Acanthodii

Long ago, spiny sharks lived in the water.

They had many sharp spines on their fins. These spines were strong and hard. They looked a lot like sharks.

These fish lived in the sea first. Later, they lived in rivers and lakes. They had tiny scales on their skin. Some scales were shaped like diamonds. These fish lived a very long time ago. They are gone now.

Long ago, a group of fish called acanthodians lived in the water.

People call them "spiny sharks." They looked like sharks because of their body shape. They had fins and a tail that turned up. But they were not true sharks. They had a skeleton made of cartilage. Cartilage is a soft, bendy material.

Most of their fins had hard spines to support them. These spines were made of dentine. Dentine is a hard part of a tooth.

These fish lived in many places. At first, they lived in the sea. Later, they moved into fresh water like rivers and lakes.

There were different kinds of these fish. Some were predators with teeth. Others were filter feeders. Filter feeders eat tiny bits of food from the water. They had no teeth. Most acanthodians died out 250 million years ago. Bony fish became very successful. This competition was too much for the spiny sharks. Now, we mostly find their tiny scales in old rocks.

Acanthodians were a group of extinct jawed fishes.

People often call them "spiny sharks." They looked like modern sharks because of their streamlined bodies. They also had paired fins and an upturned tail. However, they were not true sharks. They were a group of fish that lived before real sharks appeared. Scientists think they are related to modern cartilaginous fish. These are the living sharks, rays, and chimaeras we see today.

These fishes had a very interesting way they worked. Their skeletons were made of cartilage, which is a bendy material. Most of their fins were held up by hard spines. These spines were made of dentine, the same hard stuff found in teeth.

Their skin was covered in tiny scales. These scales were shaped like small diamonds. Each scale had a bulbous base and a flat crown. Because these scales look different for each group, they help scientists date old rocks.

Scientists have studied these fossils for a long time. The name "Acanthodii" was used to describe them. Researchers like Davis and others have looked at how they are related to other fish. They found that some acanthodians might be part of the bony fish family. Other studies, like one by Burrow in 2016, suggest they are close to sharks. Fossils like *Doliodus* show a mix of shark and acanthodian features. This helps us see how fish changed over time.

There are many facts about when and where they lived. The first clear fossils date to the Silurian Period. This was about 50 million years before the first true sharks appeared.

Some of the oldest remains are from China. These are called *Fanjingshania* and *Qianodus*. They lived around 439 million years ago. At first, these fish lived in the ocean. Later, they moved into fresh water like rivers and lakes. They lived through the Devonian and Carboniferous periods.

We can group these fishes into four different orders. One group, the Ischnacanthiformes, were predators with fused tooth plates. Another group, the Acanthodiformes, were filter feeders. These fish had no teeth and used long gill rakers to eat.

They lived until the Permian period ended. This happened about 250 million years ago. Bony fish began to take over the waters during this time. The competition was too much for the spiny sharks. Eventually, they died out completely.

Acanthodii, often called "spiny sharks," refers to an extinct class of jawed fishes known as gnathostomes.

While they share a similar shape with modern sharks, they are not true sharks. Instead, scientists view them as a paraphyletic grade. This means they are a collection of different fish lineages that are related to modern cartilaginous fish, such as sharks, rays, and chimaeras. They possess a mosaic of features. This means they show characteristics found in both bony fish, or osteichthyans, and cartilaginous fish, or chondrichthyans.

The physical structure of an acanthodian was quite unique. Their internal skeletons were made of cartilage rather than bone. However, their fins were supported by stout, largely immovable bony spines. These spines were located on the anterior margin of the fins. They were made of dentine, which is the hard material found in teeth. Their bodies were streamlined and featured paired fins and a strongly upturned tail. Their skin was covered in tiny rhomboid platelets. These scales had a bulbous base, a neck, and a diamond-shaped crown.

Scientists divide acanthodians into four distinct orders based on their physical traits. The Ischnacanthiformes were predators. They possessed tooth plates that were fused directly to their jaws. In contrast, the Acanthodiformes were specialized filter feeders. They were toothless and used long gill rakers to strain food from the water. They also had only a single dorsal fin. The Climatiiformes is a group of early acanthodians, such as the climatiids and gyracanthids. They were known for having robust bony shoulder girdles and many small, sharp intermediate spines. Finally, the Diplacanthiformes is an order that was elevated from the earlier classification of the Climatiiformes.

The history of these fishes spans many millions of years. The earliest unequivocal fossils date to the beginning of the Silurian Period. This was approximately 50 million years before the first true sharks appeared. Some of the oldest remains, such as Fanjingshania and Qianodus, come from the Early Silurian of China. These fossils date back to about 439 million years ago. While the earliest acanthodians lived in marine environments, they later moved into fresh water. They became predominant in rivers and lakes during the Devonian Period. They also thrived in the coal swamps of the Carboniferous Period.

Determining the exact relationship between acanthodians and other fish has been a major task for paleontologists. Earlier researchers thought they might be ancestors to bony fish. However, a 2012 study by Davis et al. suggested they were split between the Osteichthyes and Chondrichthyes clades. A 2016 study by Burrow et al. provided further insight by suggesting chondrichthyans were nested among acanthodians. This was supported by the discovery of Doliodus. This fossil displays a mosaic of both shark and acanthodian features. This makes it a vital transitional fossil for understanding how jawed vertebrates evolved.

Because of their unique scales, acanthodians are very important for studying Earth's history. Each order has distinctive ornamentation on its scales. Because these patterns are so specific, scientists use the scales to determine the relative age of sedimentary rocks. The scales are often the only part of the fish that remains in the fossil record. This is because the cartilaginous skeletons do not fossilize as easily as the hard spines and scales. These fossils are frequently found in ancient sedimentary rocks across the globe.

Despite their long history, the acanthodians eventually faced extinction. Their diversity remained relatively low but stable during the Carboniferous Period. However, they began to decline as they entered the Permian Period. During this time, bony fishes were becoming more successful and began to dominate the water. The competition from these bony fish proved to be too much for the spiny sharks. The youngest records of the group are isolated scales and spines found in the Middle-Late Permian strata of the Paraná Basin in Brazil. This marks the end of a group that lived approximately 250 million years ago.