Ohmdenia

 
Ohmdenia is an extinct genus of prehistoric bony fish that lived from the Toarcian stage of the Early Jurassic epoch.
"Ohmdenia" was first described in 1953 by B. Hauff, based on a fossil found in the well-known Posidonia Shale in Holzmaden, Germany. For a long time this animal has been considered a close relative of "Birgeria", a great predator typical of the Triassic period with an uncertain systematic position . Further studies have shown similarities with the pachicormiforms, a group of neopolitics considered close to the origin of teleosts and also including giant forms and planctives (e.g. "Leedsichthys"). Some studies have erroneously indicated "Ohmdenia" as a synonym of "Saurostomus", other studies have instead placed "Ohmdenia" as an important evolutionary passage between the basal pachicormiforms and the more derived planktivore pachicormiformes.
Description.
This animal is known exclusively for an incomplete fossil, but sufficient to reconstruct (at least partially) its appearance. "Ohmdenia" must have been a large fish, at least two and a half meters long. "Ohmdenia" was therefore one of the largest bony fishes of the lower Jurassic, surpassed only by the great chondrostei such as "Strongylosteus" and "Gyrosteus" . The body had to be relatively slender, with a symmetrical and slender tail. The skull was long and short, and had a jaw equipped with numerous teethsmall, rather robust, rear-facing and placed in an area that extended along the dorsal margin of the jaw. The combination of a long and slender body and a low and elongated skull is unique among the fish related to "Ohmdenia" (the pachicormiforms).
Paleobiology.
The long mouths of "Ohmdenia", together with the peculiar teeth, imply a particular diet for this animal. Usually, the pachicormiforms possess thin, needle-like teeth, or large fanged fangs, or are still totally devoid of teeth. "Ohmdenia", on the other hand, possessed numerous small backward-facing teeth; this type of dentition is commonly associated with predators that feed on Cephalopods from the soft body. Even the jaws would seem to be less robust than those of the other pachicormiforms. The presence of two fossils of belemnites in the area of the belly of "Ohmdenia" corroborates the theory that this animal is seen as a predator of molluscs; cephalopods with a soft body.