when attacked, earning them the informal name of "sea squirts."

Chordates

are bilaterally symmetric and belong to Deuterostomia.

Dinosaurs once were considered slow, sluggish creatures. However, fossil discoveries and research have led to the conclusion that many dinosaurs were

agile and fast moving. Dinosaurs had a limb structure that enabled them to walk and run more efficiently than could earlier tetrapods, which had a sprawling gait. Fossilized footprints and other evidence suggest that some species were social—they lived and traveled in groups, much as many mammals do today

Amniotes have acquired other key adaptations to life on land

amniotes use their rib cage to ventilate their lungs. This method is more efficient than throat-based ventilation, which amphibians use as a supplement to breathing through their skin. The increased efficiency of rib cage ventilation may have allowed amniotes to abandon breathing through their skin and develop less permeable skin, thereby conserving water.

The common ancestors of all gnathostomes underwent

an additional duplication of Hox genes, such that the single set present in early chordates became four. In fact, the entire genome appears to have duplicated, and together these genetic changes likely enabled the origin of jaws and other novel features in gnathostomes. The gnathostome forebrain is enlarged compared to that of other vertebrates, and it is associated with enhanced senses of smell and vision.

Caecilians

apodans, or caecilians, are legless and nearly blind, and superficially they resemble earthworms. Their absence of legs is a secondary adaptation, as they evolved from a legged ancestor. Caecilians inhabit tropical areas, where most species burrow in moist forest soil.

mineralization of the vertebrate skeleton had already begun before the chondrichthyan lineage branched off from other vertebrates. Moreover, bone-like tissues have been found in early

chondrichthyans, such as the fin skeleton of a shark that lived in the Carboniferous period. Traces of bone can also be found in living chondrichthyans—in their scales, at the base of their teeth, and, in some sharks, in a thin layer on the surface of their vertebrae. Such findings indicate that the restricted distribution of bone in the chondrichthyan body is a derived condition, emerging after chondrichthyans diverged from other gnathostomes.

The earliest fossils of vertebrates date to 500 million years ago and include those of

conodonts, a group of slender, soft-bodied vertebrates that lacked jaws and whose internal skeleton was composed of cartilage. Conodonts had large eyes, which they may have used in locating prey that were then impaled on a set of barbed hooks at the anterior end of their mouth. These hooks were made of dental tissues that were mineralized—hardened by the incorporation of minerals such as calcium. The food was then passed back to the pharynx, where a different set of dental elements sliced and crushed the food.

Amniotic egg

contains four specialized membranes: the amnion, the chorion, the yolk sac, and the allantois. Called extraembryonic membranes because they are not part of the body of the embryo itself, these membranes develop from tissue layers that grow out from the embryo. The amniotic egg is named for the amnion, which encloses a compartment of fluid that bathes the embryo and acts as a hydraulic shock absorber. The other membranes in the egg function in gas exchange, the transfer of stored nutrients to the embryo, and waste storage.

hagfishes and lampreys form a clade of living jawless vertebrates, the

cyclostomes

Most amphibians are found in

damp habitats such as swamps and rain forests. Even those adapted to drier habitats spend much of their time in burrows or under moist leaves, where humidity is high. One reason amphibians require relatively wet habitats is that they rely heavily on their moist skin for gas exchange—if their skin dries out, they cannot get enough oxygen. In addition, amphibians typically lay their eggs in water or in moist environments on land; their eggs lack a shell and dehydrate quickly in dry air.

As larvae, lancelets

develop a notochord; a dorsal, hollow nerve cord; numerous pharyngeal slits; and a post-anal tail. The larvae feed on plankton in the water column, alternating between upward swimming and passive sinking. As the larvae sink, they trap plankton and other suspended particles in their pharynx.

Early reptiles resembled lizards are were

diapsids. A key derived character of diapsids is a pair of holes on each side of the skull, behind the eye sockets; muscles pass through these holes and attach to the jaw, controlling jaw movement.

Most fishes breathe by

drawing water over four or five pairs of gills located in chambers covered by a protective bony flap called the operculum. Water is drawn into the mouth, through the pharynx, and out between the gills by movement of the operculum and contraction of muscles surrounding the gill chambers.

Pterosaurs, which originated in the late Triassic, were the first tetrapods to exhibit

flapping flight. The pterosaur wing was completely different from the wings of birds and bats. It consisted of a collagen-strengthened membrane that stretched between the trunk or hind leg and a very long digit on the foreleg. They appear to have converged on many of the ecological roles later played by birds; some were insect-eaters, others grabbed fish out of the ocean, and still others filtered small animals through thousands of fine needlelike teeth. But by 66 million years ago, pterosaurs had become extinct.

In nearly all fishes, the skin is covered by

flattened, bony scales that differ in structure from the tooth-like scales of sharks. Glands in the skin secrete a slimy mucus over the skin, an adaptation that reduces drag during swimming. Like the ancient aquatic gnathostomes mentioned earlier, fishes have a lateral line system, which is evident as a row of tiny pits in the skin on either side of the body.

As larvae, lampreys live in

freshwater streams. The larva is a suspension feeder that resembles a lancelet and spends much of its time partially buried in sediment. About 20 species of lampreys are not parasitic. These species feed only as larvae; following several years in streams, they mature sexually, reproduce, and die within a few days. In contrast, parasitic species of lampreys migrate to the sea or lakes as they mature into adults. One such parasite, the sea lamprey (Petromyzon marinus), has invaded the Great Lakes over the past 170 years and devastated a number of fisheries there.

In vertebrates (with the exception of vertebrates with limbs, the tetrapods), these slits and the pharyngeal arches that support them have been modified for

gas exchange and are called gills. In tetrapods, the pharyngeal clefts do not develop into slits. Instead, the pharyngeal arches that surround the clefts develop into parts of the ear and other structures in the head and neck.

In contrast to the shell-less eggs of amphibians, the amniotic eggs of most reptiles and some mammals

have a shell. A shell slows dehydration of the egg in air, an adaptation that helped amniotes to occupy a wider range of terrestrial habitats than amphibians, their closest living relatives. Most mammals have lost the eggshell over the course of their evolution, and the embryo avoids desiccation by developing within the amnion inside the mother's body.

Most sharks

have a streamlined body and are swift swimmers, but they do not maneuver very well. Powerful movements of the trunk and the tail fin propel them forward. The dorsal fins function mainly as stabilizers, and the paired pectoral (fore) and pelvic (hind) fins are important for maneuvering. Although a shark gains buoyancy by storing a large amount of oil in its huge liver, the animal is still more dense than water, and if it stops swimming it sinks. Continual swimming also ensures that water flows into the shark's mouth and out through the gills, where gas exchange occurs.

tetrapod body plan

head is separated from the body by a neck that originally had one vertebra on which the skull could move up and down. Later, with the origin of a second vertebra in the neck, the head could also swing from side to side. The bones of the pelvic girdle, to which the hind legs are attached, are fused to the backbone, permitting forces generated by the hind legs against the ground to be transferred to the rest of the body. Except for aquatic species the adults of living tetrapods don't have gills; during embryonic development, the pharyngeal clefts instead give rise to parts of the ears, certain glands, and other structures.

One lineage of dinosaurs, the ornithischians, were

herbivores; they included many species with elaborate defenses against predators, such as tail clubs and horned crests.

As for tunicates, several of their genomes have been completely sequenced and can be used to

identify genes likely to have been present in early chordates. Researchers have suggested that ancestral chordates had genes associated with vertebrate organs such as the heart and thyroid gland. These genes are found in tunicates and vertebrates but are absent from nonchordate invertebrates. In another example, a 2015 study found that tunicates (but not lancelets) have embryonic cells that have some of the characteristics of the neural crest, a derived trait found in all vertebrates. This suggests that embryonic cells similar to those in tunicates may represent an intermediate cell population from which the vertebrate neural crest evolved.\

Chondrichthyans (Sharks, Rays, and Their Relatives)

include some of the biggest and most successful vertebrate predators in the oceans. Skeleton composed predominantly of cartilage, though often impregnated with calcium.

Reptiles

includes tuataras, lizards, snakes, turtles, crocodilians, and birds, along with a number of extinct groups, such as plesiosaurs and ichthyosaurs. Unlike amphibians, reptiles have scales that contain the protein keratin (as does a human nail). Scales help protect the animal's skin from desiccation and abrasion. In addition, most reptiles lay their shelled eggs on land; the shell protects the egg from drying out. Fertilization occurs internally, before the eggshell is secreted.

Venomous snakes

inject their toxin through a pair of sharp teeth that may be hollow or grooved. The flicking tongue is not venomous but helps fan odors toward olfactory (smell) organs on the roof of the mouth. Loosely articulated jawbones and elastic skin enable most snakes to swallow prey larger than the diameter of the snake's head

Shark eggs are fertilized

internally. The male has a pair of claspers on its pelvic fins that transfer sperm into the female's reproductive tract

The larval stage of a frog, called a tadpole

is usually an aquatic herbivore with gills, a lateral line system resembling that of aquatic vertebrates, and a long, finned tail. Tadpole lacks legs; it swims by undulating its tail. During the metamorphosis the tadpole develops legs, lungs, a pair of external eardrums, and a digestive system adapted to a carnivorous diet. At the same time, the gills disappear; the lateral line system also disappears in most species. The young frog crawls onto shore and becomes a terrestrial hunter

Gnathostomes

jawed vertebrates. Enable gnathostomes to grip food items firmly and slice them. According to one hypothesis, gnathostome jaws evolved by modification of the skeletal rods that had previously supported the anterior pharyngeal (gill) slits. Figure 34.13 shows a stage in this evolutionary process in which several of these skeletal rods have been modified into precursors of jaws and their structural supports. The remaining gill slits, no longer required for suspension feeding, remained as the major sites of respiratory gas exchange with the external environment

Hagfishes

jawless vertebrates that have highly reduced vertebrae and a skull that is made of cartilage. They swim in a snakelike fashion by using their segmental muscles to exert force against their notochord, which they retain in adulthood as a strong, flexible rod of cartilage. Hagfishes have a small brain, eyes, ears, and a nasal opening that connects with the pharynx. Their mouths contain tooth-like formations made of the protein keratin

lancelets display key chordate characters as adults, and their lineage branches from the base of the chordate phylogenetic tree. These findings suggest that the ancestral chordate may have looked something like a

lancelet—that is, it had an anterior end with a mouth; a notochord; a dorsal, hollow nerve cord; pharyngeal slits; and a post-anal tail.

Another characteristic of aquatic gnathostomes is the

lateral line system, organs that form a row along each side of the body and are sensitive to vibrations in the surrounding water. Precursors of these organs were present in the head shields of some jawless vertebrates.

Two extant lineages of reptiles

lepidosaurs (tuataras, lizards, and snakes) and the archosaurs (turtles, crocodilians, and birds)

Early Amniotes

lived in warm, moist areas, as did the first tetrapods. Over time, early amniotes expanded into a wide range of new environments, including dry and high-latitude regions. Fossil evidence shows that the earliest amniotes resembled small lizards with sharp teeth, a sign that they were predators. Later groups of amniotes also included herbivores, as evidenced by their grinding teeth and other features.

Snakes descended from

lizards with legs- they are classified as legless lizards. Some species of snakes retain vestigial pelvic and limb bones. Snakes are quite proficient at moving on land, most often by producing waves of lateral bending that pass from head to tail. Force exerted by the bends against solid objects pushes the snake forward. Snakes can also move by gripping the ground with their belly scales at several points along the body while the scales at intervening points are lifted slightly off the ground and pulled forward.

Notochord

longitudinal, flexible rod located between the digestive tube and the nerve cord. Composed of large, fluid-filled cells encased in fairly stiff, fibrous tissue. The notochord provides skeletal support throughout most of the length of a chordate, and in larvae or adults that retain it, it also provides a firm but flexible structure against which muscles can work during swimming. In most vertebrates, a more complex, jointed skeleton develops around the ancestral notochord, and the adult retains only remnants of the embryonic notochord. In humans, for example, the notochord is reduced and forms part of the gelatinous disks sandwiched between the vertebrae.

All chordates share a set of derived characters, though many species possess some of these traits only during embryonic development

notochord; a dorsal, hollow nerve cord; pharyngeal slits or clefts; and a muscular, post-anal tail.

Charles Darwin proposed that the lungs of tetrapods evolved from swim bladders, BUT

opposite seems to be true: Swim bladders arose from lungs. Osteichthyans in many early-branching lineages have lungs, which they use to breathe air as a supplement to gas exchange in their gills. This suggests that lungs arose in early osteichthyans; later, swim bladders evolved from lungs in some lineages.

Fish reproduction vary extensively. Most species are

oviparous, reproducing by external fertilization after the female sheds large numbers of small eggs. However, internal fertilization and birthing characterize other species.

Vertebrates with additional innovations emerged during the Ordovician, Silurian, and Devonian periods. These vertebrates had

paired fins and, as in lampreys, an inner ear with two semicircular canals that provided a sense of balance. Like conodonts, these vertebrates lacked jaws, but they had a muscular pharynx, which they may have used to suck in bottom-dwelling organisms or detritus. They were also armored with mineralized bone, which covered varying amounts of their body and may have offered protection from predators. There were many species of these jawless, armored swimming vertebrates, but they all became extinct by the end of the Devonian.

The earliest gnathostomes include extinct lineages of armored vertebrates known collectively as

placoderms, which means "plate-skinned." Other jawed vertebrates, called acanthodians, emerged at roughly the same time and radiated during the Silurian and Devonian periods (444-359 million years ago). Placoderms had disappeared by 359 million years ago, and acanthodians became extinct about 70 million years later.

Many amphibians exhibit complex and diverse

social behaviors, especially during breeding seasons. Frogs are usually quiet, but the males of many species vocalize to defend their breeding territory or to attract females. In some species, migrations to specific breeding sites may involve vocal communication, celestial navigation, or chemical signaling.

bony internal skeleton was a relatively late development in the history of vertebrates. Instead, the vertebrate skeleton evolved initially as a

structure made of unmineralized cartilage. Steps toward a bony skeleton began 470 million years ago, with the appearance of mineralized bone on the outer surface of the skull in some jawless vertebrates. Shortly after that time, the internal skeleton began to mineralize, first as calcified cartilage. By 430 million years ago, some vertebrates had a thin layer of bone lining the cartilage of their internal skeleton. The bones of vertebrates underwent even more mineralization in the jawed vertebrates.

The largest sharks and rays are

suspension feeders that consume plankton

Most sharks, however, are carnivores that

swallow their prey whole or use their powerful jaws and sharp teeth to tear flesh from animals too large to swallow in one piece. Sharks have several rows of teeth that gradually move to the front of the mouth as old teeth are lost. The digestive tract of many sharks is proportionately shorter than that of many other vertebrates. Within the shark intestine is a spiral valve, a corkscrew-shaped ridge that increases surface area and prolongs the passage of food through the digestive tract

Most fishes can maintain a buoyancy equal to the surrounding water by filling an air sac known as a

swim bladder. (If a fish swims to greater depths or toward the surface, where water pressure differs, the fish shuttles gas between its blood and swim bladder, keeping the volume of gas in the bladder constant.)

Muscular, Post-Anal Tail

tail that extends posterior to the anus, although in many species it is greatly reduced during embryonic development. In contrast, most nonchordates have a digestive tract that extends nearly the whole length of the body. The chordate tail contains skeletal elements and muscles, and it helps propel many aquatic species in the water.

Two groups of invertebrate deuterostomes that are more closely related to vertebrates than they are to other invertebrates

the cephalochordates and the urochordates. Thus, along with the vertebrates, these two invertebrate groups are classified within the chordates.

The other main lineage of dinosaurs, the saurischians, included

the long-necked giants and a group called the theropods, which were bipedal carnivores. Theropods included the famous Tyrannosaurus rex as well as the ancestors of birds

The hagfishes (Myxini) and the lampreys (Petromyzontida)

the only lineages of living vertebrates whose members lack jaws. Unlike most vertebrates, lampreys and hagfishes also do not have a backbone. Even so, lampreys are classified as vertebrates because they have rudimentary vertebrae (composed of cartilage, not bone). The hagfishes, in contrast, traditionally were thought to lack vertebrae altogether; hence, they were classified as invertebrate chordates closely related to vertebrates.

Though feeble swimmers, these invertebrate chordates (lancelets) display, in a simple form,

the swimming mechanism of fishes. Coordinated contraction of muscles arranged like rows of chevrons along the sides of the notochord flexes the notochord, producing side-to-side undulations that thrust the body forward. This serial arrangement of muscles is evidence of the lancelet's segmentation. The muscle segments develop from blocks of mesoderm called somites, which are found along each side of the notochord in all chordate embryos.

The loss of chordate characters in the adult stage of tunicates appears to have occurred after

the tunicate lineage branched off from other chordates. Even the tunicate larva appears to be highly derived. For example, tunicates have nine Hox genes, whereas all other chordates studied to date—including the early-diverging lancelets—share a set of 13 Hox genes. The apparent loss of four Hox genes indicates that the chordate body plan of a tunicate larva is built using a different set of genetic controls than other chordates.

viviparous

the young develop within the uterus and obtain nourishment prior to birth by receiving nutrients from the mother's blood through a yolk sac placenta, by absorbing a nutritious fluid produced by the uterus, or by eating other eggs. The reproductive tract of the shark empties along with the excretory system and digestive tract into the cloaca, a common chamber that has a single opening to the outside

Reptiles such as lizards and snakes are sometimes described as "cold-blooded" because

they do not use their metabolism extensively to control their body temperature. However, they do regulate their body temperature by using behavioral adaptations. Many lizards bask in the sun when the air is cool and seek shade when the air is too warm. These reptiles are ectothermic, they absorb external heat as their main source of body heat. An ectothermic reptile can survive on less than 10% of the food energy required by a mammal of the same size. The reptile clade is not entirely ectothermic; birds are endothermic, capable of maintaining body temperature through metabolic activity

Some species of sharks are oviparous

they lay eggs that hatch outside the mother's body. These sharks release their fertilized eggs after encasing them in protective coats.

Ovoviviparous

they retain the fertilized eggs in the oviduct. Nourished by the egg yolk, the embryos develop into young that are born after hatching within the uterus

Tiktaalik

transitional fossil between fish and tetrapods. had fins, gills, and lungs, and its body was covered in scales. But unlike a fish, Tiktaalik had a full set of ribs that would have helped it breathe air and support its body. Also unlike a fish, Tiktaalik had a neck and shoulders, allowing it to move its head about. In addition, the bones of Tiktaalik's front fin have the same basic pattern found in all limbed animals: one bone (the humerus), followed by two bones (the radius and ulna), followed by a group of small bones that comprise the wrist. Tiktaalik's pelvis and rear fin were larger and more robust than those of a fish. Although it is unlikely that Tiktaalik could walk on land, the skeletal structure of its fins and pelvis suggests that it could prop itself up and walk in water on its fins.

The other main diapsid lineage, the archosaurs, produced

turtles, crocodilians, pterosaurs, and dinosaurs

some sharks and many skates and rays spend a good deal of time resting on the seafloor. When resting, they

use muscles of their jaws and pharynx to pump water over the gills.

Frogs

use their powerful hind legs to hop along the terrain. "Toads" are frogs that have leathery skin or other adaptations for life on land. A frog nabs insects and other prey by flicking out its long, sticky tongue. Great variety of adaptations that help them avoid being eaten by larger predators. Skin glands secrete distasteful or even poisonous mucus. Many poisonous species have color patterns that camouflage them or have bright coloration, which predators appear to associate with danger

The diapsids are composed of two main lineages. One lineage gave rise to the lepidosaurs

which include tuataras, lizards, and snakes. This lineage also produced some marine reptiles, including the giant mososaurs. Some of these marine species rivaled today's whales in length; all of them are extinct.