Cladoselache

-like pelagic predator|350px The anatomy of Cladoselache shows a mixture of derived and ancestral characteristics. The skeleton is composed of tessellated cartilage, a complex tissue unique to chondrichthyans. Tessellated cartilage combines flexible cartilage fibers with a loose mosaic of irregular calcified plates, known as tesserae. The head shape of Cladoselache shares some similarities with modern frilled sharks, while its overall streamlined body shape is reminiscent of mackerel sharks in the family Lamnidae, which likely had a similar ecology. The largest undisputed skeleton of Cladoselache was about 2.0 meters (6.6 feet) in length, though many specimens were much smaller. Some early sources have suggested that six or even seven gill slits may have been present, though this has not been confirmed. Teeth Like many other early chondrichthyans, Cladoselache had "cladodont" teeth, with a large and sharp central cusp surrounded by one or two pairs of smaller cusps. The median (central) cusp has a flat labial face (the side facing the cheek), in contrast to other "cladodonts" which had a convex labial face. The root of the tooth is broad and has a deeply arched concavity on its underside (known as a basiolabial depression), which is flanked by triangular projections. Teeth at the front of the mouth were generally taller, with a long median cusp and two pairs of small and pointed lateral cusps, with the outer pair larger than the inner pair. Further back, the median cusp becomes shorter and the inner pair of lateral cusps shrink into oblivion, leaving only the outer pair. The teeth are thickly layered with typical dentine, but lack enamel (the hypercrystalline outer layering of osteichthyan teeth) or enameloid (an enamel-like form of dentine in shark teeth). Cladoselache was one of the earliest vertebrates known to have had shark-like tooth replacement, with a series of widely spaced tooth rows constantly unfurling new teeth outward. There were around seven to nine closely packed teeth per tooth row, and about eleven or twelve rows arranged from front to back on each palatoquadrate. The oldest, outermost teeth in each row were smaller than newly formed teeth further inwards, suggesting that tooth replacement was slow enough to keep pace with the animal's overall rate of growth. Postcrania and fins The spinal column of Cladoselache was cartilaginous and poorly preserved, without calcified centra (spool-shaped main components) encompassing the notochord. Cladoselache appears to lack an anal fin. However, the underside of the tail stalk encased six pairs of forward-swept radial-like structures, with the first pair being particularly large. These structures, which may be homologous to the unpaired anal fin, extend as far back as the base of the caudal fin. While another cladoselachid Maghriboselache had fin spine in front of both dorsal fins, for Cladoselache the presence of a fin spine in front of the second dorsal is uncertain, it is considered to be hypothetically present or absent. == Classification ==

Cladoselache is considered one of the best-known early members of the Chondrichthyes: cartilaginous fish such as sharks, rays, and the deep-sea chimaeras (also known as ratfish or ghostsharks). Though it resembles modern sharks (selachians) in ecology and body shape, it is not a member of that group, which did not evolve until the Jurassic Period. When first described near the end of the 19th century, Cladoselache was immediately recognized as a plesiomorphic (ancestral) form of cartilaginous fish, and not closely related to modern sharks despite its similar appearance. while others placed it as an elasmobranch, originating slightly later than the holocephalans. though this was an uncertain and unstable position. The dorsal spine of Cladoselache appears to be homologous to the unusual head structures prevalent in symmoriiforms, Cladogram based on the phylogenetic analysis of Frey et al., 2020. Major groups are bolded: }} ==Palaeobiology==

The teeth of Cladoselache were slender and smooth-edged, making them suitable for grasping, but not tearing or chewing. and the well-preserved fossils found in the Cleveland Shale revealed a significant amount regarding their eating habits. Within the gut of most Cladoselache fossils were remnants of their stomach contents. These remains included mostly small ray-finned bony fishes ("Kentuckia" hlavini which is probably not belonging to genus Kentuckia), as well as thylacocephalan arthropods (Concavicaris) and conodonts, hagfish-like proto-vertebrates. Some of the fish remains were found tail first within the stomach, indicating that Cladoselache was a fast and agile hunter. A mystery that has yet to be resolved is its method of reproduction. Cladoselache is not known to possess claspers, organs found in modern sharks that are responsible for the transfer of sperm during reproduction. This is peculiar given that most other early shark fossils show evidence of claspers. While they may have used internal fertilization, this has yet to be demonstrated. Scales and soft tissue Placoid scales (denticles) were present in Cladoselache, but they are tiny and bluntly conical structures which are absent from much of the body. Larger denticles occur on fin margins and adjacent to the lateral line, The eye was protected by a ring of numerous (more than 20) small dermal ossifications, which were initially interpreted as denticles with a function similar to modern scleral ossicles. The exceptional preservational conditions of the Cleveland Shale allow insights into soft-tissue structures such as organs and muscles, which are not typically preserved in fossils. A pair of long organs were present between the pelvic fins and the caudal fin. These organs were packed with small tubules, and may represent kidneys, despite their unusual position. This suggests that the body cavity of Cladoselache (including the digestive and urinary systems) extends further back into the caudal peduncle relative to modern sharks. Gut content and coprolites are strongly twisted, arguing that the digestive system of Cladoselache was similar to living sharks. The musculature of Cladoselache is organized into discrete packages of muscle fibers, which are easier to differentiate than those of modern sharks. ==References==