Gnathostomata

Many animals have a jaw.

A jaw is a mouth that can bite. It helps them eat food.

Long ago, fish did not have jaws. They had parts to help them breathe. These parts changed over time. They bent to make a jaw.

This new jaw helped them suck in water. It also helped them bite.

Some old fish had hard plates for armor.

Other fish had teeth. Most animals today have jaws. This includes fish, birds, and even you!

Most animals you see today have jaws. These animals are called gnathostomes. This group makes up 99% of all living vertebrates.

Jaws were a huge change for life on Earth. Long ago, fish did not have jaws. They only had parts to help them breathe. These parts are called gill arches. Over time, the first pair of gill arches bent forward. This created a jaw.

A jaw helps in many ways. It helps animals suck water into their mouths. This helps them breathe through their gills. It also lets them bite food.

There are three main groups of gnathostomes. One group is Chondrichthyes. These are cartilaginous fishes, like sharks. Their skeletons are made of cartilage instead of bone.

The second group is Placodermi. These were armored fish that lived long ago. They are now extinct.

The third group is Teleostomi. This group includes bony fish. It also includes birds, reptiles, and mammals. This includes tetrapods, which are animals with four limbs.

Gnathostomes are a huge group of animals. They are known as jawed vertebrates. This group includes about 60,000 different species. That is 99% of all living vertebrates on Earth today.

You can find them in many forms. They include all living bony fishes and cartilaginous fishes. This group also includes tetrapods, which are four-limbed animals. Tetrapods include birds, mammals, reptiles, and amphibians.

Most gnathostomes share some very old traits. They have true teeth and a stomach. They also have paired appendages. These can be fins, limbs, or even wings.

How did these jaws first appear? Scientists believe they grew from gill arches. These are bony or cartilaginous parts that support gills. In jawless fish, these arches helped them breathe. The first pair of arches likely bent forward. This movement helped suck water into the mouth. This is called buccal pumping. It helps the animal move water over the gills for gas exchange.

Over time, using these bones helped animals bite. This was a massive change for life. It was a very important step in history. It helped jawed fish survive better than jawless ones.

We can learn about this history from fossils. One very important fossil is named Entelognathus. It is about 419 million years old. This placoderm had a bony oral skeleton. It showed details found in both cartilaginous and bony fish.

Other fossils like Guiyu oneiros and Psarolepis are also helpful. These lived at the same time as Entelognathus. They were relatives rather than direct ancestors. We also see fossils of spiny sharks. These are called Acanthodians. They were among the earliest known jawed vertebrates.

Scientists group gnathostomes into three main parts. The first group is Chondrichthyes. These are cartilaginous fishes like sharks and rays. Their skeletons are made of cartilage instead of bone.

The second group is the extinct Placodermi. These were armored fish from the Silurian and Devonian periods. Their heads and chests were covered in plates. The third group is Teleostomi. This includes bony fishes and tetrapods. Bony fishes are very successful. There are over 28,000 species in that group alone.

It is amazing to see how these animals connect. Many animals you see every day are gnathostomes. A shark in the ocean is a cartilaginous fish. A bird in a tree is a tetrapod.

Even a dog or a human is part of this group. We all share those very old traits. We have jaws, teeth, and paired limbs. This group has changed many times over millions of years. It started in the ancient seas. Now, it lives almost everywhere on land and in water.

Gnathostomata is a massive group of animals known as jawed vertebrates. This group includes roughly 60,000 different species. This number accounts for about 99% of all living vertebrates on Earth today.

The group contains all living bony fishes and cartilaginous fishes. It also includes tetrapods, which are four-limbed animals like mammals and birds. This diversity makes Gnathostomata one of the most successful groups in biological history.

Most gnathostomes share several ancient biological traits. They possess true teeth and a stomach. They also have paired appendages, such as pectoral and pelvic fins or limbs. Many species also have elastin and a horizontal semicircular canal in the inner ear. They possess myelinated neurons and an adaptive immune system. This immune system uses discrete lymphoid organs like the spleen and thymus. It also uses a process called V(D)J recombination to create antigen recognition sites.

Scientists believe jaws evolved from the first pair of gill arches. In jawless ancestors, these arches were cartilaginous elements that supported the gills. The first pair of arches likely bent forward to form the jaw. This change helped with buccal pumping, which is the movement of water into the mouth. This movement forced water over the gills for gas exchange. Over time, the repetitive use of these jaw bones allowed animals to bite.

We can trace this history through many important fossils. The placoderm *Entelognathus* is a 419-million-year-old fossil. It had a bony oral skeleton with details found in both cartilaginous and bony fish. This shows that a bony skeleton is not a new trait for all bony fish. Other fossils, like *Guiyu oneiros* and *Psarolepis*, lived at the same time. They were relatives rather than direct ancestors of modern gnathostomes.

Another important group is the extinct Acanthodii, often called "spiny sharks." These animals lived during the early Silurian. The oldest known specimen is *Fanjingshania renovata* from China. Another species, *Qianodus duplicis*, is the oldest unequivocal toothed vertebrate.

These fossils prove that the common ancestor of all gnathostomes had teeth. These teeth grew on the jawbone without roots. This means they could not be replaced once they wore down.

Traditionally, scientists divide Gnathostomata into three main groupings. The first is Chondrichthyes, which are the cartilaginous fishes. This includes sharks, rays, and chimaeras. Their skeletons are made of cartilage rather than bone.

The second group is the extinct Placodermi. These were armored fish that lived from the late Silurian to the end of the Devonian. Their heads and thoraxes were covered in articulated plates.

The third group is Teleostomi. This group includes bony fishes and tetrapods. Bony fishes, or Osteichthyes, are incredibly diverse. They include over 28,000 species across 45 different orders.

Tetrapods evolved from lobe-finned fishes about 395 million years ago. This group includes amphibians, reptiles, birds, and mammals.

This evolutionary path connects ancient sea creatures to almost all land animals today.