Animal Evolution and Diversity
Nothing Like a Place at the Beach
Vertebrates Take to the BeachFirst it's the ancestors of frogs and salamanders that crawl from the deep blue sea. Then, for a while, dinosaurs rule the earth.
Weather has always been unreliable. What do you do if water, the thing you need most, just goes away? More than 350 million years ago, a few freshwater vertebrates developed the ability to breathe air and pulled themselves around on land when water was scarce. The ancient ancestors of amphibians had features that combined some of the features of lobe-fish and lungfish: lungs to breathe air and more limb-like fins that could maneuver on land. As the Little Mermaid famously observed, fins don't work so well on land. In amphibians, fins became legs and feet with toes. Dinglehoppers, on the other hand, work everywhere.
Moving onto land opened up a whole new set of unexploited resources. Arthropods had taken to the land 100 million years earlier and were free for the snacking. As the first vertebrates on land, amphibians had the place to themselves for a while. Before reptile competition appeared, amphibians came in many more shapes and sizes than they do today, with one the size of a modern crocodile. Modern amphibians include frogs, salamanders, and caecilians.
In addition to lungs and limbs, amphibians also evolved other traits for life on land. Amphibian skeletons are stronger than those of fish, capable of supporting body weight without the help of water. Carrying weight around in water is easier. Amphibian skin better resists drying out. Amphibian's smooth skin has keratin, which helps make it waterproof. Amphibians also developed structures for hearing on land, replacing the lateral line system of fish.
Life on land led to a more efficient heart. We'll have a little heart to heart discussion. (Ouch.) Fish have a heart with one chamber for receiving blood from the body (the atrium) and one chamber (ventricle) that pushes the blood to the gills, where waste gases go out and oxygen comes in.
Amphibians have a heart with three chambers, two atria, and one ventricle. One atrium receives blood from the body, while the other receives blood oxygenated by contact with the skin or lungs. All this blood mixes together in the single ventricle before being pushed out. This is more efficient than the fish system, where blood loses some pressure as it passes through the gills after being squeezed by the heart.
Amphibians are still, however, split between two worlds: terrestrial and aquatic. Amphibians live a double life. No, not like Clark Kent or Peter Parker. Amphibian means "both kinds of life." During the first part of its life, a frog is a legless, water-breathing aquatic herbivore called a tadpole. After metamorphosis, the adult is a tetrapod (four-legged), air-breathing terrestrial carnivore. Taking off some glasses and putting on a cape can't really compare to switching from breathing water to air and changing fins for legs.
Check out the drama of becoming a tadpole here and here.
Amphibians are closely tied to water their whole lives. Their skin has to stay damp for gas exchange. Some have small lungs, but those lungs are not very effective, and amphibians rely on staying wet to breathe. Amphibian eggs also need to be in or very near water so that the young are in water when they are born. The eggs don't have shells and are covered with a sticky material so they float together in a mass while the little guys develop.
Brain SnackNothing we can dream up comes close to the real stuff. Here's one, for instance. The Surinam toad hatches its eggs through the skin on its back.
There are three extant orders of amphibians:
Urodeles: These are salamanders and newts. They look a bit like a lizard, but are smooth with no scales. Salamanders stay damp and are less able than frogs and caecilians to be away from water. Some urodeles do not go through metamorphosis and look like little adults when they are born.
Anurans: Sounds like a good name for space aliens, but this is the order for frogs and toads. These amphibians have more specializations for living on land than the others. Frogs have smooth, wet skin and toads have drier, bumpy skin. The bumps on toads can have toxins; so don't touch. Frogs and toads do not cause warts however. Anurans' distinctive features include a long, sticky tongue that is shot out of a hinged mouth to catch insects and long back legs that are very good for jumping.
Apodans: The common name probably won't help here—caecilians. It is spelled differently, but sounds a lot like Sicilian. "A-" means without and a "pod" is a foot. No feet. They look a bit like worms or a little snake (another worm-like animal, what a surprise). However, they have a backbone and a skull, unlike worms, and no tail, unlike a snake. They can hardly see and that's just fine, since they live hidden on the ground in wet forests.
- Tetrapod (except caecilians)
- Smooth, scale-less skin with waterproof keratin
- First part of life in water and second on land
- Breathes air, but at least partially through the skin
- Life-cycle includes metamorphosis
- Eggs laid in or near water and don't have a shell
- Can hibernate or be otherwise dormant in unfavorable conditions
When they came on land, amphibians kept a connection to life in the water. Reptiles were the first vertebrates to become completely terrestrial. This happened about 300 million years ago.
The earliest reptile looked like a small, stumpy lizard, but its descendants were nothing but impressive. Flying reptiles took over the air. Some reptiles went back to the ocean. Others reared up on hind legs and lorded over the land.
Big design changes were needed to become fully terrestrial. Life started in water and we are still mostly water on the inside. Reptiles developed watertight and waterproof skin. Since moving around on land is a very different process than moving in water, reptiles developed strong muscle and skeletal systems, as well as claws and nails to help get some traction on land.
Reptile ancestors were amphibians and the two can look a bit alike. However, reptiles are far more adapted to life on land.
- Reptiles have scutes or scales on their dry skin. Amphibians have smooth skin that needs to stay moist.
- Reptiles usually have nails or claws on their feet (if they have feet), while amphibians don't.
- Reptiles do not go through metamorphosis and never have gills. Even water-dwelling reptiles breathe air.
- Reptiles are herbivores or carnivores. Amphibians eat arthropods or fish.
- Reptiles have fully developed lungs. Amphibians have weak or no lungs (adults without gills rely on gas exchange through moist skin).
Another big evolutionary change is the amniotic egg. Despite living on land, life still begins in water for everyone. Reptiles, along with birds and mammals, are amniotes. An amnion is a bag of fluid where an embryo develops. All animals still start their life in water, even if the water is contained in an egg or inside the mom. Reptile eggs have an amniotic pouch inside, sealing up a private pool for the embryo. The eggs are also hard-shelled and waterproof to resist drying out.
Evolution is a response to the environmental and ecological challenges faced by an organism. In other words, if an animal has the skills it needs to get enough to eat and be able to reproduce, it's pretty well adapted to wherever it lives. But what if conditions change? That happens all the time. Animals that have the right characteristics to survive a change in conditions are the ones who survive. Fortunately for life on earth, no two organisms are exactly alike, even ones from the same group living at the same time and in the same place.
One of the many times conditions made a big change was around 250 million years ago. Weather on land was getting drier, not a good thing for amphibians. The early, small reptiles got their chance to take over. Jurassic Park, here we come. Dinosaurs evolved into many forms and took over the land. Pterosaurs, flying reptiles, took to the air. Icthysaurs and plesiosaurs, swimming (but air-breathing) reptiles, moved back to the ocean.
Brain SnackWhat did dinosaurs look like? Perhaps a bit like a crow.
A reptile-like vertebrate ancestor led to dinosaurs and modern reptiles, as well as to birds and mammals. The branches split after the development of the amniotic egg. Some of the major groups in the tree are named based on a feature of the skull that they all share, the number of openings behind the eye. Anapsids (turtles) don't have any. Synapsids have one and diapsids have two.
The synapsid line eventually leads to mammals. The turtles branch off next. The diapsid line branches into the rest of the extinct and current reptiles, as well as birds.
The reptiles are shown in green, including both extinct and modern reptiles.
The two main classes of dinosaurs are Ornithiscians and Saurischians. These groups are named after their hips (bird-like or reptile-like). The bird-hipped ones were mostly larger herbivores and the reptile-hipped ones were the speedy, carnivores.
Reptiles were the lords of all they could see for more than 200 million years. What happened? We don't have a simple answer. Possibilities include a giant collision with an asteroid or asteroids, as well as other geological events. Whatever the cause, dinosaurs disappeared and left room and board open for other vertebrates.
Brain SnackHere's some more detail about the theories on the cause of the mass, worldwide extinction that ended the dinosaurs.
Dinosaurs are gone, but we still have lots of reptiles. Currently, there is a lot of debate about how to divide them up. Some scientists feel that birds have enough similarity to some reptiles to be grouped more closely. Other scientists group modern reptiles into one class divided into a few orders and others into different classes. Let's go with the simplest. The three major classes of reptiles we have today are:
Testudines: These are the turtles. Terrestrial ones are usually called tortoises and aquatic ones include sea turtles and fresh water terrapins. The most noticeable thing about a turtle is the big shell covering most of its body. This is for protection and turtles can pull their head and limbs inside in an emergency. Turtles on land have feet with claws, while aquatic turtles have feet shaped like flippers.
Lepidoposaura: This is the biggest group of modern reptiles. This group has a unique skull called a kinetic skull. The joints in a kinetic skull are very loose and mobile. The jaws are not joined directly to the head and can open extremely wide so that lizards and snakes can swallow really large prey.
There is a simple distinction within this group: lizards have legs and snakes don't. There are some legless lizards, so another helpful distinction is that lizards have external ears and moveable eyelids, if you ever choose to get that close.
Crocodilia: Crocodiles and alligators make up this group. Crocodiles' lower teeth show when their mouth is shut and alligators' lower teeth do not. So ask them to close their mouths if you really want to know.
Crocodiles and alligators are the only reptiles with a four-chambered heart, an evolutionary development that becomes standard in birds and mammals. A four-chambered heart is more efficient, fully separating oxygenated and deoxygenated heart, giving the animal more endurance. Crocodiles share another trait with birds and mammals in that they exhibit parental care for young.
- Watertight skin with scales to prevent drying out
- Claws for digging and traction on land
- Lungs and respiration that does not need water
- Partial separation (and complete in crocodilians) of oxygenated and deoxygenated blood
- Amniotic egg