Biology of Animals & Plants - Vertebrates
Vertebrates - Chordates with             a Few "Extras"
 
 
 
Vertebrate s, 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 bird s, 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 cant 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 mammal s 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 doesnt mean that they havent 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 its 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 worlds 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 airplanes 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 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 doesnt 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       bodies 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 isnt 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 type 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.
 
 
 
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.
 
Reptile s             - 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 Bird s
 
 
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 isnt a bit stupid itself.
 
Our Fuzzy             Group - The Mammal s
 
 
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, theyre not birds,       theyre 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.