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Squalomorphi

Squalomorphi is a superorder of cartilaginous fishes within the subclass Elasmobranchii of the class Chondrichthyes, consisting of 189 species of sharks divided into five orders: Hexanchiformes, Echinorhiniformes, Squaliformes, Squatiniformes, and Pristiophoriformes (as of 2025).^[1]^[2]^[3] These sharks, often referred to as squalomorph or squalean sharks, exhibit a primitive morphology compared to other modern elasmobranchs, including the general absence of an anal fin in most taxa, lack of a nictitating membrane, and absence of suborbital shelves in the cranium.^[2] Many species display dogfish-like body forms with cylindrical or slightly depressed bodies, and dorsal fins frequently armed with spines, though variations occur across orders.^[2] Squalomorphi are predominantly deep-sea or cold-water inhabitants, with notable adaptations such as bioluminescence in several Squaliformes species, a trait unique among sharks and linked to their diversification in the Upper Cretaceous.^[4] The superorder represents about 34% of extant shark diversity (∼557 species) and includes economically important groups like the sleeper sharks (Somniosidae) and spiny dogfish (Squalidae), which are targeted in fisheries for their meat, fins, and liver oil.^[4] Evolutionarily, Squalomorphi originated in the Late Jurassic approximately 156 million years ago and form a monophyletic clade sister to the more derived Galeomorphi superorder.^[3]

Introduction

Definition and Scope

Squalomorphi, commonly known as squalomorph or squalean sharks, is a superorder of cartilaginous fishes within the subclass Elasmobranchii of the class Chondrichthyes.^[1] This superorder forms a monophyletic clade that is the sister group to the superorder Galeomorphi, together comprising the major extant lineages of sharks.^[5] As primitive neoselachians, squalomorphs retain several ancestral features relative to more derived shark groups, distinguishing them in the broader evolutionary context of elasmobranchs.^[6] The superorder encompasses approximately 163 extant species distributed across 5 orders and 11 families, accounting for about 30% of all living shark species (as of 2024). These orders include Hexanchiformes, Squaliformes, Squatiniformes, Pristiophoriformes, and Echinorhiniformes, with Squaliformes being the most species-rich.^[1] The diversity reflects a range of morphologies adapted primarily to deep-water environments, though some species inhabit shallower coastal regions.^[4] Squalomorphi includes both extinct and extant lineages, with a temporal range extending from the Late Jurassic to the present day. As a basal group among modern sharks, Squalomorphi provides critical insights into the early evolution of elasmobranchs, particularly regarding adaptations to deep-sea habitats where many extant species thrive as specialized predators.^[7]

Key Characteristics

Squalomorphi exhibit several core synapomorphies that distinguish them from other neoselachian sharks, including the absence of an anal fin, nictitating membrane, and suborbital shelves in the neurocranium.^[8] These primitive features reflect their basal position within modern sharks. Additionally, they possess 5 to 7 gill slits, with most taxa having 5 and Hexanchiformes uniquely featuring 6 or 7, and all employ internal fertilization via paired clasper structures on the pelvic fins, a trait shared with other male elasmobranchs but emphasized in their reproductive strategy.^[2] General morphology in Squalomorphi is highly variable, ranging from cylindrical bodies in deep-sea forms like squaliform dogfishes to dorsoventrally flattened shapes in angel sharks (Squatiniformes). Most taxa have one or two dorsal fins, often equipped with anterior spines for defense, and their dentition is specialized for either grasping prey with sharp, pointed teeth or crushing with molariform forms, depending on the diet.^[9] Sensory and physiological adaptations include well-developed ampullae of Lorenzini, jelly-filled pores on the head and ventral surface that detect electric fields from prey, enabling effective hunting in low-light environments. Some squaliform subgroups, such as lanternsharks in the family Etmopteridae, display bioluminescence through ventral photophores for counterillumination camouflage. Reproduction is typically ovoviviparous, with embryos developing internally without a placenta, though some species are viviparous; this contrasts with the anal fin presence in the sister group Galeomorphi.^[2]^[10] Size within Squalomorphi spans a broad range, from diminutive species under 1 m total length, such as the dwarf lanternshark (Etmopterus perryi) at about 0.21 m, to larger forms reaching up to 6.4 m, exemplified by the Greenland shark (Somniosus microcephalus) in Squaliformes.^[9]

Evolutionary History

Fossil Record

The fossil record of Squalomorphi begins in the Late Jurassic, around 150-160 million years ago, with isolated teeth and the earliest definitive representative Protospinax annectans, known from well-preserved articulated skeletons in the Late Jurassic (Tithonian stage, approximately 150 million years ago) Solnhofen Limestone of southern Germany.^[11] A key extinct taxon within Squalomorphi is the order Protospinaciformes, exemplified by the family Protospinacidae and genus Protospinax, which persisted from the Jurassic into the Early Cretaceous. Recent analyses confirm Protospinax's close relationship to Squatiniformes and Pristiophoriformes.^[11] Beyond these, the record consists largely of disarticulated remains, including isolated teeth and vertebrae from Cretaceous and Eocene deposits worldwide, which indicate adaptations to deep-sea environments through features like robust, cutting dentition suited for scavenging or predation in low-light conditions. Squalomorph diversity remained low throughout the Mesozoic, with only a handful of genera documented from Jurassic and Cretaceous strata, reflecting limited ecological radiation during this era. Diversity increased notably in the Cenozoic, particularly from the Eocene onward, as evidenced by assemblages in Antarctic sediments that include hexanchiform and squaliform taxa, suggesting expansion into polar deep-water niches. Important fossil sites include the Solnhofen Limestone in Germany for Jurassic material and Eocene outcrops on Seymour Island, Antarctica, for Paleogene records.^[11] Fossils reveal the persistence of primitive traits in Squalomorphi, such as multiple gill slits (up to seven) in hexanchiform lineages, as inferred from dental morphologies attributable to forms like Hexanchus in Eocene deposits, highlighting retention of ancestral elasmobranch features despite environmental shifts. Unlike some chondrichthyan groups, Squalomorphi show no evidence of major mass extinction events uniquely affecting the superorder, with continuous representation through the Cretaceous-Paleogene boundary.

Phylogenetic Relationships

Squalomorphi represents one of the two primary superorders within the subclass Selachimorpha, serving as the sister group to Galeomorphi; together, these superorders comprise Selachimorpha, which with Batoidea (rays) forms the infraclass Neoselachii.^[3] This division is supported by both morphological and molecular data, highlighting distinct evolutionary trajectories within modern sharks, with Squalomorphi characterized by primitive traits such as the absence of an anal fin and nictitating membrane in most members.^[3] Within Squalomorphi, the internal phylogeny places Hexanchiformes as the basal order, followed by Echinorhiniformes as the next diverging lineage; Squatiniformes and Pristiophoriformes form a sister clade, with Squaliformes emerging as the most derived order.^[12] This hierarchical structure aligns with Compagno's foundational 1973 framework, which has been corroborated by subsequent molecular analyses using mitochondrial genomes and multi-gene datasets.^[3] Key supporting evidence includes molecular phylogenies derived from whole mitochondrial DNA sequences, which robustly confirm the monophyly of Squalomorphi and its ordinal relationships, as well as morphological synapomorphies such as the coracoid bar in the pectoral girdle, featuring a distinctive posterior process that unites the group. Jurassic fossil records provide brief corroboration, aligning ancient taxa with these basal splits.^[13] Debates persist regarding certain morphological traits, such as the myology of the mandibular arches, where patterns initially proposed as diagnostic for subgroups like Hexanchiformes have been questioned for their exclusivity and reliability in delimiting monophyly; however, molecular data consistently uphold the overall coherence of Squalomorphi.^[14]

Ecology and Distribution

Habitats and Global Range

Squalomorphi exhibit a cosmopolitan distribution across all major ocean basins, ranging from the Arctic and Antarctic regions to tropical and temperate waters worldwide. They are absent from freshwater environments but occasionally venture into coastal and nearshore marine habitats. This broad range encompasses the Atlantic, Pacific, Indian, and Southern Oceans, with highest species diversity in the Indo-West Pacific and Atlantic, though they are less common in the eastern North Pacific. Depth preferences within Squalomorphi vary significantly, with many species inhabiting deep-sea environments from approximately 200 to 4,000 meters, particularly along continental slopes and seamounts. However, certain groups, such as angel sharks in Squatiniformes, occupy shallower coastal waters, often less than 50 meters. Vertical migrations occur in some species, allowing access to both midwater and benthic zones. Habitat utilization spans benthic, demersal, and pelagic realms, with associations to diverse substrates including sandy or muddy bottoms, rocky reefs, and open oceanic waters. While squaliforms dominate deep-sea benthic and benthopelagic niches, the superorder as a whole thrives in temperate to tropical settings, often linked to productive features like submarine ridges. Environmental tolerances reflect this versatility: deep-zone specialists endure cold waters (typically 0–13°C), whereas shelf-dwelling species demonstrate eurythermal adaptations across broader temperature gradients (6–26°C).

Ecological Roles and Behavior

Squalomorph sharks primarily function as mesopredators within marine ecosystems, exerting top-down control on benthic and midwater prey populations such as teleost fishes, cephalopods, and crustaceans, thereby influencing community structure and energy flow in deep-sea environments.^[16] In deeper habitats, many species also engage in scavenging, opportunistically consuming carrion to supplement their diet and contribute to nutrient recycling on the seafloor.^[16] Their feeding ecology is characterized by carnivorous habits, with diets dominated by fishes, cephalopods, and crustaceans, often pursued through ambush tactics in low-light conditions or active hunting during vertical migrations.^[16] Specialized jaw morphologies enable efficient prey capture, supporting their role in regulating invertebrate and lower vertebrate abundances across continental slopes and abyssal plains.^[17] Reproduction in squalomorphs involves internal fertilization leading to aplacental viviparity or ovoviviparity, where embryos develop lecithotrophically using yolk reserves within the mother, resulting in litters of 1 to 40 pups after gestation periods of 1 to 3 years.^[16] Life history traits include slow growth rates, late sexual maturity at ages often exceeding 10 years and up to about 150 years in species like the Greenland shark (Somniosus microcephalus), and extended lifespans up to several hundred years, such as over 400 years in the Greenland shark (Somniosus microcephalus), which contribute to low reproductive output and biennial or triennial cycles with post-parturition resting phases.^[16]^[18] Behavioral patterns typically involve solitary lifestyles or loose aggregations in small schools, with many species exhibiting diurnal depth preferences and nocturnal vertical migrations to access prey in the water column.^[16] Some deep-sea species exhibit bioluminescence for counter-illumination camouflage, aiding predator avoidance and potentially enhancing foraging efficiency during low-light activity. This includes the kitefin shark (Dalatias licha), confirmed as bioluminescent in 2021 and the largest known example among sharks.^[19]^[20] These behaviors, supported by acute sensory adaptations like enhanced olfaction and electroreception, facilitate their persistence in oligotrophic environments.^[16]

Taxonomy

Historical Classification

The classification of sharks within what would later be termed Squalomorphi began to take shape in the 19th century through the seminal work of Johannes Müller and Jacob Henle, who in their Systematische Beschreibung der Plagiostomen (1838–1841) organized cartilaginous fishes into families and genera, grouping dogfish-like forms and related taxa under the suborder Squaloidea.^[21] This framework marked a significant advance over earlier Linnaean systems that lumped most sharks into a single genus Squalus, and Müller and Henle specifically highlighted the primitive nature of sharks like Hexanchus, noting their six gill slits and single dorsal fin as archaic traits distinguishing them from more derived forms.^[9] Their descriptions included key genera such as Centrophorus and Pristiophorus, establishing morphological criteria like tooth structure and fin arrangements that influenced subsequent taxonomy.^[9] A pivotal milestone came in 1898 when Otto Jaekel proposed the higher group Squalomorphi in his catalog of Oligocene selachians, emphasizing shared morphological features among fossil and extant primitive sharks, including multi-gill-slit configurations and simplified fin supports.^[22] Following World War II, classifications evolved through morphological revisions that prioritized diagnostic traits like the number of gill slits (typically five to seven in squalomorphs versus five in galeomorphs) and the consistent absence of an anal fin, as seen in works by researchers such as Stewart Springer and others who refined subordinal boundaries in the 1950s and 1960s.^[9] Leonard J. V. Compagno's contributions profoundly shaped modern understanding, beginning with his 1973 definition of the superorder Squalomorphii (later standardized as Squalomorphi), supported by 12 synapomorphies such as the lack of a nictitating membrane, absence of suborbital shelves, and specific patterns in jaw suspension and otic capsule morphology.^[8] In his 1977 FAO Species Catalogue, Compagno further delineated orders within Squalomorphi, incorporating detailed anatomical data to distinguish groups like Hexanchiformes and Squaliformes, while his 1984 update expanded on these with illustrated keys and synonymies for over 300 species.^[9] Pre-molecular era debates, particularly in Compagno's frameworks and contemporaries like John G. Maisey (1980), focused on the positioning of Squatiniformes and Pristiophoriformes—whether as basal lineages retaining plesiomorphic traits or as derived offshoots based on orbitostylic jaw suspension and other features—without consensus until cladistic analyses in the 1990s.^[8] These morphological foundations have been integrated with molecular data in recent decades, affirming the overall coherence of Squalomorphi while adjusting internal relationships.^[23]

Modern Classification

The modern classification recognizes Squalomorphi as a monophyletic superorder of elasmobranchs, originally proposed by Compagno in 1973 and subsequently emended by Nelson et al. in 2016 to reflect updated phylogenetic insights. This hierarchy encompasses five extant orders—Hexanchiformes, Squaliformes, Echinorhiniformes, Squatiniformes, and Pristiophoriformes—distributed across 11 families and 31 genera, with approximately 189 valid species documented to date.^[1]^[24] Recent revisions have been driven by molecular phylogenetic analyses, such as those by Straube et al. (2015), which utilized mitochondrial genomes and nuclear loci to robustly confirm the monophyly of Squalomorphi, positioning Hexanchiformes as the basal lineage sister to all other orders.^[4] Concurrent updates from authoritative databases, including the IUCN Red List and FishBase, have refined species counts and taxonomic boundaries, incorporating new genetic data and resolving synonyms to yield the current totals without altering the ordinal framework. Extinct taxa are integrated into the classification, with the fossil order Protospinaciformes regarded as a stem-group to crown Squalomorphi based on morphological and phylogenetic reassessments of genera like Protospinax. Nomenclatural authorities for the extant orders include Hexanchiformes (de Buen, 1926), Squaliformes (Compagno, 1973), Echinorhiniformes (Gill, 1893), Squatiniformes (de Buen, 1926), and Pristiophoriformes (de Buen, 1926); current consensus indicates no major ongoing splits or reorganizations at this level.^[25]^[26]^[27]

Extant Orders

Hexanchiformes

Hexanchiformes represents the most basal order within the superorder Squalomorphi, comprising two families and seven extant species characterized by their retention of archaic traits among modern sharks. The family Chlamydoselachidae includes a single genus, Chlamydoselachus, with two species: the frilled shark (C. anguineus) and the southern African frilled shark (C. africana). The family Hexanchidae encompasses three genera and five species: Heptranchias perlo (sharpnose sevengill shark), Notorynchus cepedianus (broadnose sevengill shark), and three species in Hexanchus (H. griseus, bluntnose sixgill shark; H. nakamurai, bigeye sixgill shark; H. vitulus, Atlantic sixgill shark).^[28]^[29]^[30] These sharks exhibit several primitive morphological features, including six or seven gill slits—unlike the five found in most extant elasmobranchs—a single dorsal fin lacking a spine, and the presence of an anal fin. Body forms vary distinctly between families: Chlamydoselachidae species possess an elongated, eel-like body with a snake-like head and frilled gill septa, while Hexanchidae species have more robust, cylindrical bodies with blunt snouts and comb-like lower teeth adapted for grasping prey. Their dentition shows upper jaw teeth with a single cusp and lower jaw teeth with multiple slender cusps, reflecting an early evolutionary design.^[31]^[28]^[29] Hexanchiformes inhabit deep-sea environments worldwide, primarily at depths of 300–2000 m on outer continental shelves and upper slopes, though some species occasionally venture shallower. The bluntnose sixgill shark (Hexanchus griseus), a representative Hexanchidae species, is commonly found near the seafloor at 100–2000 m across temperate and tropical waters of all major oceans, demonstrating the order's adaptation to low-light, high-pressure conditions.^[32]^[33] Ecologically, these sharks function as opportunistic predators and scavengers, preying on bony fishes, cephalopods, crustaceans, and occasionally other elasmobranchs or marine mammals, with diet composition shifting ontogenetically in some species. Reproduction is ovoviviparous without a yolk-sac placenta; litter sizes are notably large in Hexanchidae, such as 47–108 pups in H. griseus, while Chlamydoselachidae produce smaller broods of 2–15 young in C. anguineus. Gestation periods are extended, contributing to their slow life histories in deep-sea niches.^[32]^[34]^[35]

Squaliformes

Squaliformes, commonly known as dogfish or spiny dogfish sharks, represent a diverse order within Squalomorphi, comprising six families, 24 genera, and more than 130 species.^[4] These families include Centrophoridae (gulper sharks), Dalatiidae (kitefin and cookiecutter sharks), Etmopteridae (lantern sharks), Oxynotidae (rough sharks), Somniosidae (sleeper sharks), and Squalidae (spiny dogfish).^[36] The order exhibits significant morphological variation, ranging from diminutive species like the spined pygmy shark (Squaliolus laticaudus) at under 30 cm to massive forms such as the Greenland shark (Somniosus microcephalus), which can exceed 6 m in length.^[37] Squaliformes occupy a derived phylogenetic position within Squalomorphi, characterized by advanced adaptations to deep-water environments.^[4] A defining feature of Squaliformes is the presence of two dorsal fins, each typically armed with prominent spines, positioned relatively low on the back, and the absence of an anal fin.^[38] Their dentition is highly varied, with many species exhibiting blade-like lower teeth suited for cutting prey, while upper teeth are often cuspidate or hook-shaped; for instance, in the family Squalidae, teeth are generally needle-like and suited for grasping.^[39] Skin texture ranges from smooth in some deep-sea forms to rough and spinous in others, such as the rough sharks of Oxynotidae.^[40] Bioluminescence is a notable adaptation in several families, particularly Etmopteridae, where lantern sharks possess photophores that produce ventral light patterns for counter-illumination and possibly intraspecific communication.^[41] Most Squaliformes inhabit deep-sea environments, typically between 200 and 3000 m, though some, like the spiny dogfish (Squalus acanthias) of the family Squalidae, occur in shallower coastal and shelf waters down to 10 m.^[42]^[4] They are distributed globally in temperate and tropical oceans, often along continental slopes and in the open ocean.^[36] Ecologically, these sharks are primarily predators of small fishes, cephalopods, and crustaceans; for example, species in Centrophoridae and Squalidae consume teleosts and squid as main dietary components.^[43] Unique feeding strategies include the parasitic behavior of cookiecutter sharks (Isistius spp., Dalatiidae), which attach to larger marine mammals, fish, and even submarines to excise circular plugs of flesh using specialized jaws and suction.^[44] Reproduction is viviparous across the order, with internal fertilization and aplacental development; litter sizes are generally small, ranging from 2 to 10 pups in many deep-sea species, though some like Squalus acanthias produce up to 15.^[45]^[46] Gestation periods can be extended, often exceeding 18 months in species such as the spiny dogfish.^[45]

Echinorhiniformes

Echinorhiniformes represents a small order within Squalomorphi, consisting of a single family, Echinorhinidae, and two extant species: the bramble shark (Echinorhinus brucus) and the prickly shark (E. cookei).^[47]^[48] These rare sharks are distinguished by their robust, flabby bodies covered in large, thorn-like dermal denticles that can measure up to 15 mm in basal diameter in adults, providing a prickly texture unique to this group. The denticles are scattered irregularly over the body and fins, with the bramble shark exhibiting denser coverage compared to the sparser arrangement in the prickly shark.^[49] Both species feature a short, blunt snout, two small dorsal fins lacking spines and positioned posteriorly near the tail, and the absence of an anal fin, contributing to their distinctive morphology.^[50] These sharks inhabit deep-sea environments, typically at depths ranging from 300 to 2000 meters, though records extend from as shallow as 11 m to over 1100 m in some cases.^[47]^[48] They are bottom-associated, occurring on continental and insular shelves and upper slopes in temperate to tropical waters, with a patchy global distribution spanning the Atlantic Ocean, Indo-Pacific region, and scattered localities in the eastern Pacific.^[50] The bramble shark is more commonly reported in the Atlantic and western Pacific, while the prickly shark predominates in the Pacific basin, from Japan and Hawaii to Chile.^[49]^[48] Ecologically, echinorhiniform sharks are sluggish, solitary bottom-dwellers that feed primarily on bony fishes, smaller sharks, cephalopods, and crustaceans, using a suction mechanism to capture prey by expanding the mouth and pharynx.^[50]^[47] They are ovoviviparous, with females giving birth to litters of 15–114 pups after internal development without a placental connection, and newborns measuring 29–90 cm in length.^[50]^[48] Due to their deep-water habits and low encounter rates, these species remain poorly studied and are infrequently observed in the wild.^[49]

Squatiniformes

Squatiniformes comprises a single family, Squatinidae, with one genus, Squatina, encompassing approximately 24 species of angelsharks, including the common angelshark (Squatina squatina).^[51]^[52] These sharks exhibit a distinctive ray-like morphology characterized by a dorsoventrally flattened body and enlarged pectoral fins that extend forward along the head and sides, creating an appearance similar to that of skates or rays.^[53] Their large eyes and spiracles are positioned on the dorsal surface for enhanced visibility while buried, while the wide mouth lies on the ventral side for bottom-feeding; the tail features a prominent caudal fin supported by the upper lobe.^[54] Angelsharks inhabit benthic environments on sandy or muddy substrates, primarily in temperate and subtropical waters from shallow coastal areas to the upper continental slope at depths of 10 to 500 meters.^[55] They are ambush predators that often bury themselves in sediment to lie in wait for prey, emerging rapidly to capture victims with a powerful suction mechanism generated by their protrusible jaws.^[56] Their diet consists mainly of small benthic fishes and crustaceans, such as crabs and shrimp, reflecting their specialized foraging strategy in these soft-bottom habitats.^[57] Reproduction in Squatiniformes is ovoviviparous, with embryos developing internally and nourished by yolk sacs until birth; females typically produce litters of 7 to 25 pups, depending on species and maternal size, after a gestation period of 8 to 12 months.^[57] Phylogenetic analyses indicate that Squatiniformes forms a sister group to Pristiophoriformes within Squalomorphi.^[58]

Pristiophoriformes

Pristiophoriformes, commonly known as sawsharks, is an order of squalomorph sharks distinguished by their elongated, saw-like rostrum equipped with sharp, tooth-like denticles along the lateral edges, which aids in prey detection and capture.^[59] This order includes a single family, Pristiophoridae, encompassing two genera—Pristiophorus and Pliotrema—and approximately 10 extant species, such as the Caribbean sawshark (Pristiophorus schroederi), which inhabits deep waters off the Bahamas.^[59] Sawsharks possess paired barbels on the underside of the rostrum for sensory purposes, five (or six in Pliotrema) gill slits positioned laterally on the head, and a slender, elongated body typically reaching less than 2 meters in length.^[59] These features set them apart from other squalomorphs, with molecular phylogenies placing Pristiophoriformes as sister to Squatiniformes.^[23] Sawsharks are primarily benthic dwellers on continental shelves and upper slopes, favoring soft substrates like sand or mud where they often burrow for concealment.^[59] Their global range spans the Indo-Pacific Ocean, from southern Africa and Australia to Japan, with some species extending into the eastern Atlantic, occurring at depths generally between 30 and 200 meters, though certain taxa like P. schroederi descend to 400–1,000 meters.^[60] They are strictly marine, showing no affinity for freshwater or brackish environments.^[59] In terms of ecology, sawsharks are ovoviviparous, with embryos developing internally and nourishing solely on yolk, resulting in small litter sizes of 3–12 pups per brood.^[59] They forage nocturnally on the seafloor, targeting benthic invertebrates such as crustaceans and small fish, employing their rostrum to stir sediment or deliver slashing strikes to uncover and immobilize prey.^[60] This specialized hunting strategy underscores their adaptation to demersal lifestyles in temperate to tropical waters.^[59]

Conservation

Threats

Squalomorphi species face significant threats from human activities, primarily overfishing and associated fisheries impacts. Targeted fisheries for meat, fins, and liver oil have depleted populations of several species, such as the spiny dogfish (Squalus acanthias) in the order Squaliformes, which has historically suffered from intense exploitation due to its slow growth, late maturity, and small litter sizes, making it highly vulnerable to overharvest. Deep-sea members of Squaliformes, including gulper sharks (Centrophorus spp.), are particularly affected by targeted trawling and longline fisheries seeking liver oil, leading to population declines exceeding 50-80% in regions like the Indian Ocean since the late 1990s. Additionally, bycatch in non-selective gears like trawls, longlines, and gillnets poses a widespread risk, with nearly all assessed chondrichthyan species, including many squalomorphs, incidentally captured and often discarded with high mortality rates. Accidental capture in non-shark fisheries, such as tuna purse-seines, further exacerbates these pressures on deep-water taxa. Habitat degradation compounds these fishery-related threats, especially for benthic and deep-sea species within Squalomorphi. Deep-sea bottom trawling damages fragile benthic habitats frequented by orders like Squaliformes and Hexanchiformes, disrupting ecosystems and reducing prey availability for species such as the bluntnose sixgill shark (Hexanchus griseus). Pollution from coastal development and industrial activities contaminates nearshore environments used by some Squatiniformes, while climate change alters ocean temperatures and currents, potentially shifting depth ranges and migration patterns for temperature-sensitive deep-water squalomorphs, affecting approximately 10% of assessed chondrichthyan species. Low reproductive rates across Squalomorphi, characterized by long gestation periods and few offspring, amplify vulnerability to these cumulative pressures, hindering population recovery even from moderate exploitation levels. Group-wide, approximately 37% of assessed Squalomorphi species are classified as threatened with extinction, reflecting the broader trend among chondrichthyans where overfishing alone drives risk for two-thirds of threatened taxa.^[61] Data deficiency remains prevalent, particularly for deep-water species accounting for a significant portion of the approximately 15% data deficient chondrichthyans, limiting comprehensive threat evaluations and conservation planning.^[62] For instance, angel sharks in Squatiniformes have experienced severe declines due to overfishing and habitat loss in the Mediterranean, underscoring the order's heightened susceptibility. Emerging threats include deep-sea mining, which as of 2025 poses risks to over 60% of deep-sea chondrichthyan species, including many Squalomorphi, by disturbing seafloor habitats.^[63]

Status and Efforts

Conservation assessments for species within the superorder Squalomorphi reveal significant vulnerabilities, with approximately 37% of assessed shark species, including many oceanic and deep-sea Squalomorphi, classified as threatened (vulnerable, endangered, or critically endangered) on the IUCN Red List as of 2024.^[61] A substantial portion, approximately 15-20%, are listed as data deficient, largely attributable to the difficulties in surveying and monitoring deep-sea habitats where most Squalomorphi reside.^[62] For instance, the angelshark Squatina squatina is categorized as critically endangered due to severe population declines driven by historical overfishing.^[64] Key conservation efforts include international trade regulations under CITES for select deep-sea species, such as the leafscale gulper shark Centrophorus squamosus, which as of November 2025 is proposed for Appendix II listing at CITES CoP20 to control unsustainable exploitation.^[65] Regionally, the European Union enforced a ban on landings of spiny dogfish Squalus acanthias from 2011 to 2023, enabling stock recovery in the Northeast Atlantic; since 2023, limited quotas have been introduced while maintaining restrictions.^[66] Additionally, deep-sea protected areas, including marine reserves like those in the Azores and Papahānaumokuākea, offer critical habitat safeguards by limiting fishing activities in vulnerable ecosystems.^[67] Research and monitoring initiatives focus on mitigating ongoing pressures, such as through global shark finning bans implemented by the EU in 2003 and strengthened in 2013, which require sharks to be landed with fins attached to reduce waste and bycatch impacts on Squalomorphi species.^[68] Genetic studies employing mitochondrial DNA analysis aid in tracking population structure and connectivity, particularly for migratory deep-sea forms like gulper sharks.^[69] The IUCN Shark Specialist Group coordinates these efforts via species-specific action plans, Red List assessments, and workshops to prioritize high-risk taxa.^[70] Addressing future conservation needs demands enhanced deep-sea surveys using advanced technologies like remotely operated vehicles to fill data gaps on elusive species distributions and abundances.^[71] Sustainable fisheries management, including ecosystem-based quotas and international agreements to curb emerging threats like deep-sea mining, is essential for long-term viability.^[72]

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[PDF] FAO species catalogue. Vol. 4. Sharks of the world. An annotated ...
The present volume includes 342 shark.species belonging to 8 orders and 31 families. It provides a comprehensive and illustrated key to all orders and families ...
[12]
Comparative morphology and systematics of the cookiecutter sharks ...
Aug 20, 2018 · Sharks of the squalomorph shark family Dalatiidae are distributed worldwide and usually occur in open water ranging from the surface to more ...
[13]
https://onlinelibrary.wiley.com/doi/abs/10.1002/gj.3965
[14]
[PDF] ASSEMBLING THE TREE OF LIFE
They first appear in the fossil record dur- ing the Late Cretaceous of Lebanon (some 90 Mya) and have an elongated rostral blade ("saw") with acute lateral ...<|control11|><|separator|>
[15]
Shark tales: A molecular species-level phylogeny of ... - ResearchGate
We used four mitochondrial and one nuclear gene to investigate the phylogenetic relationships of 229 species (all eight Orders and 31 families) of sharks.
[16]
Shark fossil diversity (Squalomorphii, Squatinomorphii, and ...
Aug 23, 2020 · The current study focuses on the fossil sharks from the Langhian Vc unit of the Brielas section, located in the Setúbal Peninsula.Missing: earliest Squalomorphi<|separator|>
[17]
Comparative myology of the mandibular and hyoid arches of sharks ...
Comparative myology of the mandibular and hyoid arches of sharks of the order hexanchiformes and their bearing on its monophyly and phylogenetic relationships ( ...
[18]
https://oceanservice.noaa.gov/facts/greenland-shark.html
[19]
[PDF] BIODIVERSITY, LIFE HISTORY AND FISHERIES. A report
Feb 15, 2007 · Elasmobranchii is divided into two superorders, the Squalomorphi and the Galeomorphi. ... reproductive life. This would further reduce the limited ...
[20]
Shark mandible evolution reveals patterns of trophic and habitat ...
May 8, 2023 · The jaw apparatus of sharks display several important morphofunctional modifications that enable specialised feeding behaviours, such as filter ...
[21]
Illuminating the evolution of bioluminescence in sharks - Ferrón - 2023
Feb 25, 2023 · The results suggest that most squalomorphs originated in neritic environments from where they colonized deep waters on several independent ...
[22]
Details - Systematische Beschreibung der Plagiostomen
Jun 24, 2008 · By. Müller, Johannes, 1801-1858 · Henle, Jacob, 1809-1885. Type. Book. Material. Published material. Publication info. Berlin, Veit und comp, ...Missing: Squaloidea | Show results with:Squaloidea
[23]
Jaekel, O. (1898) | Literature | Shark-References
Jaekel, O. (1898) Verzeichniss der Selachier des Mainzer Oligocäns. Sitzungsberichte der Gesellschaft naturforschender Freunde zu Berlin, 1898, 161–169.
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Node age estimations and the origin of angel sharks, Squatiniformes ...
Kriwet , J. and Klug , S. 2009 . “ Fossil record and origin of squaliform sharks (Chondrichthyes, Neoselachii) ” . In Biology and Management of dogfish sharks ...
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Classification List
### Classification Summary for Superorder Squalomorphi (Sharks)
[26]
Hexanchiformes - Wikispecies - Wikimedia
Feb 5, 2025 · Name. edit. Hexanchiformes F. de Buen, 1926. Type genus: Hexanchus Rafinesque, 1810. References. edit. Links. edit · Hexanchiformes in FishBase,
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https://www.marinespecies.org/aphia.php?p=taxdetails&id=105783
[28]
https://www.fishbase.se/summary/FamilySummary.php?ID=4
[29]
FAMILY Details for Chlamydoselachidae - Frilled sharks - FishBase
Nov 29, 2012 · Family Chlamydoselachidae - Frilled sharks. Order, : Hexanchiformes · Chlamydoselachidae. Class, : Elasmobranchii.
[30]
FAMILY Details for Hexanchidae - Cow sharks
### Summary of Hexanchidae Family
[31]
https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/hexanchiformes
[32]
Hexanchiformes - an overview | ScienceDirect Topics
The Hexanchiformes (Cow and Frilled Sharks) comprises two families, four genera, and seven species of morphologically distinct sharks, in that they are the only ...6 Nep Biodiversity · Chondrichthyes 1 · Fish, Biodiversity OfMissing: ecology | Show results with:ecology
[33]
Hexanchus griseus, Bluntnose sixgill shark : fisheries, gamefish
Viviparous, very large litters of 47-108 pups (Ref. 125614). Size at birth 60-75 cm (Ref. 26346). Distinct pairing with embrace (Ref. 205).
[34]
Hexanchus griseus (Bluntnose Sixgill Shark) - Animal Diversity Web
Habitat. Hexanchus griseus is mainly a deep water shark, rarely found at depths of less than 100 m. The species seems to usually stay close to the bottom, near ...Scientific Classification · Physical Description · Reproduction
[35]
https://www.fishbase.se/summary/Chlamydoselachus-anguineus.html
[36]
Chlamydoselachus anguineus, Frilled shark : fisheries - FishBase
50449), litter size ranges from 2 to 10 (Ref. 37390). Not dangerous but teeth are sharp enough to inflict lacerations on the hands of the unwary scientist ...
[37]
Echinorhinus brucus, Bramble shark : fisheries, gamefish - FishBase
A rare (Ref. 26346), large, sluggish, deepwater shark found on continental and insular shelves and upper slopes (Ref. 247). Sometimes found in shallow water.
[38]
Echinorhinus cookei summary page
### Summary for Echinorhinus cookei (Prickly shark)
[39]
Bramble Shark – Discover Fishes - Florida Museum of Natural History
Feb 5, 2025 · Biology. Distinctive Features The body of the bramble shark is stout, soft and flabby with a cylindrical trunk. The snout is short and ...Missing: Echinorhiniformes ecology
[40]
Familia Echinorhinidae - Sharks of the World - Naturalis
Habitat, Distribution and Biology: This family includes two uncommon species of large, poorly known wide-ranging, deepwater sharks in cold-temperate to ...
[41]
FAMILY Details for Squatinidae - Angel sharks - FishBase
Nov 29, 2012 · Squatiniformes · Squatinidae. Class, : Elasmobranchii. No. in FishBase, : Genera : 1 | Species : 26 Eschmeyer's Catalog of Fishes. Environment ...
[42]
Revision of the Western Indian Ocean Angel Sharks, Genus ...
The new species differs from S. africana in a number of characteristics including its coloration when fresh, smaller size at birth, size at maturity, and adult ...
[43]
Atlantic Angelshark – Discover Fishes
Feb 12, 2025 · The genus name Squatina is derived from Latin, meaning “a kind of shark”. In previous scientific literature, there are no synonyms attributed ...
[44]
Squatina squatina, Angelshark : fisheries, gamefish - FishBase
A benthic species that occurs inshore, on coasts and along the continental shelf; may enter estuaries (Ref. 247, 58085). Found mainly on sand or mud bottoms.<|control11|><|separator|>
[45]
NOVA Online | Island of the Sharks | Squatiniformes - PBS
Habitat: Shallow to moderate depths on the continental shelves and upper slopes from intertidal zones to depths of 4,500 ft. Often found during the day on sandy ...
[46]
Common Angelshark | NOAA Fisheries
Aug 31, 2016 · They eat mostly bony fish, other demersal animals, such as skates, crustaceans, mollusks, and cephalopods. Where They Live. Common angelsharks ...
[47]
Systematics and Phylogenetic Interrelationships of the Enigmatic ...
Here we revise this controversial taxon based on new holomorphic specimens from the Late Jurassic Konservat-Lagerstätte of the Solnhofen Archipelago in Bavaria ...Missing: Squalomorphi Protospinaciformes
[48]
FAMILY Details for Pristiophoridae - Saw sharks
**Summary of Pristiophoridae (Saw Sharks):**
[49]
Pristiophorus schroederi (Bahamas saw shark) - Animal Diversity Web
Bahamas sawsharks are mostly cartilaginous. Their exterior consists of 5 lateral gill slits and two dorsal fins of equal size and shape. There are no anal fins, ...
[50]
[PDF] A Comparative Analysis of Extant Oceanic Shark Species Using ...
Aug 9, 2024 · The anatomical characteristics of species in this order are extremely similar to sharks ... large number of species in the Squalomorphi superorder ...
[51]
[PDF] Deep-sea Shark Species for Consideration of a CITES Listing ...
Several shark species have been listed by OSPAR to receive greater protection, including the Portuguese shark Centroscymnus coelolepis, Gulper shark ...Missing: Squalomorphi | Show results with:Squalomorphi
[52]
Survivability of spurdog (Squalus acanthias) in an inshore otter trawl ...
A ban on landing spurdog was introduced in European Union (EU) waters in 2011 due to poor stock status and fisheries remained closed until 2023. Following ...
[53]
How Sharks Keep the Ocean Healthy - National Marine Sanctuaries
Sharks are essential to maintaining the health and balance of ocean ecosystems, playing a crucial role in regulating marine populations, helping habitats ...
[54]
EU shark finning ban at the beginning of the new millennium
Dec 29, 2013 · In June 2013, the EU Council of Ministers formally adopted a strengthened ban on shark finning (European Parliament and of the Council, 2013).
[55]
Genetype and phylogenomic position of the frilled shark ... - NIH
A dataset of available mitogenomes from closely related taxa in the Superorder Squalomorphi was constructed using available records from GenBank, in order to ...
[56]
IUCN SSC Shark Specialist Group - Home
The work of the SSC revolves around a cycle that comprises assessing the status of biodiversity, planning for conservation, and catalyzing conservation actions.Missing: Squalomorphi initiatives
[57]
Mining the deep-sea could further threaten endangered sharks and ...
many of ...
[58]
Sharks, rays, chimaeras further threatened by deep-sea mining
Oct 2, 2025 · Nearly two-thirds of these species are already threatened with extinction due to human impacts, so deep-sea mining, which will disrupt the ...