Evolution of Chordates

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  • 0:00 What Are Chordates?
  • 1:42 Craniates
  • 2:25 Vertebrates
  • 3:23 Jaws, Lungs, & Limbs
  • 4:03 Tetrapods & Amniotes
  • 5:51 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.

Chordates are a diverse and interesting group of animals. You might be surprised to find out what belongs to phylum Chordata, so check out this lesson to learn more about what sets them apart and what keeps them together.

What Are Chordates?

This may be hard to believe, but you are related not just to other mammals, but also to other animals that might surprise you like fish, snakes, and frogs! This is because we all share certain characteristics that came from a common ancestor a long, long, long time ago.

You may be aware that all animals that have a backbone are vertebrates. All vertebrates are also chordates, which is a phylum of animals that have a dorsal, hollow nerve cord, a notochord, pharyngeal slits, and a post-anal tail. This means that all of the animals mentioned above fall into the same phylum as humans, the phylum Chordata, because they all have these four features.

For many chordates, these four distinct features are present only at certain life stages. You can double check, but I bet your post-anal tail disappeared before you were even born. But you did have one during embryonic development, as well as the three other features that make chordates different from other phyla.

Non-Vertebrate Chordates

Since this lesson is about the evolution of chordates, let's start at the very beginning, with the most primitive chordates. It's easy to think that because all vertebrates are chordates, the reverse is also true. But this is not the case. Backbones didn't come until later and there are actually two very primitive chordates that share those four unique features but do not have a backbone. These are the invertebrates of the Chordata phylum and by looking at them, you'd never guess that we're related!

The first of these is the tunicate. This animal lives in the ocean and looks very much like a spouting pouch. The second is the lancelet, which looks like a knife blade floating through the water. While the lancelet does have a small 'brain' at the end of its nerve cord, neither of these animals has an actual head or a vertebral column. However, they are still chordates because they possess those four special features that define the phylum.

Craniates

The next step in the evolution of chordates was the head. That's right, still no backbone! Known as craniates, these chordates with heads still have the four features of other chordates, but now they also have a noggin on one end. The word 'cranium' means 'skull,' so 'craniates' fits quite well, don't you think?

Vertebrates

Alright, we've finally made it to the next big step - vertebrae! The first vertebrates didn't have a backbone quite as developed as ours, but rather a more rudimentary one, as well as a more extensive skull than the non-vertebrate craniates. Even so, instead of just having a nerve chord, they now have a protective bony structure that surrounds it. If we look only to the diversity of vertebrates on Earth, we can see that this must have been a very important new trait. Vertebrates are found all over the world in almost every different environment and come in all different shapes, sizes, and forms.

The animal that falls into this category still looks nothing like us, but also looks much different than our primitive relatives: the tunicate and the lancelet. Hagfish are fascinating animals that look like an eel but are quite different. They have a head but no backbone and no jaw. Hagfish have an amazing trick up their sleeve, too! They can produce incredible amounts of slime as a defense mechanism.

Because the head developed before the backbone, all vertebrates are also craniates. But one thing that not all vertebrates have is a jaw. Lampreys, which look somewhat similar to hagfish, are jawless vertebrates. Most are parasitic and feed off fish and other animals. These parasitic lampreys have sucker mouths with rows and rows of sharp teeth that help them latch on, with a rasping tongue that is used to break the skin and feed on blood and tissue.

Jaws, Lungs, & Limbs

The next step after the head was developing a jaw. This was huge because it opened up a whole new realm of dietary options for these chordates. Imagine trying to eat your regular diet without a jaw! I bet it would be pretty difficult. Of course, having a jaw is dependent upon having a head, so all of the jawed vertebrates are also craniates.

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