Brief Description: The fish are polymorphic and, in several species, the shape of the body is fusiform and laterally oblate. The typical fusiform shape appears mainly in fish of the open sea. They are anamniote animals, with body covered with scales. They are cold-blooded vertebrates, which are distinguished by the gills and radial fins. Fish are the largest genus, in terms of number of Vertebrate species. They are classified in more than 100 classes and around 720 families, 263 of which are fossilized, 224 are fossilized but alive today as well and 233 are only alive today. Osteichthyes appear in the Upper Silurian Period and still live today. About 700 different fossilized genera have been studied and a large number of their representatives live today in sea as well as brackish and fresh water. 
Bibliography: Γεωργιάδου-Δικαιούλια, Ε., Συμεωνίδης, Ν., Θεοδώρου, Γ. (2003) Παλαιο-ντολογία (μέρος Γ?), Εκδόσεις Γ. Γκελμπέση, Αθήνα.