Acanthodii

Acanthodii, also called acanthodians, were an ancient group of jawed fish that lived long ago. They are not true sharks, but they looked similar, with a streamlined body and paired fins. These fish were nicknamed "spiny sharks" because they had bony spines on their fins.

Acanthodians had a skeleton made of cartilage, like modern sharks. Their fins had a bony base and strong spines made of dentine. Because of this, scientists often find only these spines and tiny scales when they dig up ancient rocks. The earliest acanthodians lived in the sea, but later many species moved to freshwater lakes and rivers during the Devonian period.

These fish were divided into four main groups. Some were predators, while others ate small organisms. Acanthodians were around until the Permian period.

Characteristics

The scales of Acanthodii have special patterns that help scientists study old rocks. These tiny scales have a round base, a neck, and a flat or slightly curved diamond shape on top.

Even though they are called "spiny sharks," acanthodians lived long before sharks existed. The oldest clear fossils of acanthodians come from the start of the Silurian Period, about 50 million years before sharks. Later, they lived in fresh water in rivers and lakes during the Devonian and in the coal swamps of Carboniferous times. But as bony fishes became stronger, acanthodians could not compete and went extinct in the Permian Period, around 250 million years ago.

Many scientists once thought acanthodians were close to the ancestors of bony fishes. Though their inside skeletons were made of cartilage, their skin had bony-like material in the form of close-fitting scales. Some scales were very large and formed a bony cover on the head and shoulders. Others created a bony flap over the gills, similar to a feature in later bony fishes. These traits are thought to have come from shared ancient placoderm ancestors and are also found in early cartilaginous fish. The jaws of acanthodians likely came from the first gill arch of ancient jawless fishes with a gill skeleton made of jointed cartilage.

Taxonomy and phylogeny

See also: List of acanthodian genera

Scientists study how early fish with jaws are related. They found that acanthodians, an old group of fish, have traits from two main groups: bony fish and cartilaginous fish. One well-known acanthodian, Acanthodes, was grouped with bony fish even though it had some features of cartilaginous fish.

Later discoveries, like a Silurian placoderm called Entelognathus, changed this idea. Placoderms share jaw features with both bony fish and four-legged animals. This made scientists think acanthodians were a mix of different fish groups that led to modern cartilaginous fish, while bony fish came from placoderms.

More recent research shows that cartilaginous fish are closely related to two other acanthodians, Doliodus and Tamiobatis. A 2017 study of Doliodus shows it has a mix of traits from both sharks and acanthodians. It acts like a transitional fossil that helps explain these connections.

Evolutionary history

The oldest remains of acanthodians are from two types of fish called Fanjingshania and Qianodus, found in China from about 439 million years ago. These fish were part of a group that did not change much in shape or way of living compared to other fish at the time.

Acanthodians became more diverse during the Late Silurian period and were most common during the Early Devonian period. Their numbers went down a bit later but rose again. By the time of the Carboniferous and Permian periods, they were still around but in smaller numbers. The last known signs of them are from the middle to late Permian period in Brazil.