Cladoselache

Cladoselache is a genus of extinct shark. It appeared in the Devonian period.

This primitive shark grew to be up to 6 feet (1.8 m) long and roamed the oceans of North America. It is known to be a fast moving and fairly agile predator due to its streamline body and deep forked tail. Cladoselache is one of the best known of the early sharks in part to the efficiently preserved fossils that were discovered on the 'Cleveland Shale' on the south shore of Lake Erie. These fossils were so well preserved that along with teeth and fin spines, also visible were the jaws, vertebrae, crania, and muscle fibers.

Appearance
Cladoselache exhibited an extraordinary combination of derived and ancestral characteristics. It has anatomical features similar to the current mackerel sharks of the family Lamnidae. In comparison with ancestral sharks Cladoselache had a short, rounded snout that had a terminal mouth opening at the front of the skull. It had a very weak jaw joint compared with modern-day sharks, but it compensated with very strong jaw-closing muscles. Its teeth were multi-cusped and smooth-edged, making them suitable for grasping, but not tearing or chewing. Cladoselache therefore probably seized prey by the tail and swallowed it whole. Its sturdy but light-weight fin spines were composed of dentine and enamel. It was also short and resembled a blade like structure which was positioned in front of the dorsal fins. These anatomical features made swimming easier and faster. Unlike most sharks, Cladoselaches was almost entirely devoid of scales with exception of small cusped scales on the edges of the fins, mouth and around the eyes it. It also had powerful keels that extended on the side of the tail stalk and a semilunate tail fin, with the superior lobe about the same size as the inferior. This combination helped with its speed and agility which was useful when trying to out-swim its predator the heavily armored 15-foot long fish Dunkleosteus.

Diet and Reproduction
Cladoselache was a predator fish in the Devonian era. The well preserved fossils of Cladoselache found on the Cleveland Shale revealed a significant amount regarding its eating habits. Within the gut of most Cladoselache fossils were remnants of what they had eaten before they died. These remains included mostly small ray-finned bony fishes as well as shrimp-like fish and hagfish-like proto-vertebrates. Some of the fish remains were found tail first within the stomach concluding that Cladoselache was a fast and agile hunter. A mystery that has yet to be resolved is its method of reproduction. One of the most uncommon characteristics of Cladoselache was that it lacked claspers which are organs that are responsible for the transfer sperm during reproduction. This is peculiar given that most early shark fossils show evidence of claspers to prove that they were utilized in the customary method of shark reproduction. An assumption is that they utilized internal fertilization, however this is only a theory and the actuality is still unknown.