47-546: Batomorphi is a clade of cartilaginous fishes , commonly known as rays , this taxon is also known as the superorder Batoidea , but the 5th edition of Fishes of the World classifies it as the division Batomorphi. They and their close relatives, the sharks , compose the subclass Elasmobranchii . Rays are the largest group of cartilaginous fishes, with well over 600 species in 26 families. Rays are distinguished by their flattened bodies, enlarged pectoral fins that are fused to
94-534: A clade (from Ancient Greek κλάδος (kládos) 'branch'), also known as a monophyletic group or natural group , is a grouping of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree . In the taxonomical literature, sometimes the Latin form cladus (plural cladi ) is used rather than the English form. Clades are
141-479: A "ladder", with supposedly more "advanced" organisms at the top. Taxonomists have increasingly worked to make the taxonomic system reflect evolution. When it comes to naming , this principle is not always compatible with the traditional rank-based nomenclature (in which only taxa associated with a rank can be named) because not enough ranks exist to name a long series of nested clades. For these and other reasons, phylogenetic nomenclature has been developed; it
188-803: A 2011 study significantly reevaluated the phylogeny of batoids, using nuclear and mitochondrial DNA from 37 taxa , representing almost all recognized families and all of the traditional four major lineages. This is a far more numerous and diverse set of sample taxa than in any previous study, producing findings reflected in the cladogram below. Holocephali (incl. Chimaera ) [REDACTED] Selachimorpha (Sharks) [REDACTED] Rajiformes (Skates) [REDACTED] Platyrhinidae (Thornbacks) Torpediniformes (Electric rays) [REDACTED] "Guitarfishes 1" ( Trygonorrhinidae ) [REDACTED] "Guitarfishes 2" (incl. Pristidae (Sawfishes)) [REDACTED] Zanobatidae (Panrays) Myliobatoidei (Stingrays) [REDACTED] This study strongly confirmed
235-453: A 2021 study in Nature , the number of oceanic sharks and rays has declined globally by 71% over the preceding 50 years, jeopardising "the health of entire ocean ecosystems as well as food security for some of the world's poorest countries". Overfishing has increased the global extinction risk of these species to the point where three-quarters are now threatened with extinction. This is notably
282-623: A clade can be described based on two different reference points, crown age and stem age. The crown age of a clade refers to the age of the most recent common ancestor of all of the species in the clade. The stem age of a clade refers to the time that the ancestral lineage of the clade diverged from its sister clade. A clade's stem age is either the same as or older than its crown age. Ages of clades cannot be directly observed. They are inferred, either from stratigraphy of fossils , or from molecular clock estimates. Viruses , and particularly RNA viruses form clades. These are useful in tracking
329-399: A few species, like manta rays , live in the open sea, and only a few live in freshwater, while some batoids can live in brackish bays and estuaries. Most batoids have developed heavy, rounded teeth for crushing the shells of bottom-dwelling species such as snails , clams , oysters , crustaceans , and some fish , depending on the species. Manta rays feed on plankton . Batoids belong to
376-596: A number of other extinct chondrichthyans with Elasmobrachii sensu stricto /Neoselachii, to the exclusion of more primitive total group elasmobranchs, which is supported by a number of shared morphological characters of the skeleton. The 5th edition of Fishes of the World sets out the following classification of the Elasmobranchs: Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are
423-569: A nutrient when food is scarce. The oldest unambigous total group elasmobranch, Phoebodus , has its earliest records in the Middle Devonian (late Givetian ), around 383 million years ago. Several important groups of total group elasmobranchs, including Ctenacanthiformes and Hybodontiformes , had already emerged by the latest Devonian ( Famennian ). During the Carboniferous , some ctenacanths would grow to sizes rivalling
470-422: A revised taxonomy based on a concept strongly resembling clades, although the term clade itself would not be coined until 1957 by his grandson, Julian Huxley . German biologist Emil Hans Willi Hennig (1913–1976) is considered to be the founder of cladistics . He proposed a classification system that represented repeated branchings of the family tree, as opposed to the previous systems, which put organisms on
517-429: A suffix added should be e.g. "dracohortian". A clade is by definition monophyletic , meaning that it contains one ancestor which can be an organism, a population, or a species and all its descendants. The ancestor can be known or unknown; any and all members of a clade can be extant or extinct. The science that tries to reconstruct phylogenetic trees and thus discover clades is called phylogenetics or cladistics ,
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#1732781080819564-401: A ventrally located mouth and can considerably protrude their upper jaw (palatoquadrate cartilage) away from the cranium to capture prey. The jaws have euhyostylic type suspension, which relies completely on the hyomandibular cartilages for support. Bottom-dwelling batoids breathe by taking water in through the spiracles, rather than through the mouth as most fish do, and passing it outward through
611-403: A womb but without involvement of a placenta. The eggs of oviparous skates are laid in leathery egg cases that are commonly known as mermaid's purses and which often wash up empty on beaches in areas where skates are common. Capture-induced premature birth and abortion (collectively called capture-induced parturition) occurs frequently in sharks and rays when fished. Capture-induced parturition
658-499: Is also used with a similar meaning in other fields besides biology, such as historical linguistics ; see Cladistics § In disciplines other than biology . The term "clade" was coined in 1957 by the biologist Julian Huxley to refer to the result of cladogenesis , the evolutionary splitting of a parent species into two distinct species, a concept Huxley borrowed from Bernhard Rensch . Many commonly named groups – rodents and insects , for example – are clades because, in each case,
705-437: 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,
752-476: Is in turn included in the mammal, vertebrate and animal clades. The idea of a clade did not exist in pre- Darwinian Linnaean taxonomy , which was based by necessity only on internal or external morphological similarities between organisms. Many of the better known animal groups in Linnaeus's original Systema Naturae (mostly vertebrate groups) do represent clades. The phenomenon of convergent evolution
799-410: Is one of the two subclasses of cartilaginous fish in the class Chondrichthyes , the other being Holocephali ( chimaeras ). Members of the elasmobranchii subclass have no swim bladders , five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins , and small placoid scales . The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw
846-488: Is rarely considered in fisheries management despite being shown to occur in at least 12% of live bearing sharks and rays (88 species to date). Most species live on the sea floor, in a variety of geographical regions – mainly in coastal waters, although some live in deep waters to at least 3,000 metres (9,800 ft). Most batoids have a cosmopolitan distribution , preferring tropical and subtropical marine environments, although there are temperate and cold-water species. Only
893-515: Is responsible for many cases of misleading similarities in the morphology of groups that evolved from different lineages. With the increasing realization in the first half of the 19th century that species had changed and split through the ages, classification increasingly came to be seen as branches on the evolutionary tree of life . The publication of Darwin's theory of evolution in 1859 gave this view increasing weight. In 1876 Thomas Henry Huxley , an early advocate of evolutionary theory, proposed
940-489: Is still controversial. As an example, see the full current classification of Anas platyrhynchos (the mallard duck) with 40 clades from Eukaryota down by following this Wikispecies link and clicking on "Expand". The name of a clade is conventionally a plural, where the singular refers to each member individually. A unique exception is the reptile clade Dracohors , which was made by haplology from Latin "draco" and "cohors", i.e. "the dragon cohort "; its form with
987-534: The Jurassic . The oldest confirmed ray is Antiquaobatis , from the Pliensbachian of Germany . The clade is represented today by sharks , sawfish , rays and skates . Molecular evidence refutes the hypothesis that skates and rays are derived sharks. The monophyly of the skates , the stingrays , and the electric rays has long been generally accepted. Along with Rhinopristiformes , these comprise
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#17327810808191034-569: The Trygonorrhinidae , while the latter contains the remainder of Rhinopristiformes (the families Glaucostegidae , Pristidae , Rhinidae , and Rhinobatidae ). In addition, while traditional phylogenies often find electric rays to be the basalmost batoids, followed by the Rhinopristiformes, this analysis finds a polytomy between skates, electric rays, and thornbacks at the base of Batoidea, with weak support for skates being
1081-490: The hybodonts (Order Hybodontiformes), xenacanths (order Xenacanthformes) and Ctenacanthiformes . These are also often referred to as "sharks" in reference to their similar anatomy and ecology to modern sharks. The name Elasmobranchii comes from the Ancient Greek words elasmo- ("plate") and bránchia ("gill"), referring to the broad, flattened gills which are characteristic of these fishes. Elasmobranchii
1128-482: The school , gulper and basking sharks (pictured) . All three of these species have been assessed by the IUCN as vulnerable due to overfishing . 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
1175-556: The Triassic and Early Jurassic . Hybodonts were extensively present in both marine and freshwater environments. While Neoselachii/Elasmobranchi sensu stricto (the group of modern sharks and rays) had already appeared by the Triassic, they only had low diversity during this period would and only begin to extensively diversify from the Early Jurassic onwards, when modern orders of sharks and rays appeared. This co-incided with
1222-470: The actual most basal lineage, followed by a clade uniting the electric rays and thornbacks. The Mesozoic Sclerorhynchoidea are basal or incertae sedis ; they show features of the Rajiformes but have snouts resembling those of sawfishes. However, evidence indicates they are probably the sister group to sawfishes. Eschmeyer's Catalog of Fishes classigies the rays as follows: According to
1269-595: The ancient lineage of cartilaginous fishes. Fossil denticles (tooth-like scales in the skin) resembling those of today's chondrichthyans date at least as far back as the Ordovician , with the oldest unambiguous fossils of cartilaginous fish dating from the middle Devonian . A clade within this diverse family, the Neoselachii , emerged by the Triassic , with the best-understood neoselachian fossils dating from
1316-681: The case in the Mediterranean Sea - most impacted by unregulated fishing - where a recent international survey of the Mediterranean Science Commission concluded that only 38 species of rays and skates still subsisted. All sharks and rays are cartilaginous fish, contrasting with bony fishes . Many rays are adapted for feeding on the bottom. Guitarfishes are somewhat between sharks and rays, displaying characteristics of both (though they are classified as rays). Clade In biological phylogenetics ,
1363-531: The clade is unclear with respect to fossil chondrichthyans. Some authors consider it as equivalent to Neoselachii (the crown group clade including modern sharks, rays, and all other descendants of their last common ancestor ). Other authors use the name Elasmobranchii for a broader branch-based group of all chondrichthyans more closely related to modern sharks and rays than to Holocephali (the clade containing chimaeras and their extinct relatives). Important extinct groups of elasmobranchs sensu lato include
1410-648: 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. The eyes have a tapetum lucidum . The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm . These fish are widely distributed in tropical and temperate waters. Many fish maintain buoyancy with swim bladders . However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil. This stored oil may also function as
1457-619: 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 the Cretaceous . The youngest remains of hybodonts date to the very end of the Cretaceous. Elasmobranchii was first coined in 1838 by Charles Lucien Bonaparte . Bonaparte's original definition of Elasmobranchii was effectively identical to modern Chondrichthyes , and
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1504-665: The four traditionally accepted major batoid lineages, as in Nelson's 2006 Fishes of the World . However, the exact phylogeny of the major batoid lineages, internally and with respect to one another, has been subject to diverse treatments. The following cladogram is based on a comprehensive morphological assessment of batoid phylogeny published in 2004: Holocephali (incl. Chimaera ) [REDACTED] Selachimorpha (Sharks) [REDACTED] Torpediniformes [REDACTED] Rhinopristiformes [REDACTED] Rajiformes [REDACTED] Myliobatiformes [REDACTED] However,
1551-451: The fundamental unit of cladistics , a modern approach to taxonomy adopted by most biological fields. The common ancestor may be an individual, a population , or a species ( extinct or extant ). Clades are nested, one in another, as each branch in turn splits into smaller branches. These splits reflect evolutionary history as populations diverged and evolved independently. Clades are termed monophyletic (Greek: "one clan") groups. Over
1598-506: The gills. Batoids reproduce in a number of ways. As is characteristic of elasmobranchs, batoids undergo internal fertilization . Internal fertilization is advantageous to batoids as it conserves sperm, does not expose eggs to consumption by predators, and ensures that all the energy involved in reproduction is retained and not lost to the environment. All skates and some rays are oviparous (egg laying) while other rays are ovoviviparous , meaning that they give birth to young which develop in
1645-546: The group consists of a common ancestor with all its descendant branches. Rodents, for example, are a branch of mammals that split off after the end of the period when the clade Dinosauria stopped being the dominant terrestrial vertebrates 66 million years ago. The original population and all its descendants are a clade. The rodent clade corresponds to the order Rodentia, and insects to the class Insecta. These clades include smaller clades, such as chipmunk or ant , each of which consists of even smaller clades. The clade "rodent"
1692-470: The head, and gill slits that are placed on their ventral surfaces. Batoids are flat-bodied, and, like sharks, are cartilaginous fish, meaning they have a boneless skeleton made of a tough, elastic cartilage. Most batoids have five ventral slot-like body openings called gill slits that lead from the gills , but the Hexatrygonidae have six. Batoid gill slits lie under the pectoral fins on
1739-544: The last common ancestor of modern sharks and rays, a grouping which had previously been named Neoselachii by Compagno (1977). 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. The total group of Elasmobranchii includes the Cohort Euselachii Hay, 1902, which groups the Hybodontiformes and
1786-590: The last few decades, the cladistic approach has revolutionized biological classification and revealed surprising evolutionary relationships among organisms. Increasingly, taxonomists try to avoid naming taxa that are not clades; that is, taxa that are not monophyletic . Some of the relationships between organisms that the molecular biology arm of cladistics has revealed include that fungi are closer relatives to animals than they are to plants, archaea are now considered different from bacteria , and multicellular organisms may have evolved from archaea. The term "clade"
1833-518: The latter term coined by Ernst Mayr (1965), derived from "clade". The results of phylogenetic/cladistic analyses are tree-shaped diagrams called cladograms ; they, and all their branches, are phylogenetic hypotheses. Three methods of defining clades are featured in phylogenetic nomenclature : node-, stem-, and apomorphy-based (see Phylogenetic nomenclature§Phylogenetic definitions of clade names for detailed definitions). The relationship between clades can be described in several ways: The age of
1880-527: The modern great white shark with bodies in the region of 7 metres (23 ft) in length. During the Carboniferous and Permian , the xenacanths were abundant in both freshwater and marine environments, and would continue to exist into the Triassic with reduced diversity. The hybodonts had achieved a high diversity by the Permian, and would end up becoming the dominant group of elasmobranchs during
1927-412: 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
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1974-453: The sharks, are also in trouble. Elasmobranchii ( / ɪ ˌ l æ z m ə ˈ b r æ ŋ k i aɪ / ) is a subclass of Chondrichthyes or cartilaginous fish, including modern sharks (superorder Selachii), rays , skates , and sawfish (superorder Batoidea ). Members of this subclass are characterised by having five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins and small placoid scales on
2021-447: 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 elasmobranch clades . The pelvic fins in males are modified to create claspers for the transfer of sperm. There is no swim bladder; instead, these fish maintain buoyancy with large livers rich in oil. The definition of
2068-580: The spread of viral infections . HIV , for example, has clades called subtypes, which vary in geographical prevalence. HIV subtype (clade) B, for example is predominant in Europe, the Americas and Japan, whereas subtype A is more common in east Africa. Neoselachii See text Elasmobranchs lack swim bladders , and maintain buoyancy with oil that they store in their livers. Some deep sea sharks are targeted by fisheries for this liver oil , including
2115-419: The traditionally accepted internal monophyly of skates, stingrays, and electric rays. It also recovered panrays as sister to the stingrays, as older morphological analyses had suggested. However, it found the Rhinopristiformes, including the sawfishes and various "guitarfishes", to be paraphyletic , comprising two distinct clades. Referred to as "Guitarfishes 1" and "Guitarfishes 2", the former contains only
2162-401: The underside, whereas a shark's are on the sides of the head. Most batoids have a flat, disk-like body, with the exception of the guitarfishes and sawfishes , while most sharks have a spindle-shaped body. Many species of batoid have developed their pectoral fins into broad flat wing-like appendages. The anal fin is absent. The eyes and spiracles are located on top of the head. Batoids have
2209-432: Was based around gill architecture shared by all 3 living major cartilaginous fish groups. During the 20th century it became standard to exclude chimaeras 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
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