Classification of the Chordates:

The phylum is divided into three subphyla. The division is unequal with a very few primitive aquatic marine forms in the first two subphyla and the vast majority of the phylum in the last, large subphylum Vertebrata. In our discussion of the division, we will look at the evolution of the Vertebrata as a way of examining the Phylum. You should attempt to follow the changes that occurred going from; sluggish to active animals, aquatic to terrestrial animals and primitive to advanced forms.

Before proceeding with the evolution, the following is a list of the major groups of Vertebrates. Classification below the subphylum level is currently under critical review. The cladists are claiming the present evolutionary scheme is wrong and that their method is a superior indicator of phylogenetic relationships among the chordates. The more traditional taxonomists view the cladistics method as little more than a passing fad. To avoid much of the discussion and arguments, we will adopt a nonformal system of grouping of the vertebrates.

Subphylum: Urochordata. tunicates

Tunicate anatomy

Subphylum: Cephalochordata. lancelets

A lancelet

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Subphylum: Vertebrata.

The most primitive of the chordates evident in the fossil record are the jawless Ostracoderms. From this ancestral form two lines of vertebrates evolved. A jawless group, the Agnathans, and a group that developed jaws, the Gnathostomatans. The modern representatives of the Agnathans are the hagfish and the lampreys.

The first jawed fishes, the Placoderms, are extinct but from this primitive ancestor, arose the present day cartilaginous and bony fish. The cartilaginous fishes, Chondrichthyes, include the sharks, skates and rays. The bony fishes, or Osteichthyes, include two groups, the ray finned and the fleshy finned fishes. The ray-finned fishes represent the majority of the present fish. The flesh finned fishes, the lung fish and the lobe finned fish, are a small almost extinct group that is believed to have provided the ancestral stock for the remainder of the vertebrates.

The lobe finned or lungfish gave rise to a once dominant, but now marginally successful group of terrestrial animals, the amphibians. The amphibians were the ancestors of the reptiles, and highly successful birds and mammals.

This discussion will attempt to follow this line of vertebrate evolution.

The Ostracoderms:

The Ostracoderms were tiny freshwater fishes that lived during the Ordovician and Silurian periods, 500 to 400 million years ago. These animals were characterized by a number of features.

- They were small (20cm)

- They were jawless, bottom feeders.

- Their endoskeleton was cartilaginous but they were covered with small bony plates.

- They used a muscular pharynx to suck food into their mouths and to exchange gases in respiration.

- Their gill slits were permanently open with no protective operculum.

- The fins of these fish were small usually limited to median fins and the tail was hypocercal or turned downward. As the fish swam, this fin configuration drove the animal down into the bottom where its food was located. Rarely were there any lateral fins, but a few species had paired paddle like extensions.

These fish were probably erratic, inefficient swimmers. The bony plates were useful for protection but inhibiting when rapid swimming was required. Their permanently open anteroventral mouth was only good for ploughing through the bottom sediments. They probably could not digest much of their intake. The lateral eyes were better than the site organs of most insects but lacked the stereo or depth perception of other vertebrates with more frontal eyes.

Fossil Ostracoderm

The Ostracoderms existed in two groups, the more primitive heterostracans and the cephalapsids. The cephalapsids improved over the heterostracans because they had lateral stabilizers for more control of their swimming. They still had no axial skeleton and the mouth lacked jaws and teeth. These first primitive vertebrates were probably quite susceptible to predation from larger invertebrates and eventually jawed fishes. They had little protection other than limited mobility and heavy cumbersome bony armour plates.

The Agnathans:

The agnathans represent a small group of organisms that evolved from the Ostracoderms. The only extant forms of the line are the modern lamprey and hagfish. These animals are long, slender eel like organisms. They have median fins but lack the swimming control produced by lateral fins. Agnathans swim in a lateral undulating fashion that may be inefficient, but adequate for their lifestyle. These animals exist either as bottom scavengers or ectoparasites of other fish.

The most consistent feature of these animals, is their lack of jaws. Like the Ostracoderms, they are unable to grasp prey or tear up food. Instead, they ingest material by a sucking action. The Agnathans have lost the bony armour of the Ostracoderms, and utilize a cartilaginous skeleton.

Lamprey mouth

Sea Lamprey

The evolution of the jaw must have been of major importance to animals, since the jawless vertebrates are either relegated to a small group in a few habitats or they are extinct.

Gnathostomatan.

These fish represent the other major line of vertebrate evolution. The difference between the gnathostomatans and the agnathans is the presence of jaws. Following or concomitant with the evolution of the jaws, other features appeared in these animals to make them far more efficient than the jawless organisms. The jawed fishes arose at some time prior to the extensive radiation of the jawless fish (mid-Silurian) from some less specialized jawless form. The niches occupied by the ancestors of the lampreys and hagfishes were not later occupied by the jawed fish, but almost all other conceivable aquatic niches have jawed animals.

Although there is no fossil evidence to support the theory, it is believed that the jaws evolved from modifications of the gill arches (structures that support and keep the gills from closing or collapsing).

Fish paleontologists are still not in absolute agreement on the sequence of evolution of the jawed fishes. There is however, a consensus that there were several lines of evolution that radiated from one or more ancestral forms. Without being unduly concerned with precise origins, we will discuss the major groups and trends that occurred in the fish evolution. The groups include the Placoderms, Chondrichthyes, Actinopterygii and the Sarcopterygii.

Placoderms:

The Placoderms are extinct fish that are believed to be the first of the jawed fishes. There are several fossil forms of Placoderms. These fish inhabited fresh waters. They were larger than the Ostracoderms with some fossils ranging up to 10m in length. Some of the more important features of this group are:

- They developed jaws to hold prey (food) and teeth (modified dermal bones) to grip and tear the food. This meant the Placoderms could feed on much larger food, thus increasing their potential food supply. They were both carnivores and herbivores.

Dunkleosteus from the upper Devonian.

- The anterior had an armour of bony plates but the posterior of the animal had modified dermal bones forming scales. This left the posterior, where most of the movement is generated, more flexible. It also permitted the animal to raise its head, thus allowing for a greater opening of the mouth.

- The skeleton was bony.

- The muscles increased in size and efficiency.

- The placoderms had a heterocercal tail which thrust them upwards as they swam.

- The paired lateral fins evolved with the Placoderms. This allowed a much greater control or refinement of their swimming motion.

- A few developed a lung like ingrowth from the pharynx. They also retained their gills.

While the Placoderms probably represent the first jawed fish, it is suspected that they did not give rise to any of the modern fishes. Instead, the modern fish arose from some other primitive jawed ancestor that evolved some time later than the Placoderms.

Evolution of Jaws

Jaws are thought to have evolved from the modification of the primitive gill arches.

Chondrichthyes:

The Chondrichthyes are the modern cartilaginous fish. They arose from an early Gnathostomatan. They are now a small group of marine carnivores but at one time were the dominant aquatic organisms. The present Chondrichthyes include the sharks, skates and rays. They have not undergone the same extensive radiation as the bony fish, but their well developed sense organs, powerful muscles and predacious nature ensure their continued survival. Features of this group include:

- A ventral jaw with replaceable teeth. The limited mobility of the jaw means that the shark must thrash around to break up its prey.

- A heterocercal tail drives the animal upwards as it swims. This is beneficial because the fish is heavier than water and has no swim bladder for buoyancy (although the large lipid filled liver does provide some resistance to sinking). This means the sharks must continually swim to avoid sinking.

- The lateral fins are well developed to control pitch but are not for braking. In the skates and rays, the lateral pectoral fins have been greatly enlarged and the animals beat them like wings when swimming.

- The gills are not protected by an operculum.

- The extant Chondrichthyes have a well developed sensory system that permits rapid and accurate detection of danger or prey at a considerable distance.

For all their sinister and fearful reputation, the sharks, skates and rays are graceful swimming machines.

Actinopterygii: Bony Fishes

The Actinopterygii include all the modern bony, ray-finned fishes. These well adapted and successful aquatic organisms have occupied almost every conceivable aquatic niche. They are equally well suited to marine and freshwater environments. The ray-finned fishes represent the largest single group of chordates, with over 20,000 extant species. In their environment, they live as carnivores, herbivores, parasites and scavengers. Some of their more successful features are:

- The presence of well developed lateral fins with a bony peripheral skeleton to provide delicate articulations. The support for the fins comes from a series of fine delicate bony rays that are attached to and extend from the skeleton.

- The homocercal tail (caudal fin) provides a powerful lateral of forward thrust without driving the animal up or down in the water. The pectoral and pelvic fins act as stabilizers and provide a system to generate pitch, yaw, roll and braking as required for highly manoeuvrable and controlled swimming.

- The scales of the Actinopterygians are of light bone and they are small or absent in fast swimmers (reduces drag).

- Drag during swimming is also reduced by the streamlined (teardrop) shape and the production of surface mucous.

- Most of the bony fishes have a sac like structure leading from the pharynx. The connection to the pharynx may be sealed off during development. This sac, the swim bladder, is a major development to the bony fishes. It provides the fish with a neutral buoyancy. This in turn, permits the fishes to hover in the water and not have to swim constantly and thus expend energy to avoid sinking. The reduction in energy expenditure reduces the amount of time required for feeding.

- Respiration in the aquatic environment is difficult for two reasons. The medium contains far less oxygen per unit volume than air, and its viscosity makes it more difficult to move during irrigation of the respiratory surfaces or gills. The bony fishes have developed extensive gills to maximize the surface area for gas exchange. They use a counter current system where the blood in the gill capillaries flows in the opposite direction to the water flowing over the gills . This allows for a higher saturation of the blood with oxygen. Finally, the bony operculum provides a degree of protection for the delicate gill tissues and by moving the operculum, water can be forced over the gills without the fish being compelled to move.

Other systems within the bony fishes are not as well advanced. The circulatory system has only a two chambered heart and the blood flow to the body is under very low pressure. The brain is poorly developed relative to higher chordates. The sensory and to some extent the motor coordinating centres of the brain are well developed, but the overall coordinating and reasoning centres are primitive.

Sarcopterygians:

These are also a small ancient group of fishes of which the majority are extinct. These are the `fleshy-finned' fishes and the two major types are the lobe-finned fish and the lungfish. There are only four living genera of the Sarcopterygians. The significance of the group comes from their suspected contribution to tetrapod evolution. The lobe-finned fish are thought to be the forerunners of the terrestrial animals.

 a Coelacanth

The group emerged during the Devonian period when freshwater bodies were plentiful and fishes had radiated to almost every conceivable niche in the aquatic environment. Two theories prevail as to the most profound impact of the tetrapod evolution.

One theory suggests that the tetrapod development was in response to smaller bodies of water periodically drying. The fishes in those areas that had ability to move on land would crawl (walk) to other larger bodies of water. The second theory proposed that the first land tetrapods arose from lunged, air gulping fishes that, to avoid competition and predation in the aquatic habitats, began to occupy the coastal regions. As their limbs developed, they strayed further on to land. At the time of this evolution, the land plants were flourishing and only a few arthropods occupied terrestrial habitats. Therefore, food was plentiful, competitors were scarce and predators were non-existent. This would have provided all types of selection pressure in favour of moving onto land.

The Sarcopterygians had two main features that gave them an advantage over other vertebrates in this transition. They had rudimentary but functional lungs as well as gills. There was a canal connecting the lungs with the outside of the body through openings termed external nares. While the lungs were not well developed, the much greater percentage of oxygen per volume of air made it possible for these organisms to breathe.

The second major advantage of the Sarcopterygians was the development of fleshy lateral fins. The `flesh' was in fact muscle that could provide far more support and motility to the fins, particularly on land. The skeleton of the fins extended into the flesh, unlike that of the ray-finned fishes. The pattern of the bones of the fins showed a similarity to the bones of more primitive tetrapods. The only real drawback to their design was the fact that there was no girdle connecting the peripheral bones or skeleton to the axial skeleton. This would mean there was a general weakness, but at least the beginnings of the tetrapod plan were present.

 

Amphibians:

From an evolutionary point of view, the Sarcopterygians were the last of the fishes. The next chordates to evolve were true tetrapods and thus were considered Amphibians. By the end of the Devonian period, there was an animal present that had the rudiments of the Amphibian structure. This animal was an Ichthyostegalia. The main difference between this animal and the lobe-finned fishes was the development of the girdles; skeletal bones that connected the axial skeleton of the vertebral column with the bones of the lateral appendages. Other skeletal strengthening involved the rib cage and the cranium. The Ichthyostegalia still retained the caudal fin (with fin rays) and scales of its fish ancestors. The median fin still remains in the larval forms of the Amphibians.

Ichthyostegalia

The ancient and modern amphibians have features that enhance their survival on land but other features that limit or restrict this existence. The majority of the Amphibian evolution took place during the Carboniferous period. At this time, the Earth was warm, and humid. Landmasses were often swampy. There was an abundance of invertebrate insects and other forms of food. The conditions were such that there was not a great deal of pressure to develop into a truly terrestrial animal. Features of the Amphibians that were advanced include:

- The development of appendages that were well muscled, and supported by an axial and peripheral skeleton. The pectoral girdle moved back from the head to permit more head movement.

- The lungs became further developed and the skin was well vascularized to act as another site of gas exchange.

- The circulatory system had a three chambered heart which provided more pressure to the peripheral arteries than that of the two chambered heart of the fish.

All of these features increased the mobility of the Amphibians.

Other features of the Amphibians illustrate their primitive nature and suggest that they are actually transitional and not truly terrestrial animals. These include:

- The Amphibians are ectothermic. Their body temperature is determined by and varies with the external temperature. Ectothermic animals become vary sluggish during cooler temperatures and when it gets sufficiently cold, they must hibernate or die.

- Because the amphibian breathes through its skin, the skin itself must be thin and moist. Amphibians lose a considerable amount of water through their skin. They must keep it continually moist to prevent lethal desiccation.

- The amphibians must reproduce in the water. Fertilization is external and without a covering of water, the eggs (zygotes) would soon dry up. The amphibians also have an aquatic larval stage that in many ways is more closely related to a fish than a terrestrial animal.

Modern amphibians

Reptiles:

This group of terrestrial animals forms the evolutionary base of the rest of the tetrapods, including the modern reptiles, birds and mammals. The origin of the reptiles themselves is not certain. The most current thinking has them originating during the late Carboniferous to early Permian periods. Features of the reptiles allowed extensive radiation, producing several lineages. To simplify the discussion, only major features of the group will be discussed, along with their success and limitations. The first reptiles were probably small lizard like insectivorous animals. They arose from an early Amphibian. During the evolution, certain changes took place.

- The most obvious of these was the positioning of the legs more directly under the animal. This position provided more support than the splayed arrangement of the Amphibian legs.

- The jaws became better adapted for biting and tearing.

- The body of reptiles is covered with horny epidermal scales to reduce water loss and provide protection.

- Respiration is no longer through the skin, but only through internally protected and moistened lungs.

- The paired limbs usually have five toes and are variously adapted for swimming, running, climbing, although they are absent altogether in the snakes.

- With the exception of the crocodiles, the Reptile circulatory system still has a three chambered heart, but the blood from the lungs is not mixed with the deoxygenated blood.

- The excretory waste of the reptiles is uric acid unlike the dilute, water wasting urine of the Amphibians.

- The brain of the reptile shows the first cerebral cortex of the vertebrates.

The reptiles are still ectothermic animals so they must live in favourable climates or hibernate during cold periods.

One of the most successful aspects of the reptile evolution was in their reproduction. The reptiles have several features that greatly enhanced terrestrial survival.

- Fertilization is internal, therefore the gametes were not subjected to desiccation.

- The eggs (zygotes and young embryos) were covered with a tough, water resistant, leathery or calcareous shell so they may be laid on land instead of water.

Modern Reptiles

- The egg, an amniotic egg is one of the most significant features in the reptile evolution. The egg contains several extraembryonic membranes that compartmentalize the interior and give it several different functions. The chorion provides a special hard covering that is permeable to respiratory gases (O₂ and CO₂) while being impermeable to water vapour. The allantois is a storage reservoir for metabolic waste products such as nitrogenous compounds. The amnion is a fluid filled sac that acts as a cushion for the embryo and also prevents desiccation. The yolk sac contains food for the embryo, thus eliminating the need for a larval stage.

amniotic egg

In this photo you see a baby Komodo Dragon emerging from it's egg.

Zoologists believe that the reptiles provided the stem or base for all the remainder of the vertebrate evolution. From the ancestral reptiles, several lines emerged. Some produced the different types of modern reptiles, but one evolved into the birds and still another gave rise to the mammals. Fossil evidence indicates that this was not a straight-line evolution, but rather an early divergence within the reptile group. It is interesting the birds did not arise from the flying reptiles such as the Pterosaurs.

Birds:

Birds are more closely related to the reptiles than any other group. They have departed from the reptiles in only two major ways, but these two changes brought about a group that is second only to the fishes in its adaptive radiation. The birds have not stopped evolving but they have not given rise to any other group.

The two major ways that birds have changed from the reptile are in their ability to fly and the fact they are endothermic (warm-blooded). These two features have `directed' bird evolution and promoted the divergence from their reptile ancestors.

Archaeopteryx

Archaeopteryx fossil

- Feathers were one of the first and certainly the most successful structures to have appeared in the bird ancestry. It is believed the feathers first evolved from reptile scales as an adaptation to permit endothermy. As endothermic organisms, birds maintain a constant body temperature between 35 and 40 C. The insulation provided by the feathers (think of an eider down jacket) greatly reduces the amount of energy required to keep up the body heat. Without this insulation, the birds would be forced to eat impossible amounts of food and undergo cellular respiration at rapid rates just to generate the heat.

The advantage of endothermy is that it allows organisms a degree of freedom from the environment. Enzyme activity is positively correlated with temperature, at least to a certain degree. By keeping the body at a warmer temperature, independent of the outside temperature, the endothermic animals can be highly functional at temperatures that ectothermic animals are inhibited. The birds (and we shall see, mammals) can successfully occupy a wider range of habitat and often function more efficiently in shared habitats than the ectothermic animals.

Other modifications for endothermy are:

- An improved respiratory system maintains an adequate supply of oxygen for cellular respiration. The lung capacity is extremely large for the size of the animal, with air sacs even leading into the large bones.

- Birds have a four chambered heart so the blood goes through a complete double cycle. Oxygenated blood is never mixed with deoxygenated blood and the blood is always pumped through the body under high pressure. This ensures that food and oxygen are transported to the cells quickly. Birds also use a system of peripheral vasoconstrictions to keep the blood near the body core during cold periods.

The feathers are also modified or designed for flight. Their design not only gives them an insulation quality, but also the strength, flexibility and rigidity to support the animal in a medium (air) which provides almost no support itself. The `vanes' of the feathers are made of interlocking sections while the shafts are hollow. This combination provides the strength and yet lightweight for flight. Other flight modifications are:

- A skeletal system of strong often hollow bones. These bones, particularly the large sternum (keel) provide large sites for flight muscle attachment.

- The bones of the leg and ankles are fused to provide a stronger point of thrust for take off and a better cushion for landings.

- The large lungs reduce the weight per volume of the birds, as well as provide O₂ for energy. The circulatory system also supplies a rapid source of food for the cells.

- The beaks are lighter than teeth.

- The shape of the birds is aerodynamically better than that of other tetrapods.

Birds have retained some features of their reptilian ancestors. Their nitrogenous waste product is uric acid. They produce amniotic eggs, but the bird eggs have additional egg protein termed albumin. Their fertilization is internal but development of the egg is always external.

The birds differ from the reptiles in that they provide more care for the young both before and after the eggs hatch. This care may be directed by the more advanced brain development of the birds.

Some modern birds

Mammals:

The mammals represent the top of the evolutionary line of the deuterostomes. They have ascended from a reptilian ancestor as did the birds. Their evolution was also directed by a relatively few changes from the reptiles. Like the birds, the mammals are also endothermic. This gave them the same advantages over the other vertebrates as was found in the birds. Unlike the birds, the mammals did not also adapt for flight. Only a few mammals such as the bats are able to fly. Their evolution came after the mammals had already radiated from the reptiles. Adaptations found in the mammals that can be at least partially attributed to endothermy are:

- The development of hair to act as an insulation as did the feathers.

- The deposition of a fatty layer just under the skin. This not only provides an insulating blanket, but also a ready, high-energy food supply to maintain the higher body temperature.

- The mammals have sweat glands to reduce the body temperature if it should exceed the optimum level for enzyme activity.

- The four chambered heart, similar to the birds, greatly increases the efficiency of gas and food transport. It also provides heat in the form of warm blood that has been heated in the body core.

- The mammals underwent trophic specialization that favours increased efficiency in energy acquisition and the utilization of new food sources, not exploited by the insectivorous ancestors.

- The mammals developed the first muscular diaphragm that increases the ventilation of the lungs.

There are other areas that the mammals have developed during their evolution. The brain, in particular, is better developed in the mammals than in any other group of animals. All areas of the brain, sensory, motor, and reasoning centres have been specialized. The cranium has enlarged and been morphologically modified to meet this change in the brain capacity. The thought centres or cerebral cortex is particularly well developed in the mammals.

Finally, the last area of mammalian evolution to be modified is the method of reproduction.

Monotremes

- The most primitive mammals are still egg laying animals. These Monotremes, like the duck billed platypus, breed once a year, have internal fertilization and then lay amniotic eggs. The eggs are protected by the parents, and upon hatching (approximately 10 days after laying), the young immediately began to feed on milk from the mammary glands of the mother.

This is an Echinda, an egg laying mammal.

Marsupials

The second mechanism for reproduction in the mammals is found in pouched animals (Marsupials) such as the kangaroos and opossum. In these animals fertilization still occurs on an annual basis. The young begin development in the uterus, but not attached to a placenta. Instead, soon after the embryos are able to generate any form of walking on crawling movement, they leave the uterus and crawl into the pouch. Inside the pouch they attach to the mother's teats and continue development.

Some Australian marsupials.

Eutherian (Placental) Mammals

The final major variation in mammalian reproduction is the one found most frequently. After internal fertilization, the young embryo attaches to the uterus wall by an area used for the transfer of material to and from the embryo and the female parent. This area, the placenta, is unique to the mammals. The animals with internal embryo development tend to become distended as the young grow and develop themselves. This type of development affords the maximum protection for the embryo. The mammals also tend to give their young more post-natal care than do other animals.