Chondroichthyes sp. (4) Fossil Shark Vertebra Dogfishes Cartilaginous Fishes Chondrichthyes Selachians Elasmobranchs

Fossil Shark Vertebra mm 20 x 10 Dogfishes Cartilaginous Fishes Chondrichthyes Selachians Elasmobranchs, only a piece, as in photos.

The Chondrichthyes, or even cartilaginous fish, are a class of fish that includes over 1100 different species and to which belong sharks and races (included in the subclass of Elasmobranchii), chimeras (subclass of Holocephali) and forms related.
These are organisms that have the peculiarity of possessing an internal cartilaginous skeleton, hence the name of the class. Therefore they do not have dermal bones. Most species have 5 gill slits (6 or 7 in some Elasmobranchii; 4 in Holocephali). All the Chondrichthyes have placoid dermal scales and a developed electro-reception apparatus.
Reproduction is sexual with internal fertilization. The species can be oviparous, ovoviviparous or even viviparous and do not produce a conspicuous offspring, a fact for which they are considered to be at high risk of extinction.
Cartilaginous fish are believed to have evolved from the Acantodii, while bony fish evolved directly from the Placodermi.
The first unmistakable cartilaginous fish fossils appeared for the first time in the fossil record of about 430 million years ago, during the Silurian period. At the beginning of the first Devonian, 419 million years ago, fish with jaws (gnatostomes) had been divided into three distinct groups: the now extinct Placodermi (a group of ancient armored fish), bony fish and cartilaginous fish. The modern bony fish, class Osteichthyes, appeared in the late Silurian or early Devonian, about 416 million years ago. The first widespread species of shark, Cladoselache, appeared in the oceans during the Devonian period. The first cartilaginous fish evolved from shark ancestors similar to Doliodus.
An analysis of molecular data suggests that the Holocephali and Elasmoblanchii diverged in the Silurian (421 million years ago), while the races split into the Carboniferous (306 million years ago).