What is the Evolutionary History of Amphibians?
Amphibians are a class of animal that includes modern-day frogs, toads, salamanders, newts, and caecilians. They first evolved from lobe-finned fish and primitive tetrapods about 340 million years ago. Sometimes this date is incorrectly given as 400 or 380 million years ago, but fossils have not been unearthed from these periods.
About 380 million years ago, during the Devonian period, some fish began to evolve legs and digits. These early "tetrapodomorphs" lacked the characteristics that define amphibians, so they are classified as basal tetrapods. Decades ago, they were classified as amphibians, though taxonomists have changed their view on the matter. This is why the origin of this class is sometimes incorrectly cited as 380 million years ago.
Some of the earliest tetrapods include Tiktaalik, among the earliest with a weight-bearing wrist structure, and Acanthostega, which had eight digits on each foot. These early species would have been mostly aquatic, and used their limbs to navigate through swamps rather than taking extensive journeys over the land.
Between 380 and 360 million years ago is a period called "Romer's gap," in which barely any tetrapod fossils have been found, casting a cloud of mystery on the evolution of the first amphibians from the early basal tetrapods. Prior to the gap, no fossils are found, and the first known amphibian fossil appears shortly after the gap. After the gap, the world was in the Carboniferous period, where sea levels were high and the coasts were covered with flooded forests and swamps.
The first amphibians were temnospondyls, long-headed animals with a sprawling gait and distinctive look. These were the first truly terrestrial tetrapods, and would have eaten themselves silly by consuming insects that lacked specialized adaptations for defending against large vertebrate predators. The early temnospondyls were the size of large fish, ranging from about 1.6 to 5 feet (0.5 to 1.5 meters) in length. The earliest ones had stubby feet, and probably couldn't move very fast.
Throughout the Carboniferous period, temnospondyls grew in size and diversity until they occupied many of the predatory and herbivorous niches that terrestrial animals exploit today. By the late Permian, some even grew to 30 feet (9 m) in length, and resembled crocodiles. This animal, Prionosuchus is the largest amphibian known. In the Carboniferous, temnospondyls were joined by the diverse but less numerous lepospondyls. Lissamphibians, the group that includes all modern amphibians and their common ancestors, emerged about 300 million years ago.
Did axolotls evolve from salamanders? How?
@titans62 - I thought that was interesting, too. A lot of animals have gotten smaller over the last 300 million years, though. Obviously, we don't have dinosaurs anymore. Perhaps large amphibians can survive okay biologically; it's just hard for them to survive given the way the world changed and water resources were altered.
My favorite amphibian by far is the salamander. I was reading a story not too long ago about a salamander in China that is 6 feet long! It holds the record as the largest amphibian. I think salamanders are really cute, but this one was kind of gross, since it was so slimy. Most people don't know about them or have never seen one. If you just walk around near a stream and start turning over rocks and logs, though, you're bound to find one. The coolest thing about salamanders and a lot of other amphibians is that they can regrow lost body parts like tails and toes!
@JimmyT - I believe the Latin roots are more meant to represent the fact that amphibians have two distinct life stages. First they are born in the water and are dependent on it for life and then they move onto land. Even when land-based amphibians move out of the water, they still spend a good deal of time around it for survival.
I did find it very interesting that the first amphibians were so big. I was also aware of their respiratory system and had always heard that as being one of the main reasons for why there were never any huge amphibians around. Does anyone have any idea what the biggest amphibian alive today is? The biggest one I have ever seen what a newt that my friend used to have when I was in college. It might have been a foot long or so.
@TrogJoe19 - I wasn't aware of what the etymology of the word amphibian was. I find that find of interesting, because plenty of other animals spend relatively equal proportions of time on land and in water.
There are quite a few different types of water snakes. Around where I live, we have something called a cottonmouth that lives in swamps and very wet places. You also have sea turtles, which are reptiles that spend the majority of their time underwater.
Aquatic mammals aren't as common but you can find things like beavers, muskrats, and even platypus that spend a lot of time in the water.
@BostonIrish - I would say that a lot more than the amphibian life cycle separates them from reptiles. We are learning about them right now in class. Yes, it is true that the young of amphibians have to go through stages, but scientists usually go by a lot of different things.
Probably the most defining feature of amphibians is that they have smooth, wet skin most of the time. This is because they don't have lungs like reptiles and mammals have. They have to get most of their oxygen through little pores in their skin. Having water on their skins makes it easier for oxygen to diffuse to their lungs and circulate around their bodies.
There are a lot of other differences having to do with the way the eggs develop as well as other parts of the anatomy.
The main difference between amphibians and reptiles is their life cycle. Amphibians morph over time while reptiles are born looking like a miniature version of their parents.
Amphibians are quite unique in that they are both aquatic and land animals. They do not give live birth, but lay eggs, much like reptiles. Amphibian is from ambi- bios, which is Greek for both sides- life. They live their lives in both water and land. As young tadpoles, they are much like fish, and grow legs to live on the land.
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