Elasmobranchii (/ɪˌlæzməˈbræŋkiaɪ/[1]) is asubclass ofChondrichthyes or cartilaginous fish, including modernsharks (division Selachii), andbatomorphs (division Batomorphi, includingrays,skates, andsawfish). Members of this subclass are characterised by having five to seven pairs ofgill slits opening individually to the exterior, rigiddorsal fins and smallplacoid scales on the skin. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper. The details of this jaw anatomy vary between species, and help distinguish the different elasmobranchclades. The pelvic fins in males are modified to createclaspers for the transfer of sperm. There is noswim bladder; instead, these fish maintain buoyancy with large livers rich in oil.
The definition of the clade is unclear with respect to fossil chondrichthyans. Some authors consider it as equivalent toNeoselachii (thecrown group clade including modern sharks, rays, and all other descendants of theirlast common ancestor). Other authors use the name Elasmobranchii for a broaderbranch-based group of all chondrichthyans more closely related to modern sharks and rays than toHolocephali (the clade containingchimaeras and their extinct relatives).[2] Important extinct groups of elasmobranchssensu lato include thehybodonts (Order Hybodontiformes),xenacanths (order Xenacanthformes) andCtenacanthiformes. These are also often referred to as "sharks" in reference to their similar anatomy and ecology to modern sharks.
The name Elasmobranchii comes from theAncient Greek wordselasmo- ("plate") andbránchia ("gill"), referring to the broad, flattened gills which are characteristic of these fishes.
From a practical point of view the life-history pattern of elasmobranchs makes this group of animals extremely susceptible to over fishing. It is no coincidence that the commercially exploited marine turtles and baleen whales, which have life-history patterns similar to the sharks, are also in trouble.[7]
Members of the Elasmobranchii subclass have noswim bladders, five to seven pairs ofgill clefts opening individually to the exterior, rigiddorsal fins, and smallplacoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper.
Extant elasmobranchs exhibit several archetypal jaw suspensions: amphistyly, orbitostyly, hyostyly, and euhyostyly. In amphistyly, the palatoquadrate has a postorbital articulation with the chondrocranium from which ligaments primarily suspend it anteriorly. The hyoid articulates with the mandibular arch posteriorly, but it appears to provide little support to the upper and lower jaws. In orbitostyly, the orbital process hinges with the orbital wall and the hyoid provides the majority of suspensory support.
In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid.[8][9][10] The eyes have atapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute aclasper for the transmission ofsperm. These fish are widely distributed intropical andtemperate waters.[11]
Many fish maintain buoyancy withswim bladders. However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil.[12] This stored oil may also function as a nutrient when food is scarce.[7][13]
The oldest unambiguoustotal group elasmobranch,Phoebodus, has its earliest records in the MiddleDevonian (lateGivetian), around 383 million years ago.[14] Several important groups of total group elasmobranchs, includingCtenacanthiformes andHybodontiformes, had already emerged by the latest Devonian (Famennian).[15] During theCarboniferous, some ctenacanths would grow to sizes rivalling the modern great white shark with bodies in the region of 7 metres (23 ft) in length.[16] During the Carboniferous andPermian, thexenacanths were abundant in both freshwater and marine environments, and would continue to exist into theTriassic with reduced diversity.[17] The hybodonts had achieved a high diversity by the Permian,[18] and would end up becoming the dominant group of elasmobranchs during the Triassic and EarlyJurassic. Hybodonts were extensively present in both marine and freshwater environments.[19] While Neoselachii/Elasmobranchisensu stricto (the group of modern sharks and rays) had already appeared by the Triassic, they only had low diversity during this period and only began to extensively diversify from the Early Jurassic onwards, when modern orders of sharks and rays appeared.[20] This co-incided with the decline of the hybodonts, which had become minor components of marine environments by the Late Jurassic but would remain common in freshwater environments into theCretaceous.[21] The youngest remains of hybodonts date to the very end of the Cretaceous.[22]
Elasmobranchii was first coined in 1838 byCharles Lucien Bonaparte. Bonaparte's original definition of Elasmobranchii was effectively identical to modernChondrichthyes, and was based around gill architecture shared by all 3 living major cartilaginous fish groups. During the 20th century it became standard to excludechimaeras from Elasmobranchii; along with including many fossil chondrichthyans within the group. The definition of Elasmobranchii has since been subject to much confusion with regard to fossil chondrichthyans. Maisey (2012) suggested that Elasmobranchii should exclusively be used for the last common ancestor of modern sharks and rays, a grouping which had previously been named Neoselachii by Compagno (1977).[2] Other recent authors have used Elasmobranchii in a broad sense to include all chondrichthyans more closely related to modern sharks and rays than to chimaeras.[14]
Thetotal group of Elasmobranchii includes thecohort Euselachii Hay, 1902, which groups the Hybodontiformes and a number of other extinct chondrichthyans with Elasmobrachiisensu stricto/Neoselachii, to the exclusion of more primitive total group elasmobranchs, which is supported by a number of shared morphological characters of the skeleton.[23][24][25][26]
The 5th edition of Fishes of the World sets out the following classification of the Elasmobranchs:[29]
Infraclass Elasmobranchii
Division Selachii (sharks)
Superorder Galeomorphi
†Order Synechodontiformes
Order Heterodontiformes
Order Orectolobiformes
Suborder Parascyllioidei
Suborder Orectoloboidei
Order Lamniformes
Order Carcharhiniformes
Superorder Squalomorphi
Series Hexanchida
Order Hexanchiformes
Series Squalida
Order Squaliformes
Series Squatinida
†Order Protospinaciformes
Order Echinorhiniformes
Order Squatiniformes
Order Pristiophoriformes
Division Batomorphi (rays)
Order Torpediniformes
Order Rajiformes
Order Pristiformes
Order Myliobatiformes
Suborder Platyrhinoidei
Suborder Myliobatoidei
Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are a monophyletic superorder within Elasmobranchii that shares a common ancestor with the selachians.[30][31]
^Bigelow, Henry B.; Schroeder, William C. (1948).Fishes of the Western North Atlantic. Sears Foundation for Marine Research, Yale University. pp. 64–65.ASINB000J0D9X6.
^Bone, Q.; Roberts, B. L. (2009). "The density of elasmobranchs".Journal of the Marine Biological Association of the United Kingdom.49 (4): 913.doi:10.1017/S0025315400038017.S2CID85871565.
^Schultze, H.-P., Bullecks, J., Soar, L. K., & Hagadorn, J. (2021). Devonian fish from Colorado's Dyer Formation and the appearance of Carboniferous faunas in the Famennian. In A. Pradel, J. S. S. Denton, & P. Janvier (Eds.), Ancient Fishes and their Living Relatives: a Tribute to John G. Maisey (pp. 247–256.). Verlag Dr. Friedrich Pfeil.
^Rees, J. A. N., and Underwood, C. J., 2008, Hybodont sharks of the English Bathonian and Callovian (Middle Jurassic): Palaeontology, v. 51, no. 1, p. 117–147.
^Ebert, David A.; Fowler, Sarah; Dando, Marc (2021).Sharks of the world: a complete guide. Princeton: Princeton University Press.ISBN978-0-691-20599-1.
^Douady, Christophe J.; Dosay, Miné; Shivji, Mahmood S.; Stanhope, Michael J. (2003). "Molecular phylogenetic evidence refuting the hypothesis of Batoidea (rays and skates) as derived sharks".Molecular Phylogenetics and Evolution.26 (2):215–21.Bibcode:2003MolPE..26..215D.doi:10.1016/S1055-7903(02)00333-0.PMID12565032.
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