Fossil range: Devonian
Artist's rendition of Cladoselache.
| Scientific classification
Cladoselache is a genus of extinct shark that first appeared in the Devonian period. Cladoselache was characterized by broad-based paired fins, a short stout bony spine preceding the first of the two dorsal fins, and a large lunate tail-fin. Often cited as among the best known, earliest, and/or most primitive of the early chondrichthyans (cartilaginous fishes including modern chimaeroids, sharks and rays), Cladoselache now appears ill defined and poorly diagnosed. Most of the best fossil specimens of Cladoselache are from the Upper Devonian Cleveland Shales, and include complete individuals of around 1 m (3.3 ft) in length. Several of these have mineralized traces of soft tissues, including much of the superficial jaw and gill musculature, and even apparent traces of internal organs such as the kidneys.
The dentition consists of small tricuspid teeth in which the central cusp is largest; the body is mostly naked except for compound scales on the head and fins. No specimens of Cladoselache are known to have claspers—the sex organs of male chondrichthyans used for internal fertilization (otherwise preserved as skeletal rods extending from the rear of the hind set of paired fins).
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 well preserved fossils that were discovered on the 'Cleveland Shale' on the south shore of Lake Erie. In addition to the skeleton, the fossils were so well-preserved that they included traces of skin, muscle fibers, and internal organs, such as the kidneys.
Scientists found that Cladoselache exhibited several anatomical features that are exclusive to modern sharks. This fish had a very streamlined body structure and grew up to 2 meters (6.5 ft) in length. It had five gill slits and was equipped with all of the same fins, with the exception of an anal fin, as modern sharks. It had an elongated snout and a terminal mouth at the front of the head. It also had strong spines composed of dentine and enamel that were positioned in front of the two dorsal fins. The positioning of these spines indicates that they functioned to cut water in front of the dorsal fin, making swimming easier and faster. These spine structures became more common in later sharks and are still found in some species today. Cladoselache had a large pectoral fins and a heterocercal or asymmetrical tail structure, with the top was larger than the bottom, this suggests that it was an excellent swimmer and highly maneuverable. This resembles the modern sharks of the family Lamnidae, a group which includes the white shark and makos which are two of the most efficient swimmers of all pelagic shark species. Remnants of fish fossilized swallowed hole, tail first are indicative of this early sharks speed and agility. The teeth of Cladoselache also displayed similarities in structure to modern sharks. They had a shallow root and a crown with a central cusp flanked by two smaller cusps, designed to prey on small fish. Unlike most modern and ancient sharks, Cladoselache was almost completely devoid of scales, with exception of a few cusped scales on the edges of the fins, mouth and eyes. One particular characteristic of Cladoselache that was unusual for a shark of that time was that it lacks claspers. The claspers are organ that transfer sperm during copulation. These structures were present on most early sharks, including xenocanths, and evident on all modern shark species. Also Cladoselache had a jaw that was fused with the cranium under the snout and the eye. Modern sharks have a hinged jaw that is connected the skull through several series of ligaments. The latest fossil of Cladoselache have led scientists to believe that it went extinct about 250 million years ago.
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, Cladoselache 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.
The high-aspect-ratio (tall and narrow) tail is like that of only a subset of Paleozoic sharks, all of which have a transformed stretch of the vertebral column supporting the upper lobe, but this is far from primitive and completely absent from most contemporaneous chondrichthyan and nonchondrichthyan fishes. The pectoral fins are remarkably large for the total body size, and are supported by a series of broad straplike cartilages—most of which extend directly from the shoulder girdle. This pattern was once considered primitive because it resembled a hypothesized archetypical fin condition (the continuous lateral fin-fold model) for which there is now little fossil and embryological support. Comparisons with paired fins among other early fishes now highlight the cladoselachian pattern as unusual. The braincase (neurocranium) is known in outline only; it is peculiarly wide, although this might result from postmortem compression.
Few other aspects of the paleobiology of Cladoselache are known. The Cleveland Shale was deposited in a shallow inland sea covering what is now northeastern Ohio. Cladoselache was probably a medium-sized pelagic member of the fauna, dwarfed by large arthrodire placoderms (such as Dunkleosteus) known from the same fossil localities. Stomach contents of Cladoselache include the intact remains of small ray-finned (actinopterygian) fishes (Kentuckia), preserved with heads directed toward the front of the enclosing gut-trace. This rare detail from a fossil fish suggests that they (the ray-finned prey) were engulfed from the rear, tail-first, implying that in this instance Cladoselache behaved as an active, pursuit predator.
The 'Cleveland Shale' on the south shore of Lake Erie have provided paleontologists with some of the most remarkable - and fortunate - geological accidents ever: about 100 specimens of a 370-million-year-old, 4-foot (1.2-meter) long shark called Cladoselache, some of which are so exquisitely preserved that not only teeth and fin spines, but also jaws, crania, vertebrae, muscle fibers, and even kidney tubules are discernible to varying degrees.
Cladoselache was a predatory shark, and the well preserved fossils found on the Cleveland Shale revealed a significant amount regarding its eating habits. Within the gut of most Cladoselache fossils were remnants of their stomach contents. 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, indicating that Cladoselache was a fast and agile hunter.
These extremely well-preserved Cladoselache specimens support the notion - inferred from its tail shape - that it was a fast-swimming hunter. Paleontologist Mike Williams has studied many of the superbly preserved fossil specimens of Cladoselache excavated from the Cleveland Shale. Astonishingly, 53 of these specimens had identifiable traces of their last meal preserved in their gut regions. These allowed Williams to glean some insights into the predatory habits of Cladoselache. He found that 65% of specimens examined had eaten small ray-finned bony fishes, 28% shrimp-like Concavicaris, 9% conodonts (peculiar hagfish-like proto-vertebrates with complex, comb-like teeth), and one specimen had eaten another shark. (These percentages add up to more than 100 because some specimens had eaten more than one kind of prey.)
The orientation of food items in the body cavity suggests that Cladoselache was swift enough to catch its prey on the fin. 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.
There may have been another reason for Cladoselache to adopt a high-speed lifestyle. It shared the Devonian seas with Dunkleosteus, a 20-foot (6-metre) long predatory placoderm with huge teeth and massive, heavily armored jaws.
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.
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