An Online Introduction to the Biology of Animals and Plants

Key Concepts

Section 3

Chapter  10

The Vertebrates

Vertebrates - Chordates with a Few "Extras"

Vertebrates, being a subphylum of the Chordates, have the basic list of chordate features, but have a few others as well:

- Vertebrate ectoderm forms a skin with two layers, an epidermis over a dermis.

- A vertebral column / backbone, which is both the central component of the skeletal system and a protector of the spinal cord, the trunk of nerves that runs under it.

 - A skull, usually, to protect the brain.

- Usually limbs or fins in matched pairs, which usually show serial homology.

- A circulation system with heart chambers, red blood cells to carry oxygen, and white blood cells to aid immunity.

- An endocrine system of hormone-producing glands.

There are Many Vertebrate Subgroups

Here is a list of the basic vertebrate subgroups with some distinguishing features:

- The jawless fish, not quite full-fledged vertebrates but much closer than lancelets are.  These lack jaws (their teeth are embedded in tough rings around the mouth opening) or paired fins, and have other features not found in advanced vertebrates (for example, they retain their notochords through their lives).

- The cartilage fish, whose skeleton is made of cartilage, making it less rigid than the bone skeletons of other vertebrates.  Although thought to be more primitive than other groups, some of the cartilage fish have been shown by recent research to be much more "evolved" than folks really expected.  There are comparatively few fresh water species - this is considered a marine group.

- The bony fish, with bone skeletons.  This group appears to have evolved in continental fresh water systems and then moved back out into the oceans.  They developed the water-proofing and shallow-water adaptations that made land vertebrates possible.  At a critical point in evolutionary history, this group could have either become slow-moving but heavily-armored, or quick but more vulnerable - the former seemed to be the most "popular" construction.  However, the survivors were mostly in the last group, but who knows why?  Of the vertebrates, the bony fish have the most named species.

- The amphibians, the first vertebrates to really exploit land environments.  However, like early plants, they remained limited by a need to find open water for the spread of sperm to egg cells.  Eggs are fertilized in water and usually an aquatic larval form develops, which may or may not go through metamorphosis to become a land-living adult.  Many amphibians require wet environments (and breathe through their thin, moist skin), but many have adapted to dry conditions where open water is available only when mating is done.  One odd characteristic of the group is that adults usually have relatively huge mouths.

- The reptiles are one of the vertebrate groups fully adapted to land - their epidermis produces a nice waterproofing layer of scales, breathe with internal lungs, transfer sperm into the females body with no need for open water (this is called internal fertilization), and produce amniote eggs that have special membranes to minimize water loss.  Reptile breathing, with a rib cage around the lungs, is more efficient than most amphibian respiration.  Reptiles have been a very successful land group - especially if one includes the dinosaurs as reptiles - and has been limited only by cold climates. 

- The birds, which are quite likely a surviving subgroup of the dinosaurs (who themselves were probably not still reptiles), who produce a highly specialized scale type, feathers, and have many other adaptations that allow them to fly.  It is unclear how many features used in flight might have developed in dinosaurs as a way of making very large animals light enough to move quickly - did birds evolve traits for flight, or just modify traits that were already there?  In any case, birds are also internal fertilizers and produce amniote eggs.  They also are endotherms (commonly called warm-blooded, but that term can be misleading), which means that their bodies work at a set internal temperature that energy is consumed to keep stable.  Endotherms get some advantages, the biggest being a permanently-high metabolism, very good if you might need to take to the air at any moment (the feathers also serve as insulation to minimize heat loss, and may have evolved in ancestral dinosaurs for this reason).  The downside is that a lot of food (and oxygen) is needed to generate that energy, much more than similar-sized ectotherms ("cold-blooded," who mostly work at temperatures matching the environment around them).  Birds have a heart with two separate sides to keep their lung circulation separate from their body circulation and maximize the oxygen-carrying function of the blood.  Birds move on the ground on their back legs, as their dinosaur ancestors did, and their front legs have evolved into wings.  The need to produce wing power has fused most of the bones of the body - birds are flexible in their neck, wings, and legs but can't twist their bodies like other vertebrates - and the front of their rib cages are modified to anchor very large wing-pulling muscles.  Birds are also full of air spaces, not just in their bones (you probably knew that birds "have hollow bones," but so do you - only theirs' have air sacs in them to make them super-light) but in many body cavities - air is much lighter than the fluids which would otherwise be there.  Some of these air spaces are connected to the lung system, further increasing oxygen capacity.  The lack of teeth, which are denser and heavier than any other vertebrate part, is probably also also for weight purposes - beaks are made of a much lighter cartilage-type material.

- The mammals are also endotherms, also with insulation, in the form of fur or hair, and a similar circulation pattern.  Mammal skin is often embedded with glands, and a particular type has become a marking feature of the group:  mammary glands produce nourishment for the young mammals.  The vast majority of the group also do not produce eggs, but rather produce live young that develop either in a pouch or inside a specialized internal organ, the uterus, a structure which is used for egg processing in other animals.  A less well-known feature is that mammals, unlike other animals, can have a variety of different types of teeth in each mouth (and usually get just two full sets during their lives), making them perhaps more evolutionarily flexible.

Jawless Fish - Some Primitives Hang On

There are only two small groups of jawless fish still around, but long ago there were many more.  In the distant past, these fish commonly had armor coverings, but all of those fish went extinct.  The surviving jawless fish (also called Agnathans) are -

- The hagfish or slime eels.  These marine fish are primarily scavengers.  They can protect themselves by producing amazing amounts of nasty mucus when threatened.  They can be a nuisance when trapped in nets or lobster traps, but are a popular food fish in Korea.

 - The lamprey eels.  These are found in both salt and fresh water, and are one of a very few types of vertebrates that are attachment parasites.  They attach to larger fish with a mouth that is like a many-toothed suction cup and rasp away at the hosts tissues, sucking in blood and whatever else comes loose.

Cartilage Fish - Not as Primitive as Their Reputation

People swimming often cant help but worry about whats hidden beneath them, and in the ocean that fear often takes on the image of a shark.  And they are out there, but remember that your chances of being lunch are much tinier than your chance of being crunched in your car on the way home.

Sharks belong to the cartilage fish subgroup.  This group, and sharks in particular, have been around a very long time, probably an example of a successful form in a fairly constant environment requiring little obvious change.  However, just because sharks of today look like their ancestors doesn't mean that they haven't been evolving.  Recent research has found that sharks can be quite bright, that they may have complex social structures, and that some have endothermic brains.  They do have cartilage skeletons, which are perhaps less efficient than bony skeletons, but they lack the buoyancy that allows bony fish to have harder and heavier skeletons.  When a cartilage fish stops swimming, it sinks (which is not so bad if it's close to the bottom...).

There are many types of sharks, from the freely-swimming predators that everyone thinks of to many bottom-dwelling species.  They range from fairly small species up to the world's largest fish, the whale shark, a huge animal that filter feeds on tiny plankton.  A swimming shark keeps itself "aloft" while swimming with fins that act in many ways like an airplane's wings.

The other subgroup of cartilage fish is the rays and skates, which swim with a flying motion similar to a birds.  For the most part, though, these are bottom-dwellers, laying camouflaged on the ocean floor.  Although there are differences between rays and skates, they look pretty much the same.

Bony Fish - A Success Story

It is thought that the bony fish evolved in fresh water.  Adapting to a very dilute environment may have led to some dilution of tissue and cell fluid (of course, it couldnt function if it was as dilute as fresh water), and even the modern marine species still have tissue fluid more dilute that the water around them (most marine invertebrates are in a dilution / osmotic balance with their surroundings).

Bony fish are the largest vertebrate group, with by far more named species than any of these other groups.  They have a couple of important features that have helped them to prosper.  

Obviously, bony fish have a calcified bone skeleton, which provides better muscle leverage than cartilage.  It is heavier, but the next feature offsets the extra weight.

Bony fish usually have a gas-filled internal organ called a swim bladder.  This is used to adjust the buoyancy of the fish to the water around it (deeper water is denser, affecting buoyancy) so that, unlike cartilage fish, a bony fish doesn't sink when it stops swimming, it can just "hang," reducing the amount of energy it has to expend.  Oddly enough, the swim bladder is thought to have arisen as a lung, used by fish whose water body homes dried out periodically.  This seems unlikely, but the progression of evolution of this group is pretty confusing.

Whereas cartilage fish have gill slits, bony fish usually have an operculum (opercle),  a rounded cover over the gills.  This may make water flow across the gills more efficient, and it often seems to make pumping water through the gills (essential if a fish isn't swimming) work better.

The two major subgroups of bony fish include the lobe-finned fish, whose fins are supported by limb-like structures that protrude from the body.  Few species in this group still exist, although at one time it was quite a successful group.  The other group is the ray-finned fish, whose fins have no such limb-like structures.  At one time, the lungfish were considered a third subgroup, but rarely are any more - strangely enough, some books now include them with the lobe-finned, some with the ray-finned fish.

The progression from fish to 4-legged land vertebrates (tetrapods) is a subject of much controversy.  Lungfish were once the leading candidate as ancestor types, but there seem to be too many anatomical differences between them and early tetrapods.  Lobe-finned fish are thought to be the ancestral type by some, with the lungs evolved from the swim bladder (which some think evolved from a lung - told you it was confusing!), ray-finned by others, with lungs evolving from brood pouches, "cheeks" where eggs and babies could be protected and supplied with oxygen from the linings.

Amphibians - Part-Timers

Probably the first vertebrates to really live in the world beyond the lakes and seas were the amphibians.  For a while, when they had the run of the place, some got to be as big as cars, but many species were eventually replaced by later animals with better abilities for a terrestrial (land-living) existence.  Amphibians, like early plant groups, were able to handle some aspects of life on land, but some problems kept them mostly in moist areas with at least periodic open water available.

The amphibians have two major subgroups:  the frogs and toads and the salamanders and newts.  Frogs and toads are hoppers, with large back legs and short front legs.  In general, frogs favor wetter environments and are less resistant to drying than toads.  Toads tend to use their skin less as a respiratory surface, allowing the skin to be more waterproofed.  These amphibians usually go through a fish-like larval stage, commonly called a tadpole.  Salamanders and newts are lizard-shaped crawlers, and sometimes confused with lizards, although they are not scaly.  Salamanders usually live on land as adults, while newts often spend just some time on land before living in water as adults - they usually have flatter tails, used for swimming, than salamanders.

Amphibians skin is very glandular - the glands help keep the skin moist.  But these glands have often been adapted for defensive purposes, and many amphibians are noxious if not actually poisonous if eaten.  Some South American frogs produce poisons so nasty that a person can get ill from just handling them.  And like many nasty creatures, including many in other groups, they are colored to stand out in their environments (called warning colors / displays);  predators tend to avoid "prey" that can be easily seen - or they soon learn to.  Warning colors tend to be very contrasty and bright, and can be found in things like millipedes, centipedes, bees, and skunks.

A current scientific mystery is why, in recent decades, worldwide amphibian populations seem to be shrinking.  All sorts of explanations have been given:  global warming, greater sun ultraviolet radiation due to loss of atmospheric ozone, pollution, parasites / disease, and others.  They may all be in play.

Reptiles - Multiple Groups Under the Same Heading?

With lots of four-legged critters running about on land those many years ago, did the amphibians give rise to the reptiles and the reptiles to the dinosaurs, the birds, the mammals?  That seems  to be the accepted story.  The transition from amphibians to reptiles has not been found in the fossil record, but may have involved small salamander-type amphibians that were unlikely to fossilize.

The amniote eggs, waterproof but able to breathe, that made real land-living possible are a feature found in all of these groups.  Reptile eggs (and mammal eggs in the couple of species that make them) are leathery.  The internal structure used by live-bearing vertebrates, including the mammals, is still based on amniote membranes and connections.

The subgroups of reptiles are a motley crew that share the same type of scaly skin and some skeletal features, but are different enough that they could easily all be considered separate classes.  And the more that is discovered about dinosaurs, the more it seems that they, too, were a distinctly separate group.

Probably the reptile prototype is the lizards and snakes.  Lizards are long, slung between the "elbows" of their legs, and during the ages of dinosaurs included forms that flew through the air, the pterosaurs, and swam in the seas, the mosasaurs and plesiosaurs.  The snakes are obviously related to the lizards, but how the transitions happened is not clear - one of the more recently-proposed theories makes some subgroup of mosasaurs the ancestors of snakes.

The crocodilians, which include the alligators as well as crocodiles and several other similar animals, moved back from the land into the water, where they have become formidable predators.  This is an extremely old group of reptiles.

The turtles are the least like the other reptiles, with several unique features besides their shells, which are actually highly modified scales supported by a modified rib cage.  In general, the water-living turtles are predators, and the land-living tortoises are plant-eaters.

The Ups and Downs of Birds

The idea that the birds were descended from dinosaurs is both fairly new and very old.  One of the oldest true bird fossils, Archaeopteryx, had been classified as a dinosaur until a low-angle lighting of the fossil showed the imprints of feathers around it.  Recently, it has become more and more accepted that the two-legged dinosaurs, of the group that includes Tyrannosaurus rex, often, maybe always, had feathers, and they had an offshoot group that took to the air (the "flying dinosaurs," pterosaurs, as mentioned earlier, were really lizards).  Some fossils found recently in China seem to indicate that feathers were fairly common in 2-legged dinosaurs and may have started as insulation or camouflage and been put to work on wings later.  By the end of the Age of Dinosaurs, several groups of birds had evolved.  Their size and flying ability seems to have gotten most of them past the mass extinction that wiped out the others, making them the only surviving dinosaurs.

Ironically, much later, in some isolated parts of the world such as New Zealand and South America (before it connected to North America), huge flightless birds that "in person" would have been very similar to tyrannosaurs were the top predators, living a very tyrannosaur-type existence.

A feature of many modern birds is migration.  This movement from one place to another may be done for breeding purposes, but usually its just to follow the food.  The birds that stay behind during the temperate winters can still find food, while the migrants depend on something that isnt around (bugs) or accessible (worms or algae or fish) during the cold season.  Migrant birds find their ways using many navigation methods and types of programming.  For some, the direction and duration of flight is almost all instinctive, while for others its learned from elders (which makes it an epigenetic trait).  Some navigate using landmarks, some use the night sky, some have internal compasses.

Birds can be quite intelligent and have complex social groups.  Some parrots can not only mimic human speech but seem to recognize how the language works on a simple level.  Crow babies stay in a family unit for years, learning how things are done.  In captivity, they have been shown to have a complex understanding of tool use. They do a lot with very small brains, which makes one wonder if the assumptions about dinosaurs being pretty stupid isn't a bit stupid itself.

Our Fuzzy Group - The Mammals

At one time, the reptiles that would go on to be "reptiles" but the ones that would go on to be mammals were very similar, so much so that fossils had to be divided up based on a kind of artificial benchmark.  The bones that are in a modern mammals middle ear are on the jaw of a modern reptile, so it was decided that fossil classification would use that detail to decide what was what.  Sometimes that produces some very lizardy-looking mammals (the famous fin-backed Dimetrodon that seems to be in every kiddie bag of plastic dinosaurs is an example) and some doggy-looking reptiles.  It took a bit of evolution for the groups to pick up their distinctive features.

Through the Age of Dinosaurs, mammals succeeded by staying out of the Big Boys' way - our ancestors from that time period were small, probably night-hunting bug eaters and scavengers.  When the dinosaurs became extinct, our ancestors had the perfect jobs to survive those bad times, eating the dead or eating the insects that ate the dead.  Afterward, mammals evolved into many of the niches that the dinosaurs had previously occupied, although they never quite got as big (on land, that is).

The early mammals were almost certainly egg-layers, and there still exist a couple of mammals that lay eggs:  the famous platypus and the less-famous echidna.  In these primitive mammals, newly-hatched babes lap up a milky secretion produced by the mother;  more modern mammals collect the milk ducts at the surface in nipples.

The egg-laying mammals, called monotremes, were eventually succeeded by mammals whose young stayed in the mothers reproductive system for a while, then crawled out and up into a pouch, where they latched on to a nipple and continued to develop.  These were the marsupials, which once were "state of the art" mammals.  Later, placental mammals evolved, who could keep their young in the females system and feed them with connections to the mothers blood though a placenta.

On the wandering continents of Australia and South America, marsupials were pretty successful.  In the rest of the world, placentals arose and replaced almost all of the marsupials.  When South America collided with North America and a land bridge, Panama, allowed mixing of the animals, the placentals moving south seem to have driven the remaining marsupials (except for the opossum) out of business.  Australia is still fairly isolated, dealing mostly with late-arriving placentals that flew in (bats) and those that humans have brought in, but that causes problems as rabbits and rats and dogs compete with native mammals.

There are several orders of placental mammals.  Some are small, with one or just a few species, such as the elephants or aardvarks.  The larger orders include such odd mammals as the bats (no, they're not birds, they're furry) and the whales, as well as flippered orders such as the seals.  The biggest orders are the bats (that flying ability is still quite an advantage - they just needed to find the right work shift that the birds had not used much) and the rodents, which include mice, rats, porcupines, and beavers.  There are a few different orders of mammals with hooves, divided by how many functional hooves each foot has.  And there is our group, the primates, characterized by our opposable thumbs (and opposable big toes, usually), good eyesight, and relative intelligence.

Informative Links

KEY CONCEPTS -
Click on term to go to it in the text.
Terms are in the order they appear.

Vertebrate Features  
Epidermis & Dermis  
Backbone / Vertebral Column w/ Spinal Cord  
Skull  
Serial Homology  
Blood Cells  
Endocrine System  
Jawless Fish - Basic Features  
Cartilage Fish - Basic Features  
Bony Fish - Basic Features  
Bony Fish - Evolution  
Amphibians - Basic Features  
Metamorphosis  
Reptiles - Basic Features  
Internal Fertilization  
Amniote Eggs  
Birds - Basic Features  
Endotherms / "Warm-Blooded"  
Mammals - Basic Features  
Jawless Fish - Subgroups  
Lamprey Eels  
Slime Eels / Hagfish  
Cartilage Fish - Subgroups
Sharks  
Rays & Skates    
Swim Bladder  
Operculum  
Bony Fish - Subgroups  
Lobe-Finned Fish  
Ray-Finned Fish  
Tetrapods  
Vertebrate Lungs - Evolution  
Amphibians - Subgroups  
Frogs & Toads  
Tadpole  
Salamanders & Newts  
Warning Colors 
Reptiles  - Subgroups  
Lizards & Snakes  
Crocodiles & Alligators  
Turtles & Tortoises  
Birds - Evolution & Dinosaurs  
Archaeopteryx  
Migration  
Mammals - Evolution  
Mammals - Subgroups  
Monotremes  
Marsupials  
Placentals  
Placental Subgroups