Selachii

Fossil record

The oldest total-group chondrichthyans, known as acanthodians or "spiny sharks", appeared during the Early Silurian, around 439 million years ago. The oldest confirmed members of Elasmobranchiisensu lato (the group containing all cartilaginous fish more closely related to modern sharks and rays than to chimaeras) appeared during the Devonian. Anachronistidae, the oldest probable representatives of Euselachii, the group containing modern sharks (Selachii) and rays (Batomorphi) to the exclusion of most extinct elasmobranch groups, date to the Carboniferous. Selachii and Batomorphi are suggested by some to have diverged during the Triassic. Fossils of the earliest true sharks may have appeared during the Permian, based on remains of "synechodontiforms" found in the Early Permian of Russia, but if remains of "synechodontiforms" from the Permian and Triassic are true sharks, they only had low diversity. Modern shark orders first appeared during the Early Jurassic, and during the Jurassic true sharks underwent great diversification. Sharks largely replaced the hybodonts, which had previously been a dominant group of shark-like fish during the Triassic and Early Jurassic.