Fishes and Sharks: Origins of Jaws in Vertebrates

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  • 0:03 The Origin of Jaws
  • 1:26 Class Chondrichthyes
  • 2:45 Ray-Finned Fishes
  • 3:42 Lobe-Finned Fishes
  • 4:48 Lesson Summary
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Lesson Transcript
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.

While it may appear to be a minor feature, the hinged jaw led to major changes in the evolution of animals. In this lesson, you'll discover some of the earliest jawed animals, the sharks and fishes of the world.

The Origin of Jaws

Have you ever thought about your jaw? I mean, really thought about it? Your jaw is a pretty important part of your life. Without it, you wouldn't be able to chew up that steak you grilled, or talk to your best friend on the phone.

But not all animals have jaws. Not even all vertebrates have them! So where did they come from, and how did they help increase the diversity of animals on Earth?

In order to understand jaws we need to understand a little bit more about our evolutionary history. Just like all other vertebrates, we are chordates. And chordates share four unique features: a dorsal hollow nerve cord, a notochord, pharyngeal slits and a post-anal tail. Not all vertebrates retain these features into adulthood (last time I checked I didn't have a tail!), but they do have them at some point during development.

One of these features, the pharyngeal slits, is believed to be the origin of the hinged jaw that allows us to do so much with our mouths. These are gill openings in the throat, and the main function of these gill slits in our ancestors was to filter food that was floating through the water.

Over time though, skeletal rods in the pharyngeal slits evolved into the hinged jaw that we now know and love. Amazingly, some animals have both jaws and gills, which are now used for breathing instead of feeding. Collectively, these animals are known as the fishes of the world.

Class Chondrichthyes

Sharks, rays and skates - oh my! You are well aware of these fish, known as the chondrichthyes. The name of this group means cartilaginous fish, which quite accurately describes them because these animals have flexible 'skeletons' made of cartilage instead of bone.

Some chondrichthyes, like the giant manta ray, eat plankton that floats through the water. Others, like stingrays, are predatory. Their wide, flat bodies make it easy for them to settle into the sand and hide, patiently waiting to attack their dinner when it comes along.

Most sharks are also voracious predators. No doubt you've already imagined a large shark mouth with its rows and rows of sharp teeth! Sharks' bodies are made for swimming through the water instead of sitting at the bottom. Sharks have excellent vision and smell, and even have electrosensory organs that can detect electric fields produced by other animals.

In contrast to the cartilaginous skeleton of our chondrichthyes friends, osteichthyes are the bony fish (the word 'oste' means 'bone'). But this includes a lot of fishes! So we can further break this group into two more specific classes: the ray-finned fishes and the lobe-finned fishes.

Ray-Finned Fishes

The first class is the ray-finned fishes, called actinopterygii, which comes from the Greek word for 'wing.' These fishes constitute more than half of all vertebrates on Earth and are by far the most numerous of the fish world. Familiar players of this group include tuna, goldfish, trout, flounder and salmon (just to name a very scant few!).

As mentioned, they have bony skeletons, which you've likely encountered if you've ever eaten one of these fishes for dinner. Their fins are soft but supported by thin skeletal structures. Most of these fishes also have scales covering their bodies, as well as a hard bony structure that covers the gills called an operculum.

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