Euselachians

Euselachians are a group of ancient cartilaginous fishes that lived during the Early Devonian period, around 416 million years ago. They are considered to be one of the earliest known groups of jawed vertebrates, and are seen as a link between the earlier jawless fishes and the modern cartilaginous fishes.

The name "euselachians" means "true sharks" and is used to distinguish this group from the more primitive squaliomorpha, which are thought to be their ancestors. Euselachians were characterized by the presence of a symmetrical arrangement of tooth-like denticles on their jaws, a more complex brain structure, and a more advanced skeleton compared to squaliomorpha.

Euselachians were some of the first fish to have a more advanced jaw structure, which allowed them to feed on a wider variety of prey. Their skull bones were more compressed and their spiracles were moved to the dorsal side of their head. Some species of euselachians were also covered in leathery skin and had a more robust body compared to modern cartilaginous fishes.

Several fossil specimens of euselachians have been found in rocks dating back to the Early Devonian period. One of the most famous species is Leptodus, which was a relatively large euselachian with a body length of around 3 meters. Despite their relatively short history, euselachians played a significant role in the evolution of fish diversity and paved the way for the development of modern cartilaginous fishes.