Vertebrates are the most advanced organisms on Earth. The traits that make all of the animals in this section special are their spinal cords, vertebrae, and notochords. It's all about having a series of nerves along your back (dorsal side). If you are an organism, you can't just have the nerves sitting there. You need to give those nerves support and protection. That need brings us to the backbones and a rod of cartilage called the notochord.
Fifty thousand species might seem like a lot. Compared to the invertebrates, there are not that many species of vertebrates. You might be asking why. One reason is that vertebrates are usually larger than invertebrates. They need more space. Another reason is that, even though they are more advanced, there are many limitations on the environments that are available to them.
Think about it this way. If you are smart mammal, would you rather live near the ocean or in the frozen tundra of the arctic? Many land animals can make that decision and move to more desirable areas for living. Those nicer areas can only support so manyspecies of animals.
Vertebrates are smart. Some of them are very smart. We mean you. Most vertebrates have very advanced nervous systems. While a goldfish might not compare to your intelligence, when you compare a goldfish to a sea anemone, a goldfish is like Einstein. Octopi are probably the smartest invertebrates and may equal or be smarter than some vertebrates. Octopi are the exception in the invertebrate category.
More cool traits about vertebrates are that they have muscles and skeletons. While the materials may vary, muscles allow vertebrates to move around very efficiently and perform complex moves. That ability to move and the intelligence to go with it gives vertebrates such as reptiles and birds an advantage in the natural world.
Fish have had an evolution all their own. When you think of a fish today, you are actually thinking about an advanced organism. They started their development over 500 million years ago as fish-like organisms without jaws. Over many years they developed bones and skeletons. There are four main types of fish.
(1) Jawless fish: Like we just said, the first fish didn't have jaws. It is tough to eat and even harder to survive in the long run. Even with a mouth and a series of teeth to cling to their food, it's still hard to compete with fish that have jaws and mouths. Some species made it to the modern world. One good example is a Lamprey. These fish suck on the sides of other fish. Not a very exciting life.
(2) Fish with cartilage: In our opinion, though not as advanced as fish with real bones, fish with cartilage are the coolest fish out there.Cartilaginous fish include species of sharks, rays, and skates. Sharks are the ultimate hunters of the ocean. They are big, fast, and have very sharp teeth that rip their prey apart. Skates and rays are a lot more docile or non-aggressive. They are usually bottom feeders. The bodies of rays and skates have developed large wings that allow them to glide through the water using their tail as a rudder.
(3) Bony fish with lobe fins: And then the fish with real bones came along. No longer was cartilage the skeleton of choice. Bony fish were able to out-compete most of the fish that had cartilage for skeletons. Scientists classify them into two groups - the LOBE-finned and the RAY-finned. Lobe-finned bony fish include examples such as lungfish and coelacanthus. Until about 30 years ago, scientists thought the coelacanthus was long extinct. One day a fisherman found one in the net and voila!
(3) Bony fish with ray fins: You probably think of these fish when you think of a classic fish. The bony fish with ray shaped fins include goldfish, tuna, and trout, all tasty fish you can eat. Goldfish are for the college guys. They have complex skeletons and are built to move. Some move really fast and are very strong. Something like a tuna can move extremely quickly through the water.
Most of the amphibians are slimy. They have a very good reason. Amphibians are the evolutionary step that happened when animals left the oceans and lakes and came on to land. Some fish (lungfish) can survive out of the water for a while, but amphibians were designed for it. One hitch: they need the water when they are kids. Amphibians also use their skin to absorb oxygenfrom the air. The absorption process is more efficient when the skin is moist all of the time.
It's hard for us to explain what a frog or a toad is. It has four legs. Most amphibians also have moist skin (toads do not) for absorbing oxygen. They have extensions that are like fingers and toes. Like all amphibians, they have a three-chambered heart. We do want to tell you that frogs (and amphibians in general) are dying off. Since they have moist skin that helps them breathe, they are very vulnerable to poisons.
Man's pollution is one of the deadliest poisons of all. As we create more pollution, the delicate way that amphibians breathe is being upset. They are slowly dying off. Unfortunately, they live in areas man may not be, so we will never know.
Salamanders are like frogs, but their body shape is more like a lizard. They are longer and have shorter legs. Often their bodies lay flat on the ground because their limbs (legs) are attached on the side of their bodies, not underneath as in mammals. You will often find salamanders around ponds and other small bodies of fresh water.
Caecilians are wild. They look kind of like snakes but are actually amphibians that found an advantage in not having legs or feet. You might be thinking that doesn't sound like an advantage. You need to remember that when amphibians were the first creatures on land, there were no snakes. There was an entire ecological niche to be filled. Some amphibians evolved to slither along the ground and eat insects and worms. When you have a body like a snake you can get into tight spaces and find tasty insects for your lunch. Those slithering amphibians filled a specialized ecological niche that snakes filled millions of years later.
Let's start with alligators and crocodiles. Of the reptiles on Earth, the alligators, crocodiles, and others of this type are the largest around. Did you know that some of them get larger than 18 feet long (over 6 meters)? This group of reptiles is called Crocodilia even though they aren't all crocodiles. Some scientists think that they are more closely related to birds than any other reptile. If you're out looking for one, you'll find them near the water in warmer climates. Reptiles don't do well in the cold.
Both snakes and lizards are in the order calledSquamata. Snakes are special because they have no legs. You knew that fact. But did you know that snakes don't have ears? They feel vibrations and smell with their tongues. Did you know that snakes can unhook their jaws so that they can swallow prey that is actually wider than they are? Very cool. Don't worry about snakes being slimy. They aren't. They are just very shiny because of their specialized scales.
With so many types of lizards, we don't know where to begin. Lizards have scales just like snakes. They also have legs. They can dig, climb trees, and grab things. It's very handy to be able to grab on to things. Those legs and feet are also placed under the body, so they can walk around without hitting the ground.
Turtles and tortoises are the special species that have shells. Some are found in forests, some are in the water, and some are in the deserts. Turtles and tortoises are able to live anywhere that the temperatures are warm enough. They are also able to live a very long time, sometimes over 100 years. Turtles are usually the ones found near water. Tortoises are found on land and dry areas. Scientists group turtles and tortoises together in the order Chelonia.
Alligators, dinosaurs, lizards, turtles, and snakes. All of them are one type of reptile or another. Dinosaurs, as you know, are extinct. They were killed off millions of years ago, and that extinction allowed the rise of mammals and birds. What makes reptiles so special? Dry skin. While amphibians are dependent on water at some point in their lives, reptiles made it away from the water. Let's look at some of the advantages that reptiles have over amphibians.
Dry Skin: The dry skin allows reptiles to move around for periods of time without water. The dry skin stops evaporation of the organism's moisture.
Eggs: We're not talking about eggs like a chicken egg, but reptile eggs are tough enough to survive on land. Amphibians needed moist areas for their eggs to develop and for fertilization to occur. Reptiles have made the jump to laying eggs in the open and on land.
Skeleton: It is true that fish and amphibians have skeletons. As with other traits, reptiles took it to the next level. They developed larger skeletons that could hold larger organs and help them increase in size. Some of the biggest creatures to ever walk on the planet have been the dinosaurs. See? Those better skeletons paid off.
Leg shift: We didn't have a cute phrase to explain it. Basically the legs of reptiles shifted to the bottom of the body. We told you that amphibians have legs on the sides. Reptiles' legs are directed down towards the ground. That change in direction allows reptiles to pick their bellies off the ground and helps them move more efficiently. It's obvious that this trait doesn't apply to snakes that have no legs. They have their belly on the ground or on a tree limb all day long.
Being in this section, you already know that birds are vertebrates. What is the other major thing to know about birds? They fly. Sure there are other vertebrates that fly (bats), but birds rule the sky. There are small hummingbirds that suck nectar from flowers. There are midsized crows with high levels of intelligence. Then there are massive eagles that are some of the most capable predators on Earth.
Birds weren't always here. Scientists believe special reptiles began to develop bird-like characteristics towards the end of the dinosaur era. A species calledArchaeopteryx had traits such as feathers that may have allowed it to fly. Inside those dinosaurs, scientists also discovered a wishbone (it's big in the bird world). As we continue to discover more fossils, more species with feathers and small forelimbs are appearing in the evolutionary tree.
Over time, bird species developed very efficient respiratory systems, circulatory systems, and the ability to keep warm. The body heat of reptiles changes with the temperature of the environment while birds are endotherms, regulating their own body temperature.
We have a section that goes into overall bird anatomy such as laying hard-shell eggs, no teeth, feathers, and hollow bones for a lighter skeleton. As with all nature, there is also a lot of specialization. Not all birds fly. Then what do they do? Ostriches (one of the oldest bird species) have very strong legs and can run very fast. Penguins have dense feathers and a waterproofing that allow them to swim underwater very efficiently. Something that has always interested us is the fact that there are hopping birds and walking birds. You should go out and look one day. You'll find little sparrows that hop on two legs and birds such as pigeons that walk like you and I. Take some time to figure out the advantage of that specialization.
As Charles Darwin traveled the world, he studied many bird species and saw how they were able fill specific ecological niches that allowed different species to survive. Darwin looked at finch species and compared their food to the shape of their beaks. There were finches with long beaks that ate nectar from flowers and some with stubby strong beaks that allowed then to eat hard seeds. As with all species, birds developed different characteristics so that they could capitalize on available food sources and environments. Once they were able to establish themselves, they could reproduce and succeed as unique species.
There are actually more species of bird than any other vertebrate except fish. Just like mammals, you can find them anywhere. Because of their body heating system, reptiles are limited to warmer habitats. Birds are homoeothermic. Homoeothermic means they regulate their own temperature in the way mammals do. As we were saying, birds are everywhere. You'll find them from pole to pole and from the mountaintops to under the ocean.
How were birds a step up from reptiles? Their closest relatives were animals from the Crocodilia order. Birds were able to make some physiological improvements.
Feathers: Feathers are just specialized scales. When they first appeared, they weren't used for flying. Scientists really aren't sure what they did. One use other than flying is to attract other birds. They may have been developed to attract the ladies.
Hard shell eggs: While reptiles were the first to have eggs that could survive on land, birds gained an advantage when they developed eggs that had a hard shell. They were stronger and could support the embryo inside through harsher conditions (like rolling out of a nest).
Hollow bones: When it comes time to fly, the lighter you are the better. As time passed, bird-like creatures that had lighter bones were able to do more. Eventually those lighter bones had big empty spaces that were hollow. They are more fragile than regular bones in mammals, but much better for flying.
We mentioned you could find birds everywhere. They are found in so many habitats because they are great at adapting. There is a classic example that Charles Darwin used when studying finches. Even though all the birds he was always looking at were finches, he noticed that their beaks were different, depending on the food they ate. It's that kind of adaptation that allowed some birds to feed on the ground and others to eat fruit from trees. Adaptation is not limited to beaks. It also happened with wings and feet. Ducks in the water have webbed feet; raptors (eagles/hawks) that hunt animals from the air have talons. Those changes are all based on the same idea.
You are a mammal. Your pet dogs and cats are mammals. An elephant is a mammal and so is a whale. When you learn about animals, the first you learn about are probably mammals. Not all mammals are made the same way. Most of the mammals you see will be placental. If you are in Australia you will see a lot of marsupials. Monotremes are tough to find. There aren't many of those left in the world.
Monotremes were the first mammals. How do scientists know? They actually lay eggs. Monotremes are more closely related to reptiles than any other mammal. They have not yet evolved a way to have their babies live. Examples of monotremes are a duck-billed platypus and the spiny anteater.
There are many more marsupials than monotremes. Kangaroos, koalas, bandicoots, and possums. You'll find a lot of them in Australia. Australia is an island continent. Because of its isolation, placental mammals didn't take hold in their ecosystems. Australia is like a mammalian time capsule. Marsupials are special mammals that give birth to their young live, but the babies mature in pouches. While they are in the pouch they suckle on the mother's milk for nourishment.
Placental mammals are the dominant form of mammal on the planet. Placental mammals deliver their young live and ready for action. Although the babies might still need some work, much of the basic development is done inside of the female's placenta. When the baby is born, it still needs some raising and education. Mammalian mothers will usually stick around and help in that learning process. There are no pouches. The baby must walk or be carried.
Placental mammals are everywhere, even in the oceans. A group called cetaceans includes dolphins and whales. They are mammals that evolved and returned to the ocean. They still breathe air and even have tiny hairs like other mammals, but they live their whole lives in the water.