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Hybodontiformes
Temporal range: 360.7–66 Ma Late Devonian to Late Cretaceous
Fossil and life restoration of Asteracanthus, from the Late Jurassic of Europe
Scientific classification Edit this classification
Kingdom: Animalia
Phylum: Chordata
Clade: Eugnathostomata
Class: Chondrichthyes
Clade: Euselachii
Order: †Hybodontiformes
Patterson, 1966
Families

See text

Hybodontiformes, commonly called hybodonts, are an extinct group of shark-like cartilaginous fish (chondrichthyans) which existed from the late Devonian to the Late Cretaceous. Hybodonts share a close common ancestry with modern sharks and rays (Neoselachii) as part of the clade Euselachii. They are distinguished from other chondrichthyans by their distinctive fin spines and cephalic spines present on the heads of males. An ecologically diverse group, they were abundant in marine and freshwater environments during the late Paleozoic and early Mesozoic, but were rare in open marine environments by the end of the Jurassic, having been largely replaced by modern sharks, though they were still common in freshwater and marginal marine habitats. They survived until the end of the Cretaceous, before going extinct.

Etymology

[edit]

The term hybodont comes from the Greek word ὕβος or ὑβός meaning hump or hump-backed and ὀδούς, ὀδοντ meaning tooth. This name was given based on their conical compressed teeth.

Taxonomic history

[edit]

Hybodonts were first described in the nineteenth century based on isolated fossil teeth (Agassiz, 1837). Hybodonts were first separated from living sharks by Zittel (1911).[1] Although historically argued to have a close relationship with the modern shark order Heterodontiformes, this has been refuted.[2] Hybodontiformes are total group-elasmobranchs and the sister group to Neoselachii, which includes modern sharks and rays. Hybodontiformes and Neoselachii are grouped together in the clade Euselachii, to the exclusion of other total-group elasmobranchs like Xenacanthiformes.[3] Hybodonts are divided into a number of families, but the higher level taxonomy of hybodonts, especially Mesozoic taxa, is poorly resolved.[4]

Simplified cladogram of chondrichthyan relationships following several studies.[5][6][7]

Chondrichthyes

Holocephali (chimaeras and relatives)

Total group Elasmobranchii

†Xenacanthiformes

†Ctenacanthiformes (possibly paraphyletic)

†Phoebodontiformes

Euselachii

†Hybodontiformes

Neoselachii

Selachii (modern sharks)

Batomorphi (rays, etc)

Description

[edit]
Specimen of "Hybodus" fraasi from the Late Jurassic of Germany, which some studies have included in Egertonodus
Skeletal diagram of Hybodus sp.

The largest hybodonts reached lengths of 2–3 metres (6.6–9.8 ft),[4] while some other hybodonts were much smaller, with adult body lengths of around 25 centimetres (0.82 ft).[8] Hybodonts had a generally robust bodyform. Due to their cartilaginous skeletons usually disintegrating upon death like other chondrichthyans, hybodonts are generally described and identified based on teeth and fin spine fossils, which are more likely to be preserved.[4] Rare partial or complete skeletons are known from areas of exceptional preservation.[9][4][10]

Restoration of Hybodus hauffianus showing sexual dimorphism with fin claspers and cephalic spines present in males (below) but absent in females (above)

Hybodonts are recognized as having teeth with a prominent cusp which is higher than lateral cusplets.[11] Hybodont teeth are often preserved as incomplete fossils because the base of the tooth is not well attached to the crown.[11] Hybodonts were initially divided into two groups based on their tooth shape.[12] One group had teeth with acuminate cusps that lacked a pulp cavity; these are called osteodont teeth. The other group had a different cusp arrangement and had a pulp cavity, these are called orthodont teeth.[13] For example, the hybodont species Heterophychodus steinmanni have osteodont teeth with vascular canals of dentine which are arranged vertically parallel to each other, also called 'tubular dentine'.[14] The crowns of these osteodont teeth are covered with a single layer of enameloid. Hybodont teeth served a variety of functions depending on the species, including grinding, crushing (durophagy), tearing, clutching, and even cutting.[4]

Hybodonts are characterized by having two dorsal fins each preceded by a fin spine. The fin spine morphology is unique to each hybodont species. The fin spines are elongate and gently curved towards the rear, with the posterior part of the spine being covered in hooked denticles, typically in two parallel rows running along the length of the spine, sometimes with a ridge between them. Part of the front of the spines are often covered in a ribbed ornamentation, while in some other hybodonts this region is covered in rows of small bumps. The spines are mineralised, and primary composed of osteodentine, while the ornamentation is formed of enamel.[15] Similar fin spines are also found in many extinct chondrichthyan groups as well as in some modern sharks like Heterodontus and squalids.[16] Male hybodonts had either one or two pairs of cephalic spines on their heads, a characteristic distinctive to hybodonts.[17] These spines, while of variable placement,[13] were always placed posterior to the eye socket,[18] and were composed of a base divided into three lobes, with the main part of the spine being backwardly curved, most specimens of which had a barb near the apex.[13] These spines, like the fin spines, were mineralised, with the base composed of osteodentine,[19] while the main part of the spine was covered in enamel. Male hybodonts possessed fin claspers used in mating, like modern sharks.[13] Hybodonts had a fully heterocercal tail fin, where the upper lobe of the fin was much larger than the lower one due to the spine extending into it.[20] Like living sharks and rays, the skin of hybodonts was covered with dermal denticles.[21] Hybodonts laid egg cases, similar to those produced by living cartilaginous fish. Most hybodont egg cases are assigned to the genus Palaeoxyris, which tapers towards both ends, with one end having a tendril which attached to substrate, with the middle section being composed of at least three twisted bands.[22]

  • Teeth of Durnovariaodus, a member of the family Hybodontidae
    Teeth of Durnovariaodus, a member of the family Hybodontidae
  • Jaw fragment of Strophodus a specialised durophagous hybodont
    Jaw fragment of Strophodus a specialised durophagous hybodont
  • Jaws of Asteracanthus, showing the arrangement of the teeth in jaws. The teeth were designed for grasping
    Jaws of Asteracanthus, showing the arrangement of the teeth in jaws. The teeth were designed for grasping
  • Preserved dentition of the acrodontid Acrodus, which had low, rounded teeth used in durophagy
    Preserved dentition of the acrodontid Acrodus, which had low, rounded teeth used in durophagy
  • Teeth of Planohybodus, a hybodontid, whose teeth were designed for tearing
    Teeth of Planohybodus, a hybodontid, whose teeth were designed for tearing
  • Diagram of Palaeoxyris egg cases, thought to have been produced by hybodonts
    Diagram of Palaeoxyris egg cases, thought to have been produced by hybodonts

Ecology

[edit]
Hybodont egg cases (Palaeoxyris) attached to a Neocalamites stem in an estuarine environment. Art by Michael Rothman

Hybodont fossils are found in depositional environments ranging from marine to fluvial (river deposits).[23] Many hybodonts are thought to have been euryhaline, able to tolerate a wide range of salinities.[24] Hybodonts inhabited freshwater environments from early in their evolutionary history, spanning from the Carboniferous onwards.[25] Based on isotopic analysis, some species of hybodonts are likely to have permanently lived in freshwater environments,[26][27] while others may have migrated between marine and freshwater environments.[28][29] One genus of hybodont, Onychoselache of the lower Carboniferous of Scotland, is suggested to have been capable of amphibious locomotion, similar to modern orectolobiform sharks such as bamboo and epaulette sharks, due to its well-developed pectoral fins.[19] It has been suggested that male hybodonts used their cephalic spines to grip females during mating.[30] Preserved egg cases of hybodonts assigned to Palaeoxyris indicate that at least some hybodonts laid their eggs in freshwater and brackish environments, with the eggs being attached to vegetation via a tendril. Laying of eggs in freshwater is not known in any living cartilaginous fish.[31][22][32] At least some hybodonts are suggested to have utlilized specific sites as nurseries, such as in the Triassic lake deposits of the Madygen Formation of Kyrgyzstan, where eggs of Lonchidion are suggested to have been laid on the lakeshore or upriver areas, where the juveniles hatched and matured, before migrating deeper into the lake as adults.[31]

Life restoration of Strophodus rebecae with other contemporary organisms from the Early Cretaceous (Valanginian-Hauterivian) Rosa Blanca Formation of Colombia

Hybodonts are thought to have been generally relatively slow swimmers, though capable of fast bursts of locomotion.[33] Some hybodonts like Hybodus are thought to have been active predators capable of feeding on swiftly moving prey,[2] with preserved stomach contents of a specimen of Hybodus hauffianus indicating that they fed on belemnites (a type of extinct squid-like cephalopod).[34] Hybodonts have a wide variety of tooth shapes. This variety suggests that they took advantage of multiple food sources.[11] It is thought that some hybodonts which had wider, flatter, teeth specialized in crushing or grinding hard-shelled prey (durophagy),[23] with some hybodonts like Asteracanthus probably consuming both hard and soft bodied prey.[4] Often multiple species of hybodonts with different prey preferences coexisted within the same ecosystem.[35][14]

Evolutionary history

[edit]
Fossil of Hamiltonichthys a primitive hybodont from the Carboniferous of North America

The earliest hybodont remains are from the latest Devonian (Famennian, ~ 360 million years ago) of Iran, belonging to the genus Roongodus,[36] as well as remains assigned to Lissodus of the same age from Belgium.[37] Carboniferous hybodonts include both durophagous and non-durophagous forms, while durophagous forms were dominant during the Permian period.[37] By the Permian period, hybodonts had a global distribution.[37][38][39] The Permian-Triassic extinction event only had a limited effect on hybodont diversity.[33] Maximum hybodont diversity is observed during the Triassic. During the Triassic and Early Jurassic, hybodontiforms were the dominant elasmobranchs in both marine and non-marine environments.[23] A shift in hybodonts was seen during the Middle Jurassic, a transition between the distinctly different assemblages seen in the Triassic – Early Jurassic and the Late Jurassic – Cretaceous.[23] As neoselachians (group of modern sharks) diversified further during the Late Jurassic, hybodontiforms became less prevalent in open marine conditions but remained diverse in fluvial and restricted settings during the Cretaceous.[23] Possible reasons for the replacement of hybodonts by modern sharks include more effective locomotory and jaw movement mechanisms of the latter group.[40] By the end of the Cretaceous, hybodonts had declined to only a handful of species,[41] including members of Lonchidion[42], and Meristodonoides.[43] The last hybodonts disappeared, seemingly abruptly, as part of the Cretaceous-Paleogene extinction event approximately 66 million years ago.[41]

Families and genera

[edit]

The taxonomy of hybodonts is considered poorly resolved,[4] so the classification presented should not be taken as authoritative.

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Hybodontiformes

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Hybodontiformes, commonly referred to as hybodonts, are an extinct order of elasmobranch chondrichthyans that represent a stem group of shark-like cartilaginous fishes, distinct from modern neoselachians such as , skates, and rays. These ancient predators are characterized by key morphological features including two dorsal fins supported by prominent, ornamented spines, dentition adapted for various feeding strategies (from crushing to slicing), and in some taxa, cephalic spines on the head of males. The temporal range of Hybodontiformes spans from the Late Devonian (approximately 382 million years ago) through the Mesozoic Era, with peak diversity during the Triassic and Jurassic periods (252–145 million years ago), before a decline in the Cretaceous. Fossil evidence, primarily consisting of isolated teeth, fin spines, dermal denticles, and rare articulated skeletons, indicates they occupied a wide array of ecosystems, including fully marine, brackish, and freshwater habitats across all continents. This ecological versatility allowed hybodonts to thrive as dominant predators and scavengers, filling niches later dominated by neoselachians, with some genera exhibiting specialized durophagous (shell-crushing) adaptations. Hybodontiformes went extinct by the end of the (around 66 million years ago), likely due to competitive exclusion by more advanced neoselachians and environmental perturbations associated with the end-Cretaceous mass extinction event. Their evolutionary significance lies in bridging primitive chondrichthyans with modern elasmobranchs, providing insights into the diversification of cartilaginous fishes during key geological transitions. Over 50 genera have been described, with well-known examples including (a versatile coastal predator) and Strophodus (a specialized durophagous form), highlighting their morphological and ecological diversity.

Overview

Etymology

The name "Hybodontiformes" derives from the Greek roots hybos (ὕβος), meaning hump or hump-backed, and odous (ὀδούς), meaning tooth, reflecting the characteristic compressed, conical or hummocky morphology of the teeth in this group of extinct chondrichthyans. This nomenclature originates with the genus Hybodus, the type genus of the order, which Louis Agassiz established in 1837 within his foundational multivolume work Recherches sur les poissons fossiles, where he systematically named many chondrichthyan genera based on distinctive dental features, as isolated teeth often represented the primary fossil evidence for these cartilaginous fishes. The order Hybodontiformes itself was formalized taxonomically by Colin Patterson in 1966, building on Agassiz's groundwork to classify these shark-like forms within the broader context of elasmobranchs.

Definition and general characteristics

Hybodontiformes is an extinct order of shark-like cartilaginous fishes (chondrichthyans) belonging to the subclass Euselachii, positioned as the to Neoselachii, which encompasses modern sharks, skates, and rays. These ancient elasmobranchs are distinguished from other chondrichthyans by key skeletal features, including the presence of two spines and cephalic spines, particularly prominent in males. The group originated in the Late Devonian period, approximately 360 million years ago, and persisted until the , around 66 million years ago, with peak diversity during the and before a decline during the . Hybodontiformes exhibited oviparous , laying eggs encased in leathery capsules known as Palaeoxyris, which often featured tendrils for attachment to substrates. Body sizes varied from small forms reaching about 25 cm in length to larger species up to 3 m, reflecting adaptations to diverse ecological roles. Hybodontiformes encompassed approximately 50 genera, demonstrating significant taxonomic diversity across global deposits. This order occupied a wide array of environments, including marine, freshwater, and brackish habitats, which contributed to their ecological success throughout much of the era.

Taxonomy

Taxonomic history

The taxonomic history of Hybodontiformes begins with the initial recognition of hybodont-like fossils in the early 19th century, primarily through isolated teeth. first described the genus in 1837, based on distinctive crushing and cutting teeth collected from strata in , such as those from , , marking the earliest formal documentation of these shark-like chondrichthyans. These discoveries highlighted the group's prominence but were limited to dental material, as complete skeletons were scarce even then. By the late , broader classifications emerged. In , Karl Alfred von Zittel grouped hybodont forms under the subclass Hybodontes in his comprehensive Handbuch der Palaeontologie, encompassing a range of to elasmobranchs characterized by fin spines and specialized dentition, though without a formal ordinal rank. This grouping reflected the era's reliance on fragmentary evidence, including teeth and spines, due to the poor preservation of cartilaginous skeletons in the fossil record. Early 20th-century studies continued this trend, often lumping diverse taxa under Hybodus and related genera without resolving interrelationships. Over 50 genera have been described in total, reflecting the group's extensive diversity across its geological range. A significant advancement occurred in 1966 when Colin Patterson formalized the order Hybodontiformes in his monograph on British Wealden sharks, defining it by synapomorphies such as the presence of labial root cartilages, tuberculate spines, and a specific enameloid microstructure, distinguishing it from neoselachians. Subsequent revisions refined this framework. John G. Maisey, in 1984, examined hybodonts and emphasized their primitive features, linking forms like Tristychius to later radiation while addressing inconsistencies in earlier classifications. In 2008, Jan Rees and Charlie Underwood described hybodont sharks from the English and (), identifying 15 species across nine genera in marine and brackish habitats and proposing ecological specializations based on dental morphology, which underscored the order's adaptive success. Modern debates center on the of Hybodontiformes, with challenges arising from incomplete fossils that prioritize teeth and spines over skeletal data, perpetuating outdated groupings. Studies in the , such as those by Jan Rees, suggested multiple lineages within hybodonts based on dental morphology and enameloid variations, indicating possible convergence rather than a single and influencing ongoing phylogenetic revisions. These discussions highlight persistent difficulties in integrating fragmentary evidence, as rare articulated specimens continue to reveal discrepancies in traditional .

Phylogenetic position

Hybodontiformes are classified within the subclass and the infraclass Euselachii, where they are recognized as the sister taxon to Neoselachii, the clade encompassing all modern sharks and rays. This positioning reflects their shared euselachian characteristics, such as an orbitostylic jaw suspension and advanced hybodontiform dentition, distinguishing them from more basal chondrichthyans like symmoriiforms or xenacanthiforms. Cladistic analyses have consistently supported the monophyly of Hybodontiformes, based on several key synapomorphies, including a reduced hyoid arch with limited ceratotrichia support and distinctive fin spine histology featuring tuberculate ornamentation and internal vascularization patterns. These features, evident in both skeletal and dental remains, underscore their cohesive evolutionary lineage from the Late onward. Integration of evidence with molecular phylogenies of extant chondrichthyans further corroborates this, estimating the divergence of Hybodontiformes from Neoselachii in the period, around 360–400 million years ago, aligning with the earliest isolated teeth and spines. Debates persist regarding the exact boundaries of Hybodontiformes, particularly concerning potential . Some taxa traditionally assigned to the group, such as Tristychius arcuatus, exhibit primitive traits like absent cephalic spines and a distinct palatoquadrate morphology, positioning them as basal euselachians rather than true hybodontiforms. This reclassification excludes groups like Xenacanthiformes, which lack euselachian synapomorphies and are now placed outside the , reinforcing the of a more restricted Hybodontiformes comprising primarily forms. Ongoing cladistic revisions continue to refine these relationships through enhanced fossil sampling and comparative morphology.

Description

Body morphology

Hybodontiformes exhibited a robust, body plan, characterized by a broad anterior region that tapered posteriorly, facilitating efficient swimming in diverse aquatic environments. The was heterocercal, with the upper lobe significantly larger than the lower due to an extension of the vertebral column, providing propulsion and stability. This body shape, combined with a streamlined profile, allowed for agile maneuvering, as evidenced in well-preserved specimens exceeding 2 meters in length. The fins were distinctive, with two dorsal fins positioned along the back, each supported by prominent spines that featured longitudinal ridges, posterior denticles, and sometimes stellate tubercles for structural reinforcement and defense. Pectoral fins were broad and rounded at the apex, lacking an apical lobe (aplesodic), while pelvic fins showed a continuous puboischiadic bar; both paired fins possessed a metapterygial axis, where the metapterygium formed the primary supportive element extending posteriorly. Males possessed claspers for and pairs of cephalic spines with T-shaped bases and recurved cusps, likely aiding in reproductive behaviors. An anal fin was present, often teardrop-shaped, and the caudal fin included a well-developed hypochordal lobe supported by radials. The was cartilaginous and rarely preserved in the record due to its poor mineralization, though isolated elements like robust scapulocoracoids, elongate , and pleural with rounded cross-sections have been documented. The body and fins were covered in small, placoid-like scales known as monodontode dermal denticles, featuring a circular base and a cone-shaped cusp with vertical folds, providing protection and hydrodynamic advantages. Body size varied widely, from juvenile forms or small species around 25 cm, such as Lissodus, to large adults reaching 3 m, exemplified by species of like H. hauffianus. Proportions could differ by , with some freshwater taxa exhibiting more elongate bodies adapted to riverine conditions.

Dentition and feeding adaptations

The teeth of Hybodontiformes are typically small, featuring multicuspid crowns that are laterally compressed, with a central principal cusp flanked by smaller lateral cusps, facilitating efficient prey capture and processing. These teeth are arranged in imbricated rows within the jaws, allowing for a conveyor-belt-like continuous replacement throughout the animal's life, which ensures a functional despite wear or damage. This replacement mechanism involves unique root resorption, where the roots of functional teeth are gradually eroded to accommodate erupting successors, a trait exclusive to Hybodontiformes among elasmobranchs. Dental morphology varies significantly across taxa, reflecting specialized feeding strategies. Predatory forms, such as Hybodus, possess cutting-oriented teeth with pointed main cusps and sharp, continuous edges on asymmetrical anterior and lateral crowns, adapted for grasping and slicing soft-bodied prey. In contrast, durophagous species like Strophodus exhibit broad, low-crowned, pavement-like teeth with domed, subtriangular or subrectangular occlusal surfaces ornamented by reticular patterns and strong lingual folds, enabling the crushing of hard-shelled . Some taxa, including Priohybodus arambourgi, further display serrated cutting edges along the cusps, enhancing tearing capabilities against tougher tissues. The apparatus supports these dental specializations through a hyostylic suspension, where the upper (palatoquadrate) is primarily connected to the cranium via the hyomandibula, permitting a wide gape and flexible protrusion for enhanced bite versatility. evidence, such as embedded teeth and puncture marks on ammonite shells attributed to hybodonts like Planohybodus, demonstrates this adaptability, indicating that even with primarily tearing could inflict damage on robust, shelled prey. The crowns are capped by a durable enameloid layer, characterized by large, fluoride-rich crystallites that resist abrasion and maintain sharpness during repeated use. replacement occurs rapidly, at rates averaging about 2.6 days per row in some , further supporting sustained feeding efficiency.

Paleobiology

Habitats and distribution

Hybodontiform sharks exhibited a euryhaline lifestyle, inhabiting marine, freshwater, and brackish environments throughout their history. Isotopic analysis of teeth from species such as Lissodus and Hybodus in the Late Triassic Rhaetian Sea reveals oxygen isotope (δ¹⁸Oₚ) values ranging from 15.4‰ to 18.6‰ across regions, indicating adaptation to salinity gradients from brackish conditions (less than 16‰ salinity in eastern areas) to fully marine settings near western and southern marine gateways. For instance, fossils from the Jurassic Solnhofen lagoon in Bavaria, Germany, represent shallow marine to brackish paleoenvironments, where well-preserved skeletons like Asteracanthus ornatissimus suggest lagoonal habitats. In contrast, freshwater occurrences are documented in Triassic red beds, such as the Dockum Group in Texas, where Lonchidion humblei teeth indicate fluvial systems. Their global distribution was extensive during the Paleozoic, with fossils reported from both Gondwanan and Laurussian landmasses, reflecting widespread presence in coastal and inland waters. Diversity peaked in the Mesozoic across Europe, Asia, and North America, where hybodontiforms dominated marine and non-marine assemblages, as seen in deposits from the Jurassic of southern Germany and Cretaceous of Kansas. Post-Triassic records in the Southern Hemisphere were rarer, though notable exceptions include Cretaceous sites in Brazil, highlighting a predominantly northern hemisphere bias in later distributions. Juveniles often utilized shallow, freshwater or brackish nursery grounds, providing protected environments for early development. In the Carboniferous Mazon Creek locality of Illinois, juvenile Bandringa specimens, along with egg cases, indicate one of the earliest known shark nurseries in a deltaic setting adjacent to coal swamps, with brackish to marine influences. Key fossil sites preserve articulated skeletons and isolated elements that illuminate these habitats. The Carboniferous Bearsden locality in Scotland has yielded articulated hybodontiform remains in shallow-water deposits suggestive of estuarine conditions. Similarly, the Cretaceous Santana Formation in Brazil's Araripe Basin contains Tribodus limae fossils, including dermal denticles, from lagoonal paleoenvironments with freshwater influences.

Diet and ecological roles

Hybodontiform exhibited predominantly predatory diets, targeting a variety of prey including and cephalopods. Direct evidence from preserved gut contents demonstrates that species such as Hybodus hauffianus consumed belemnites, with one exceptional specimen from the containing over 100 rostra of the belemnite Passaloteuthis bisinuata, suggesting whole ingestion without mastication. Specialized forms like Asteracanthus adapted to durophagous feeding, using crushing to process hard-shelled prey such as bivalve mollusks, small ammonites, and crustaceans, as inferred from the robust, blunt morphology of their teeth. In marine ecosystems, hybodonts occupied mid-level trophic positions as versatile predators, contributing to dynamics in reef-associated environments where they preyed on smaller and . In freshwater habitats, some species acted as opportunistic , supplementing their diet with carrion alongside active hunting, which allowed them to exploit nutrient-rich but prey-scarce settings. Evidence for these habits includes coprolites containing scales and bone fragments from lacustrine deposits, indicating carnivorous predation, as well as bite traces on ammonite shells, such as a Orthaspidoceras specimen from with an embedded Planohybodus tooth and puncture marks. Niche partitioning with emerging neoselachians is evident in the , where hybodonts maintained dominance in nearshore and reefal zones while neoselachians increasingly occupied deeper-water or more specialized roles. Hybodontiforms were oviparous, producing leathery egg cases known as Palaeoxyris, which provided protection during embryonic development. These capsules, often found in clusters, featured tendrils for anchoring to vegetation such as horsetail stems (Neocalamites or Equisetites), facilitating attachment in shallow riverine or vegetated spawning grounds. Freshwater nurseries, as indicated by mass occurrences of egg cases alongside juvenile remains in deposits, likely reduced predation risk by isolating developing embryos from marine threats.

Evolutionary history

Origins and diversification

Hybodontiformes originated in the Late Devonian, approximately 360 million years ago, during the Famennian stage, evolving from primitive chondrichthyans. The earliest known fossils, including isolated teeth of the genus Roongodus, come from Famennian strata in regions like and the Timan-Pechora Basin (), marking the initial appearance of this group as basal euselachians with shark-like features. These early forms exhibited basic chondrichthyan traits, such as placoid scales and cartilaginous skeletons, adapted to shallow marine environments, and represented a key step in the diversification of elasmobranch-like fishes from more archaic stem-group chondrichthyans. Following the Permian-Triassic mass extinction around 252 million years ago, Hybodontiformes showed initial low diversity in the but underwent a significant during the , diversifying into numerous genera as they recolonized vacated niches in both marine and freshwater habitats. This post-extinction recovery was relatively rapid for chondrichthyans, with hybodontiforms showing limited diversity loss compared to other marine groups, allowing them to exploit ecosystems disrupted by the event. Fossils from Lower Triassic deposits in and reveal an expansion into coastal and lacustrine settings, where they filled predatory and durophagous roles amid the slow rebound of competing taxa. During the , particularly the and periods, Hybodontiformes achieved dominance among elasmobranchs, forming a significant portion of chondrichthyan diversity in assemblages. This era saw innovations in , including specialized crushing and piercing morphologies that enabled exploitation of diverse prey, from shelled to smaller fishes, enhancing their ecological versatility. The group's success was driven by recovery from earlier extinctions, which reduced competition from bony fishes—whose modern lineages experienced a prolonged hiatus—allowing hybodontiforms to occupy a broad range of trophic levels without intense rivalry.

Decline and extinction

The diversity of Hybodontiformes, which peaked during the with approximately 30 genera, began a marked decline in the and accelerated through the , reducing to only a handful of genera by the stage. This decline is evidenced by increased extinction rates from the Santonian to the middle , with regional disappearances noted across various paleoenvironments. By the , surviving taxa such as Lonchidion were largely confined to non-marine or marginal marine habitats, with fossil records including teeth and spines from deposits in , such as the in and the Navarro Group in . The primary drivers of this decline appear to be ecological competition from increasingly dominant neoselachians, the modern group of and rays, which outcompeted hybodontiforms through superior adaptations like enhanced swimming efficiency, more advanced sensory systems, and broader niche exploitation in marine ecosystems. Niche overlap in coastal and open marine realms intensified during the , as neoselachians diversified rapidly while hybodontiforms, once versatile across freshwater and marine settings, became progressively marginalized. This competitive pressure is supported by the stratigraphic distribution, where hybodontiform fossils become rarer in fully marine sequences compared to their abundance. The Cretaceous-Paleogene (K-Pg) mass extinction event at approximately 66 Ma delivered the final blow, eliminating the few remaining hybodontiform populations alongside many other marine vertebrates. No post-Cretaceous fossils of the group have been documented, confirming their complete during this crisis, which was triggered by the Chicxulub asteroid impact and associated environmental perturbations. Although competition had already weakened hybodontiforms, the K-Pg event's global effects, including and plankton collapse, likely prevented any recovery. Despite their extinction, hybodontiforms left a lasting influence on modern evolution as a stem group to neoselachians, with retained ancestral traits such as spines appearing in basal neoselachian lineages before their reduction in more derived forms. This phylogenetic legacy underscores their role in bridging chondrichthyans to the dominant elasmobranch radiation that followed.

Systematics

Families

The order Hybodontiformes encompasses several families, traditionally numbering 4–6, though higher-level taxonomy remains contentious, particularly for Mesozoic representatives, owing to the predominance of isolated dental remains in the fossil record and persistent systematic revisions. Classification relies heavily on tooth morphology, such as cusp arrangement, enameloid structure, and root histology, which often limits resolution of familial boundaries and phylogenetic relationships. Hybodontidae, the most prominent and species-rich family with approximately 20 genera, dominated from the through the . These featured versatile, characterized by multicusped teeth with a tall central cusp flanked by smaller lateral cusplets, an orthodont , and a porous , enabling adaptations for grasping and cutting prey across diverse habitats. The genus exemplifies this family, with teeth displaying symmetrical or asymmetrical cusps and flared cusplets in some species. Lonchidiidae ranged from the to the and comprised relatively slender forms with specialized piercing suited to marine environments. Diagnostic traits include minute, arrowhead-shaped teeth with a prominent labial protuberance on the main cusp, few ornamental folds on the crown, and a low-angle lingual inclination, facilitating penetration of soft-bodied prey. Key representatives such as Lonchidion and Parvodus highlight the family's prevalence in coastal and open-marine settings. Acrodontidae represents a more basal lineage, extending from the Late Devonian or to the Jurassic, with some extensions into the . This family is defined by durophagous adaptations, including broad, low-crowned teeth with acrodin cusps, domed occlusal surfaces, and reticulate ornamentation for crushing hard-shelled organisms; many taxa inhabited freshwater systems. The genus Acrodus illustrates these features through its sub-rhomboidal or parallelogram-shaped teeth with vertical edges and irregular ridges. Additional families include the durophagous Palaeobatidae, known from the and characterized by specialized crushing dentitions similar to advanced acrodontids, and the early Protacrodontidae from the , featuring primitive multicusped teeth indicative of basal hybodonts. Unresolved basal groups, such as Tristychiidae (, with phalacanthous spines) and Distobatidae (, with unique vertebral and dental traits in genera like Aegyptobatus), further complicate the framework, underscoring the need for integrated skeletal and dental analyses in future revisions.

Genera

Hybodontiformes encompass approximately 50 genera, most of which are known primarily from isolated teeth due to the poor preservation of their cartilaginous skeletons, with incomplete skeletal material documented for only about 10 genera, including Akmonistion from the Biota in . The taxonomic framework is complicated by numerous junior synonyms arising from historical descriptions based on fragmentary remains, alongside recent additions such as the Columnaodus, described in 2024 from durophagous teeth in the Burlington and Keokuk Limestones of and , . Among the most prominent genera is , a widespread predatory form documented from to deposits across , , , and , distinguished by sharp, pointed teeth adapted for grasping and tearing prey, alongside two dorsal fin spines and cephalic hooks typical of hybodontiforms. Asteracanthus, known primarily from strata in and , represents a large-bodied genus (up to 3 meters) with durophagous dentition featuring low-crowned, crushing teeth suited for hard-shelled , often associated with robust fin spines. Strophodus, prevalent in marine and marginal environments worldwide, is characterized by specialized crushing-type teeth with a low, rounded crown and coarse ornamentation, indicating a durophagous lifestyle focused on mollusks and crustaceans. Meristodonoides, restricted to occurrences in and , exhibits teeth with a prominent central cusp flanked by small lateral cusplets and fine striations, suggesting to coastal or brackish habitats. Planohybodus, from to sites in and , features low-crowned, multicusped teeth with a flattened labial profile and short folds, reflecting durophagous feeding in shallow marine settings. Durnonovariaodus, a recently described genus from the Formation of , displays specialized cutting dentition with serrated edges on a high main cusp, highlighting lingering diversity in marginal marine ecosystems.

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