Cladoselache

Cladoselache was a genus of early sharks that lived during the Late Devonian (363-358 million years ago). With a streamlined body well-adapted to its carnivorous lifestyle, Cladoselache probably preyed on squid, fish, and crustaceans (Palmer 1999). At nearly 2 meters (7 feet) long (Palmer 1999), this shark was about one third the size of the average mature great white shark. Many well fossilized specimens of Cladoselache have been found, some with muscle and soft tissue imprints, in the Cleveland Shale (Palmer 1999).

During the Late Devonian (363-358 million years ago), all land mass was concentrated in one single super-continent named Pangea. Surrounding Pangea was the Panthalassa sea, a gigantean global sea teeming with life (Van der Meer et al. 2012). This sea was the home of Cladoselache. In popular culture, the Devonian is often called 'The Age of Fish'. This is an apt title; the oceans of the Devonian were home to incredible biodiversity. Fish had radiated into many different types; some of the most recognizable are the placoderms, or armor plated fish. This group included Dunkleosteous, one of the dominant predators of the Devonian. Acanthodians (jawed fish), squids, crustaceans, and jawless fish were also common in Devonian oceans (Palmer 1999). In deep waters, large coral beds flourished, while the shallow costal areas were home to Stromatoporoids, reef builders related to sponges (Embry III & Klovan 1972).

Most early sharks were not well suited to fossilization; they were Chondrichthyes, with skeletons made out of cartilage. Cartilage is a flexible soft tissue found in noses. It decays too fast to fossilize, and, as a consequence, the fossil record of Chrondrichtyes is comprised of mostly teeth. Cladoselache, though Chondrichthyes, fossilized extremely well. This was most likely do to the genus' population density; there were so many Cladoselache in the Devonian that some were well fossilized by chance. These specimens were found in the Cleveland Shale, a site where fine sediment was deposited on top of organisms with little to no oxygen present. Shale can be an incredible preserver of soft tissue. Several fossils have even been found with impressions of internal organs, skin, and muscles (Palmer 1999).

In appearance, Cladoselache would have looked very similar to modern oceanic sharks. It had a sleek body, larger near the head and narrowing down towards the tail. It was about 1.8 meters long (Palmer 1999), about one third the length of the average mature great white shark. Interestingly, it had scales only around its eyes and jaw. Cladoselache had teeth with a single high peak and a number of surrounding accessory cusps throughout its jaw that allowed it to catch and hold soft bodied fish (Carroll 1988).
Unlike many modern sharks such as great whites, Cladoselache's two dorsal fins (extending upward from the top of the animal, one near the head and one further down the body) were of an equal length. Its caudal fin (tail fin) was equally sized on the top and bottom. It had keels (horizontal fins near the caudal fin) characteristic of sharks and swift fish, such as tuna (Carroll 1988). Cladoselache's pectoral fins (extending downwards near the head) were unusually large for a shark of its size (Carroll 1988). Both fast and agile, Cladoselache was a ferocious predator. There was no shortage of food sources in the Late Devonian seas, 363-358 million years ago. It ate squids, crustaceans, and fish (Palmer 1999).It is unlikely that Cladoselache's jaw could have punctured through the tough armor of the contemporary Placoderms (armored fish).
Like all Chrondrichthyes (the class that contains sharks, rays, skates, and chimaeras), Cladoselache had a skeleton composed of cartilage (flexible organic material found in noses), not bone. (Palmer 1999, Carroll 1988). It breathed with five sets of gills located along the head (Palmer 1999).
Unlike all other sharks, Cladoselache had no claspers (specialized male reproductive organs) (Carroll 1988). This has led to some confusion about Cladoselache's reproductive cycle, though it is known that the genus fertilized internally, like mammals and pelagic (ocean) sharks (Carroll 1988).