Cladoselache ("branch shark") is an extinct genus of shark from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks (such as mako sharks and the great white shark), but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers.
Some 20th century studies considered Cladoselache to be a basal (early-diverging) member of Elasmobranchii, the fork of cartilaginous fish which leads to modern sharks and rays. More recent studies have identified distinctive traits of the chondrocranium (cartilaginous braincase), dorsal fin spines, and pectoral fin bases. These newly identified features support a close relationship to symmoriiforms, a small group of bizarre chondrichthyans such as the bristle-spined Stethacanthus. Cladoselache and symmoriiforms may be more closely related to chimaeras (a modern group of unusual deep-sea fish) than to true sharks and rays.
Growing to several meters in length, Cladoselache is considered to have been a fast-moving and fairly agile marine predator due to its streamlined body and deeply forked tail. From both an anatomical and historical perspective, is one of the best known of the early chondrichthyans in part due to an abundance of well-preserved fossils, discovered in the Cleveland Shale on the south shore of Lake Erie. In addition to the cartilaginous skeleton, the fossils were so well preserved that they included traces of skin, muscle fibers, and internal organs, such as the kidneys.
Description
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.
Like other chondrichthyans, the "skull" of Cladoselache consists of a central cranial cartilage protecting the brain and sensory organs (the chondrocranium, a cartilaginous equivalent of the neurocranium or braincase) alongside an upper and lower toothed jaw cartilage (the palatoquadrate and Meckel's cartilage, respectively). The eyes and nasal capsules were large and shifted forwards.
The gill basket is poorly known, but available evidence points towards the presence of at least five pairs of slender and well-separated branchial arches, and therefore five gill slits on either side of the body. Some early sources have suggested that six or even seven gill slits may have been present, though this has not been confirmed. The gill slits were separated by muscular flaps packed with fibrous overlapping gill filaments.
There are a pair of large pectoral fins behind the gills and low pelvic fins further back along the underside. Cladoselache has two dorsal fins, with the first being larger and preceded by a stout, curved fin spine. The caudal peduncle (tail) is tapered and keeled, ending at a crescent-shaped caudal fin.
Chondrocranium
The chondrocranium was low, with a broad and triangular sphenoid region (above and between the eyes) followed by a somewhat smaller otic region (above the hindbrain and balance organs). The ethmoid region (between the nostrils) at the front of the chondrocranium is unknown, but it was likely far shorter than the overhanging rostrum ("snout") of modern sharks. The underside of the chondrocranium is perforated by various nerve and blood vessel openings. A deep excavation is present at its rear tip, below the connection to the spinal column. This excavation contains a pair of closely spaced openings, where the two lateral branches of the dorsal aorta (a major artery) enter the braincase. Cladoselache's condition is intermediate between symmoriiforms (which have a single opening within the excavation) and other chondrichthyans (which have widely separated openings not confined to an excavation). Each branch of the dorsal aorta reemerges from the chondrocranium further forwards.
There are many other braincase traits shared between Cladoselache and symmoriiforms. The exits for the orbital arteries (which supply the eye and its vicinity with blood) are directly in front of the exits for the dorsal aortae. The postorbital process (which separates the eye socket from the ear region) is elongated (from side-to-side) and followed by a waisted area. A distinct notch develops on the outer edge of the wide supraorbital shelf (a plate which overhangs the eyes). However, in Cladoselache the exits for every branch of the facial nerve (nerve VII) are widely separated from the orbital arteries. This contrasts with symmoriiforms, in which some branches of the facial nerve are bundled with the orbital arteries in a groove or shared opening.
Jaws The palatoquadrate and Meckel's cartilage were somewhat lightly built and hosted a series of transverse tooth rows sitting within scalloped grooves. Fossils reconstruct a terminal mouth (opening at the front of the head), similar to Chlamydoselachus (the modern frilled shark) but unlike other modern sharks. The joint between the upper and lower jaw is weakly connected, but facets for strong, sheet-like muscles can be found on the outer surface of the jaw. The otic process (rear portion) of the palatoquadrate expands upwards to articulate with the postorbital process of the chondrocranium. The palatine ramus (front portion) also expands to a lesser extent, presumably articulating with the ethmoid region of the chondrocranium. A similar two-point articulation is also found in xenacanthid sharks. The rear edge of the palatoquadrate would have been braced by the rod-shaped hyomandibula. With stable connections to both the chondrocranium and the hyomandibula, the jaw suspension of Cladoselache can be classified as amphistyly. Amphistyly is common in early chondrichthyans, but it is only present in hexanchiforms (frilled and sixgill sharks) among modern members of the group. Most modern sharks and rays have a looser jaw suspension which only involves the hyomandibula (a condition known as hyostyly). Chimaeras, on the other hand, have a palatoquadrate fused to the chondrocranium (known as holostyly).
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. This is supported by the presence of worn surfaces on some teeth, indicating prolonged tooth retention.
Postcrania and fins
The spinal column of Cladoselache was cartilaginous and poorly preserved, without calcified centra (spool-shaped main components) encompassing the notochord. Typically only the pointed, closely spaced neural arches can be observed in fossils. All of the fins (except the caudal fin) have the same basic structure: a wide base which is continuous with the torso, and a subtriangular profile supported along nearly the whole length of the fin by cartilaginous rods known as radials. Only a few radials are segmented, with thicker radials concentrated at the front of each fin and thinner radials concentrated further back.
The pectoral fin is large and elongated, connecting to a strong and broad pectoral girdle. A tall scapula projects a fair distance upwards from the base of each pectoral fin. The innermost cartilage in the pectoral girdle are the procoracoids, a pair of L-shaped structures also found in most other Paleozoic chondrichthyans. Each connects outwards to a wing-shaped coracoid plate which forms the base of the pectoral fin. The front half of the pectoral fin consists of eleven radials (slender fin components) which attach directly to the coracoid plate. The first six are long and unsegmented while the following five are segmented, separated into a short inner portion (proximal radials) and a longer outer portion (distal radials). In the rear half of the pectoral fin, a triangular cartilaginous structure, the metapterygium, expands out from the rear of the coracoid plate. More than six particularly slender radials attach to the metapterygium. Many traits of Cladoselache's pectoral girdle are characteristic of symmoriiforms, including large paired procoracoids, proximal radials, and a distinct metapterygium at the rear of the coracoid. Cladoselache also has a unique trait when compared to other Paleozoic chondrichthyans: the rear edge of the metapterygium does not send out a string of small cartilage structures known as axials.
The pelvic fins are small and low, about three times longer (from front-to-back) than high. Their backswept radials connect to rows of basals (small linking structures), which are embedded into the torso above each pelvic fin. Basals are also present below the dorsal fins. A pair of pointed rod-like structures, which appear to be an early form of pelvic girdle, project forwards from each row of pelvic basals. Claspers have never been found in Cladoselache, despite their presence in close relatives and even other Devonian chondrichthyans. This may be due to its reduced pelvic region, a reliance on external fertilization, or simply a lack of male fossils preserving the area. 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.