Sarcopterygii

Some fish have thick, fleshy fins.

These fins have bones inside them. They look like little arms or legs. Long ago, these fins changed into real limbs. This helped animals move onto dry land.

These fish also had parts that became lungs. Lungs help animals breathe air. Most of these fish are gone now. Only lungfish and coelacanths are still alive.

They are very special fish to find!

Some fish have special fins. These are called Sarcopterygii, or lobe-finned fishes.

Unlike other fish, their fins are fleshy. They have strong bones inside. These bones look like little arms or legs.

These fish changed over a long time. Their fins slowly became limbs. Their body parts also became lungs. Lungs help animals breathe air. This helped them move from water to land. This is why we call them tetrapods.

Tetrapods are animals with four limbs.

Many of these fish lived long ago. Some were very big. One fish, Rhizodus hibberti, was 6 meters long. Most are now extinct. Only two groups live today. One group is the coelacanths. The other group is the lungfishes.

The largest lungfish is the marbled lungfish. It can grow to 2 meters long. These living fish are very special to study.

Sarcopterygii are a special group of bony fishes. People often call them lobe-finned fishes.

Most fish have fins made of thin spines covered in skin. Sarcopterygians are different because they have fleshy, muscular lobes on their fins. These lobes contain strong bones that act like tiny limbs. This makes them very important to study. They show us how animals changed over millions of years.

These fish have a very interesting way they work. Their fins are joined to their bodies by a single bone. Inside the fin, the bones have joints that look like arms or legs.

This structure helped some groups move toward a new way of life. Their fins slowly turned into limbs for walking. Even their breathing changed. A part of their gut turned into lungs to breathe air. This process helped them move from water to land.

We can look back at history to see these changes. The earliest known bony fish fossils are from the Late Silurian period.

That was about 419 million years ago. During the Devonian period, these fish split into two main paths. One path led to the coelacanths. The other path led to the rhipidistians. These rhipidistians eventually became the lungfish and the tetrapods. Tetrapods are the four-limbed animals we see today.

There are many amazing facts about these creatures. In the Carboniferous period, they were top predators in freshwater.

One huge fish named Rhizodus hibberti lived in Great Britain. It could grow up to 6 meters long.

Today, only a few types remain. The West Indian Ocean coelacanth is a large living species. It can weigh up to 50 kilograms. The marbled lungfish is also large. It can reach 2 meters in length.

You can see the link between these fish and yourself. Even though mammals and birds look different, we share ancestors with Sarcopterygii.

We share similar bone structures in our limbs. We also share the same kind of enamel on our teeth. This shows that the very first steps toward land life happened in the water. The fleshy fins of these ancient fish were the start of every leg and arm on land.

Sarcopterygii is a major group of vertebrate animals known as lobe-finned fishes.

This clade is essential to understanding the history of life on Earth. It includes bony fishes that possess prominent muscular limb buds called lobes. These lobes are supported by articulated appendicular skeletons, which are internal bone structures. This is very different from the Actinopterygii, or ray-finned fishes. Ray-finned fishes have fins supported only by skin-covered bony spines. Sarcopterygians are significant because they are the ancestors of all tetrapods. Tetrapods are the four-limbed vertebrates that live mostly on land.

The biological mechanism of the sarcopterygian fin is quite unique. Each fin is joined to the body by a single bone. The fin grows from a fleshy, scaly stalk that resembles a limb bud. Inside this lobe, the bones have articulations that resemble the joints in limbs. This structure allowed for a massive evolutionary shift from water to land. Over time, the paired pectoral and pelvic fins evolved into actual limbs. Additionally, a part of the foregut, called a diverticulum, evolved into air-breathing lungs.

This transition allowed ancestors to move from swimming to walking and breathing air.

Scientists divide the sarcopterygian lineage into two distinct evolutionary paths. These paths are known as the Actinistia and the Rhipidistia. The Actinistia clade is represented today by the coelacanths. These fish remained in the ocean and kept many ancient features. The Rhipidistia clade includes the lungfish and the Tetrapodomorpha. The Tetrapodomorpha are the group that includes tetrapods and their closest relatives. Some fossils in this group, like Tiktaalik, are called "fishapods." They look half-fish and half-tetrapod in their body shape and limb structure.

The history of these animals stretches back hundreds of millions of years. The earliest known bony fish fossils, such as Guiyu oneiros, date to the Late Silurian. This was approximately 419 million years ago. During the Devonian period, sarcopterygians began to split into their main lineages. Many lobe-finned fishes were dominant predators in freshwater during the Carboniferous and Permian periods. However, they suffered a massive decline after the Permian–Triassic extinction event. This event changed the course of life for many aquatic species.

There are many impressive measurements regarding these ancient and modern creatures. During the Carboniferous period, the Rhizodus hibberti lived in Great Britain. This massive fish could reach up to 6 meters in length. Today, the West Indian Ocean coelacanth is a notable living species. It can weigh up to 50 kilograms. Another large survivor is the marbled lungfish. This species can grow to 2 meters in length and weigh up to 30 kilograms.

These numbers show how diverse and large these animals can become.

Sarcopterygians also have very specific physical traits that set them apart. They possess two dorsal fins with separate bases. This is a contrast to ray-finned fish, which have a single dorsal fin. Their scales are called true scaloids, made of lamellar bone and cosmine. Cosmine is a substance similar to dentin found in teeth. All sarcopterygians also have teeth coated with genuine enamel.

Even their braincase originally had a hinge line, though this was lost in later groups like lungfish.

Finally, the study of sarcopterygians connects us to the broader tree of life. Even though mammals and birds look very different from fish, they are highly derived sarcopterygians. We share similar bone structures in our limbs and the same enamel on our teeth. Lungfish and tetrapods even share a divided atrium in their hearts. This means that the very foundation of land-dwelling life was built within the fins of these ancient fishes.