Cladoselache
Cladoselache Temporal range:
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Complete C. fyleri specimens in the American Museum of Natural History | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Chondrichthyes |
Subclass: | Holocephali |
Order: | †Cladoselachiformes |
Family: | †Cladoselachidae |
Genus: | †Cladoselache Dean, 1894 |
Species | |
List of species
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Cladoselache is an
Some 20th century studies considered Cladoselache to be a
Growing to several meters in length, Cladoselache is considered to have been a fast-moving and fairly agile marine
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.[7][8]
The head shape of Cladoselache shares some similarities with modern
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).[10][2] The eyes and nasal capsules were large and shifted forwards.[11][10]
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.[10][12] Some early sources have suggested that six or even seven gill slits may have been present, though this has not been confirmed.[11][10] The gill slits were separated by muscular flaps packed with fibrous overlapping gill filaments.[11][12]
There are a pair of large
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).[13][2] 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.[14] The underside of the chondrocranium is perforated by various nerve and blood vessel openings.[14] 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.[15][2] 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.[2]
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.[2]
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.[11][10][14] 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.[12] 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).[12][2][16]
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.[4] 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).[10]
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,[10][14] and about eleven or twelve rows arranged from front to back on each palatoquadrate.[10][17] 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.[17] This is supported by the presence of worn surfaces on some teeth, indicating prolonged tooth retention.[11]
Postcrania and fins
The
The
The
The
There are two dorsal fins: the first dorsal fin is located above the pectoral fins and is significantly taller than the second dorsal fin, which is located above the pelvic fins. Cladoselache was initially considered to lack fin spines, unlike most prehistoric sharks.[10] However, a stout spine-like structure is now known to have been present in front of the first dorsal fin, according to specimens which were first described in 1938.[9] The dorsal fin spine was recurved and nearly as long (from front-to-back) as it was tall, in contrast to the far more slender spines of ctenacanthiforms and other chondrichthyans. There is no ornamentation apart from randomly-oriented furrows which extend over its entire area. The base of the spine was probably only shallowly implanted into the skin, and not closely appressed to the succeeding dorsal fin.[14] The fin spine slots onto a plate of calcified cartilage, which is gilded with layers of bone-like cellular and acellular dentine. This internal structure is most similar to the sexually dimorphic spine brush complex of symmoriiforms, rather than the enamel-coated fin spines of other sharks.[3] While another cladoselachid Maghriboselache had fin spine in front of both dorsal fins,[19] for Cladoselache the presence of a fin spine in front of the second dorsal is uncertain,[9] it is considered to be hypothetically present[20] or absent.[13]
Classification
Cladoselache is considered one of the best-known early members of the
Chondrichthyes splits into two branches early in the evolution of the group.
Traditional interpretations
The earliest publications regarding the genus noted that both Cladoselache and
The position of Cladoselache as an early chondrichthyan near ctenacanths and other cladodont “sharks” was generally unchallenged in the 20th century. Most disagreements related to precisely how early Cladoselache branched off from the rest of the chondrichthyan family tree. Some studies argued that it was the earliest-diverging member of the group,
Relation to symmoriiforms and holocephalans
More recent information on Cladoselache’s anatomy has led many authors to propose close relations between Cladoselache and symmoriiforms such as Stethacanthus and Falcatus.[24][25][26][5] The dorsal spine of Cladoselache appears to be homologous to the unusual head structures prevalent in symmoriiforms,[3] and many similarities are also present in the braincase and pectoral region.[2][4] Symmoriiforms and Cladoselache may form an early-diverging clade at the base of Chondrichthyes or Elasmobranchii, similar to traditional interpretations of their affinities.[24][25][26]
However, an alternative hypothesis has caught traction: Symmoriiforms (including Cladoselache) may lie on the opposite side of Chondrichthyes, as an early part of the
Chondrichthyes |
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Palaeobiology
The teeth of Cladoselache were slender and smooth-edged, making them suitable for grasping, but not tearing or chewing.[17] Members of the genus were predators, 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),[30][31] as well as thylacocephalan arthropods (Concavicaris)[32][33] and conodonts, hagfish-like proto-vertebrates.[34] 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
The exceptional preservational conditions of the
References
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- ^ a b c d Harris, John Edward (1938). "The dorsal spine of Cladoselache". Scientific Publications of the Cleveland Museum of Natural History. 8: 1–6.
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- ^ a b c d e Harris, John Edward (1938). "The neurocranium and jaws of Cladoselache". Scientific Publications of the Cleveland Museum of Natural History. 8: 7–12.
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- ^ S2CID 85858435.
- ^ Trinajstic, K.; Boisvert, C.; Long, J.A.; Johanson, Z. (2018). "Evolution of vertebrate reproduction". In Johanson, Z.; Underwood, C.; Richter, M. (eds.). Evolution and Development of Fishes. Cambridge: Cambridge University Press. pp. 207–226.
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- ^ a b Zangerl, R. (1981). Chondrichthyes I – Paleozoic Elasmobranchii. Handbook of Paleoichthyology. Stuttgart: Gustav Fischer Verlag. pp. i–iii, 1–115.
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Sources
- Ferrari, Andrea; Ferrari, Antonella (2002). Sharks. Buffalo: Firefly Books.
- Maisey, John. G. (June 1998). "Voracious Evolution". Natural History. 107 (5): 38–41.
External links
- Cladoselache ReefQuest Centre for Shark Research. Retrieved 10 February 2012.
- Monastersky, Richard (1996) The first shark: to bite or not to bite Science News, 149 (7): 101.
- The Evolution of Sharks