Tetrapods: Development of Limbs in Vertebrates

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  • 0:03 Two Pairs of LImbs
  • 2:02 Living a Double Life
  • 3:12 Amniotic Eggs
  • 4:02 Lesson Summary
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Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

Having four limbs allowed for an incredible amount of diversification in the animal kingdom. But where did those limbs come from, and what types of diversifications arose from animals moving onto land?

Two Pairs of Limbs

Having arms and legs makes life SO much better, don't you think? Personally, I see this as the best way to be. I can still swim in water, but a fish can't walk on land, or drive a car, or play in a baseball game.

We should be thankful to the fishes though, because without them we really wouldn't be here. A long time ago, there were some fishes that had fins with a long bone in them that was attached to the main body. A few of these fishes remain today, but some of these sturdy fins eventually evolved into what we now recognize as limbs.

Tetrapods, which are four-limbed vertebrates ('tetra' means 'four'), evolved from those lobe-finned fishes and over time made their way onto land. The earliest tetrapods probably couldn't walk around like we do because their limbs just weren't strong enough. But over time, those limbs became stronger and more supportive, turning into the legs and arms modern tetrapods use for getting around.

Modern tetrapods are a very diverse part of the Animal Kingdom. They include things like amphibians, reptiles, and mammals.

But now you may be wondering if you should believe what I say because some animals, like snakes, have no limbs at all! This is a valid concern, and indeed some tetrapods have lost some or all of their limbs.

Still other tetrapods have given up the luxury of land life and returned to the water. However, at one point they fit the criteria for belonging to the tetrapod club, so there they remain.

The development of limbs is a crucial part of the diversity of tetrapods. Being mobile allowed for novel feeding opportunities as well as the ability to inhabit new environments. Having limbs also allowed some animals to escape predation by climbing a tree, while others could take flight to avoid getting eaten. As you can see, having limbs led the way for a substantial increase in mobility and diversification.

Living a Double Life

While some tetrapods saw land and never looked back, others continue to spend part of their lives in aquatic environments. In fact, the word 'amphibian' comes from the Greek 'double life.'

This term refers to the metamorphosis of amphibians that spend much of their life on land but lay their eggs and grow into adults in the water. As larvae they have gills that allow them to breathe underwater and a tail that helps them swim around. But as they mature, they develop lungs and limbs that allow them to breathe air and move around on land.

Many of these animals return to the water to lay their eggs because these eggs are not adapted to developing in terrestrial environments. Amphibian eggs are not surrounded by a hard shell, so they must remain in wet environments in order to keep from drying out.

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