This is an Eocene dogfish (in the South, these are called "mudfish"), Cyclurus (Amia) kehreri, from the Messel Shale, around Messel, Germany. The "bituminous claystone" (think "oil-shale") around Messel constitutes a lagerstatte (plural: lagerstatten) of Eocene fossils, both land mammals and freshwater fish.
Other fish associated with Amia in these lacustrine deposits include Lepisosteus, Amphiperca, Palaeoperca, and Thaumaturus. Along with these freshwater fish, plants, insects, crustaceans, mollusks, sponges, reptiles, mammals, birds, and amphibians are collected here.
Fish from Messel take special preparation because of the crumbly nature of the shale. This fish was prepared on one side, then a layer of resin was poured over the prepared side. The now-stabilized specimen was then completely prepared from the other side, removing all the shale matrix, but leaving all the bones!
Less-fragile fossils come out of the shale intact. There is a mind-boggling display of these Messel fossils in the natural history museum in Basel, Switzerland, which includes a number of articulated mammal skeletons. Worth a visit.
From Oilshale: Class: Actinopterygii Family: Amiidae Messel Formation Hessia International Age: Ypresian
Cyclurus kehreri, originally assigned to the recent genus Amia, was placed in Cyclurus by Gaudant (1987). Lit.: GAUDANT J. 1999a. — Cyclurus kehreri (Andreae) : une espèce clé pour la connaissance des Amiidae (Poissons actinoptérygiens) du Paléogène européen. Courier Forschungsinstitut Senckenberg 216: 131-165.
L. Grande and W. E. Bemis. 1998. A comprehensive phylogenetic study of amiid fishes (Amiidae) based on comparative skeletal anatomy. An empirical search for interconnected patterns of natural history. Society of Vertebrate Paleontology Memoir 4. Journal of Vertebrate Paleontology 18(1, suppl.):1-690