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Cladoselache

Cladoselache is an extinct genus of primitive elasmobranch shark that inhabited marine environments during the Late Devonian Period, approximately 370 million years ago.^[1] Known primarily from exceptionally preserved fossils in the Cleveland Shale Formation of Ohio, United States, it represents one of the earliest chondrichthyans with a distinctly modern body plan, adapted for fast swimming as an active predator.^[1] These sharks typically measured about 1.2 meters in length, with the largest specimens reaching up to 2 meters.^[1] Anatomically, Cladoselache possessed a short rounded snout, a terminal mouth positioned at the front of the head (unlike the ventral mouths of most modern sharks), and small, pointed cladodont teeth suited for grasping slippery prey rather than tearing or crushing.^[2]^[1] Its body was streamlined with prominent lateral keels on the tail stalk, large pectoral fins for maneuverability, and a crescent-shaped caudal fin featuring nearly equal upper and lower lobes, indicative of high-speed propulsion.^[1] The skin was largely smooth, lacking the extensive dermal armor or denticles common in other Paleozoic sharks, except for small multi-cusped scales on the fins, mouth, and around the eyes; it also had short, blade-like bony spines anterior to the dorsal fins.^[1] The braincase was platybasic, with features such as a flat orbital floor, no interorbital septum, a wide postorbital arcade, and an expansive jugular canal.^[3] Phylogenetically, Cladoselache is classified within the extinct family Cladoselachidae, as a sister group to symmoriiform sharks within basal chondrichthyans, based on shared apomorphic cranial characters like the division of the dorsal aorta and a narrow otic region.^[3]^[4] Several species are recognized, including the type species C. fyleri and C. clarki, both from the Famennian Stage.^[1] Recent discoveries include related taxa such as Maghriboselache from Morocco (2023).^[5] The genus's fossils, often found articulated due to the anoxic conditions of the shale deposit, have been crucial for understanding early shark evolution, revealing transitions from armored acanthodian-like ancestors to the diverse modern chondrichthyan radiation.^[1]

Discovery and nomenclature

History of discovery

The fossils of Cladoselache were first formally described in 1889 by paleontologist John S. Newberry as Cladodus fyleri based on isolated teeth and fragments from the Late Devonian Cleveland Member of the Ohio Shale Formation in the Cleveland Shale of Ohio, United States. These early finds highlighted the abundance of well-preserved chondrichthyan material in the black shales, often exposed in ironstone concretions that protected soft tissues from decay.^[6] In 1894, Bashford Dean formally established the genus Cladoselache and redescribed C. fyleri using articulated skeletons collected from the same locality, emphasizing its streamlined body, paired fins, and lack of placoid scales as distinguishing features from other Paleozoic sharks. Dean's work, drawing on specimens from ongoing quarrying operations, marked a pivotal advancement in understanding early shark morphology, with the exceptional preservation allowing reconstruction of the full skeletal framework.^[1] The bulk of Cladoselache specimens reside in major institutions, including the Cleveland Museum of Natural History, which holds the largest collection with over 1,700 examples primarily from the type locality, and the American Museum of Natural History, featuring notable articulated individuals that have facilitated detailed anatomical studies. These collections, amassed through 19th- and 20th-century field efforts in northeastern Ohio, include rare soft-tissue impressions and gut contents, underscoring the site's role as a Lagerstätte for Devonian marine life. Key 20th-century research advanced interpretations of Cladoselache's biology, including Otto Jaekel's 1918 skeletal reconstruction that emphasized its predatory adaptations and Erwin H. Barbour's concurrent analyses of fin morphology. In 1982, John G. Maisey conducted ontogenetic studies on growth stages using serial sections of juvenile and adult specimens, revealing incremental calcification patterns in the endoskeleton and supporting its position as a basal neoselachian.

Etymology and species

The genus name Cladoselache derives from the Greek words klados (branch) and selachē (shark), alluding to the multi-cusped, branched appearance of its teeth.^[7] The genus was formally established in 1894 by Bashford Dean, who reassigned several species from the earlier genus Cladodus—a form genus based on isolated cladodont teeth—based on detailed morphological studies of Devonian body fossils. The type species, C. fyleri, was originally described as Cladodus fyleri by John S. Newberry in 1889 based on isolated teeth from the Cleveland Member of the Ohio Shale Formation (Famennian, Late Devonian). Dean's redescription utilized articulated specimens to define the genus.^[6] Another valid species, C. clarki, was named by E. W. Claypole in 1893, also from the Cleveland Shale, with its holotype (NHMUK PV P 9271) representing a larger form.^[8] These two species are the most well-documented and valid within the genus, with C. fyleri reaching approximately 1.8 m in total length and exhibiting a more slender body profile, while C. clarki attained up to 2 m and featured a broader head region.^[9] Synonymy includes Cladodus fyleri and Cladodus clarki for the respective type species, reflecting the initial misclassification under Cladodus. Other proposed names, such as C. kepleri, are treated as distinct but closely related taxa in current revisions, though some fragmentary referrals have been rejected due to insufficient diagnostic material.^[6]^[8]^[10]

Taxonomy

Classification

Cladoselache was originally classified by Dean in 1894 within the family Cladoselachidae and the order Cladoselachiformes, placing it among the extinct chondrichthyans of the Late Devonian.^[11] Modern classifications place Cladoselachidae within the order Symmoriiformes under the subclass Holocephali, positioning Cladoselache as a basal chondrichthyan within the total-group Holocephali, characterized by primitive features that bridge early jawed vertebrates and more derived sharks.^[5] Historically, in the early 20th century, Cladoselache was regarded as a primitive shark within the subclass Selachi (Elasmobranchii), exemplifying an archetypal early elasmobranch form due to its streamlined body and grasping teeth. This view persisted until the 1980s, when Zangerl (1981) reassigned the Cladoselachidae to the newly erected order Symmoriida (later Symmoriiformes), grouping it with other cladodont sharks based on shared dental and skeletal traits, such as multicusped teeth and a slender body plan.^[3] Modern consensus, derived from cladistic analyses, regards Cladoselache as a basal chondrichthyan or the sister group to symmoriiforms plus holocephalans, supported by neurocranial and postcranial synapomorphies like a short otic capsule and upturned caudal axis. Maisey's 1986 phylogenetic study emphasized its position near the base of chondrichthyans, while updates in the 2000s and 2010s, including analyses by Coates and Sequeira (2001) and recent works, refine this to a clade uniting Cladoselachidae with Symmoriiformes as successive outgroups to crown chondrichthyans.^[12]^[11] The family Cladoselachidae, which includes the genera Cladoselache and Maghriboselache, is diagnosed by key traits such as the apparent lack of claspers in adult specimens—suggesting possible primitive reproductive strategies—and the presence of multiple radials supporting the large pectoral and dorsal fins, which enhance maneuverability. Debates persist regarding its subordinal status among Devonian sharks, with some proposing elevation to a distinct suborder Cladoselachii to reflect its transitional morphology between basal chondrichthyans and more specialized elasmobranchs.^[3]

Phylogenetic relationships

Cladoselache is positioned as a stem chondrichthyan, basal to the crown-group divergence of elasmobranchs (modern sharks and rays, or neoselachians) and holocephalans (chimaeras).^[5] This placement reflects its primitive features, such as uncalcified cartilage in the vertebral column, which distinguish it from acanthodians—now often regarded as more basal stem chondrichthyans with partial calcification—and align it with the unmineralized endoskeleton typical of derived chondrichthyans.^[3] Fossil evidence from Late Devonian deposits, dated to approximately 372–359 million years ago, underscores this basal role, with Cladoselache representing one of the earliest well-documented members of the chondrichthyan lineage.^[13] Phylogenetic analyses consistently recover Cladoselache within or sister to Symmoriiformes, a group of Paleozoic shark-like chondrichthyans characterized by shared traits such as robust dorsal fin spines and a similar braincase morphology.^[12] For instance, the braincase of Cladoselache exhibits unusual features, including a narrow occipital cotylus and specific otic capsule proportions, that closely resemble those in symmoriiform taxa like Cobelodus, supporting a close evolutionary affinity.^[3] This clade, in turn, forms the sister group to holocephalans, linked by features of jaw suspension (hyostylic to amphistylic transitions) and dentition, including pleurodont attachment where teeth fuse laterally to the jaw rather than replacing in distinct families.^[14] Seminal work by Patterson (1965) highlighted these dental similarities, arguing that early forms like Cladoselache bridge shark-like ancestors to the specialized holocephalan lineage.^[14] Recent cladistic studies, incorporating Bayesian tip-dating and expanded character matrices, reinforce this topology. For example, Frey et al. (2023) analyzed an emended dataset from prior works, placing Cladoselachidae (including Cladoselache) as the immediate sister to symmoriiforms, with the combined group basal to crown holocephalans and distant from neoselachians.^[5] These 2020s analyses, building on 3D reconstructions of fossils from sites like the Cleveland Shale, emphasize synapomorphies in the pectoral girdle and caudal fin, further solidifying Cladoselache's position near Cobelodus in the basal chondrichthyan tree.^[12] The phylogenetic placement of Cladoselache has significant implications for understanding chondrichthyan origins, illustrating an early divergence around 380 million years ago that bridges basal jawed vertebrates (gnathostomes) to the modern elasmobranch radiation.^[13] By exemplifying primitive conditions—such as the absence of specialized scales or claspers seen in later forms—it highlights how symmoriiform-like sharks contributed to the split between holocephalan and elasmobranch lineages in the Devonian, informing broader evolutionary patterns in vertebrate skeletal and sensory systems.^[12]

Anatomy

Cranial features

The chondrocranium of Cladoselache is cartilaginous and exhibits an amphistylic jaw suspension, in which the palatoquadrate is primarily supported by the hyomandibula, allowing flexibility in jaw movement. The ethmoid region includes large nasal capsules that contribute to the anterior expansion of the skull, while the overall chondrocranium is perichondrally mineralized with a single layer of tesserae and lacks a well-defined interorbital septum, resulting in a flat floor that completely separates the orbits. This structure is classified as platybasic, though some uncertainty persists due to compression in fossil specimens.^[3]^[15] The braincase features a short otico-occipital region with a persistent otico-occipital fissure and an extensive postorbital arcade, including a primary postorbital process and lateral commissure. Orbits are notably large, occupying approximately 50% of the preserved braincase length and suggesting enhanced visual acuity, with broad suborbital shelves extending to the orbital articulation. In adult specimens, the skull reaches lengths of up to about 30 cm, with the jaws positioned far forward to facilitate prey capture. The otic region is compact, and the occipital cotylus is narrow, separated by the fissure.^[3]^[16] Sensory structures on the skull include variable lateral cranial canals associated with the lateral line system, which likely aided in mechanoreception, and foramina for the palatine ramus of the facial nerve. The vestibular apparatus follows the standard gnathostome pattern, with an inner ear labyrinth comparable to that in related taxa such as Cladodoides, and large orbits further emphasize reliance on vision over specialized low-frequency phonoreception. A median fossa for the dorsal aorta is present below the occipital region, and pituitary foramina mark the extent of the polar cartilage.^[3]^[15] Compared to contemporaries like Doliodus, Cladoselache displays a more derived braincase morphology, including a longer otico-occipital region, separate palatine foramina, and a broader internasal plate, reflecting evolutionary advancements in cranial robustness and sensory integration within early chondrichthyans.^[3]

Jaws and teeth

The jaws of Cladoselache exhibit an amphistylic suspension, characterized by a long palatoquadrate cartilage with distinct otic and orbital processes that loosely articulate with the neurocranium, enabling a wide gape for prey capture.^[17] The palatoquadrate displays a cleaver-like shape typical of early chondrichthyans, with the otic process shorter than the palatine process and the jaw articulation positioned just posterior to the occipital condyle.^[5] This loose attachment, supplemented by ligaments, allowed flexibility in jaw protrusion during feeding.^[3] The branchial arches consist of at least five pairs of slender, well-separated cartilages, with preserved elements showing a ventral exposure and posterior inclination, decreasing in size caudally.^[11] The teeth of Cladoselache are small, typically measuring up to 5 mm in height, and arranged in families forming whorls perpendicular to the jaw axis, with pleurodont attachment via lingual positioning and side-based fusion to the jaw cartilage.^[18] They possess a cladodont morphology, featuring a tall, sigmoidal central cusp flanked by 1–3 pairs of smaller lateral cusps (yielding 3–5 points total), smooth edges without serrations, and a tooth base with a deep basolabial depression bordered by projections.^[5] Tooth crowns show continuous material between cusps and orolingual buttons, with significant wear patterns evident in fossils, where cusps become rounded and blunt, indicating abrasive contact during use.^[18] Replacement occurs slowly in whorls, estimated at 75–85 days per row, with size increments of approximately 24.7% between successive teeth in C. clarkii.^[18] This dentition facilitated a primitive grasping and cutting mechanism, as inferred from the multi-cusped design and wear, suited to holding soft-bodied prey without tearing.^[18] Compared to modern sharks, Cladoselache teeth lack advanced features like serrations or rapid replacement, reflecting an early evolutionary stage in chondrichthyan feeding adaptations.^[5] Species variations include more robust, larger teeth in C. clarkii than in the smaller C. fyleri, though overall morphology remains consistent without pronounced heterodonty in shape.^[19]

Postcranial skeleton and fins

The postcranial skeleton of Cladoselache consists primarily of cartilage reinforced by tessellate prismatic calcifications, a characteristic feature of early chondrichthyans that provided lightweight structural support without full ossification.^[20] This material forms the axial and appendicular elements, enabling a streamlined form suited to agile swimming in Devonian seas. The vertebral column features a persistent notochord sheathed in calcified cartilage, with paired neural and haemal arches providing lateral stability but lacking fully developed, spool-shaped centra typical of more derived vertebrates.^[20] In adult specimens, the column comprises numerous segmental elements—estimated at 150–200 based on preserved impressions—arranged in an elongate series that underscores the fish's fusiform body plan.^[3] These elements exhibit hourglass-like constrictions in cross-section, reflecting incremental growth through annual deposition rings visible in the calcified layers, indicative of rhythmic expansion during ontogeny.^[21] The pectoral and pelvic girdles are broad and robust, with the scapulocoracoid measuring up to 194 mm in length in large individuals, supporting expansive paired fins via metapterygial axes that extend posteriorly for enhanced maneuverability.^[21] The fins themselves are triangular in outline, with broad bases and radials segmented into short proximal elements and elongate, strap-like distal rays that fan outward, promoting hydrodynamic efficiency.^[21] Two dorsal fins are preceded by short, blade-like spines, while a single anal fin occupies a posterior position, contributing to overall body balance.^[20]^[1] The overall body is elongate and fusiform, with the postcranial skeleton comprising the majority (~75–80%) of the total body length and a lightweight construction that minimized drag and facilitated rapid locomotion; total lengths reach up to 2 meters in mature examples.^[20] The caudal fin is lunate with nearly equal upper and lower lobes, supported by upturned axial elements in the upper lobe, forming a high-aspect-ratio structure for powerful propulsion.^[3]^[1]

Paleobiology

Locomotion and sensory systems

Cladoselache exhibited a highly streamlined body, with a fusiform shape, reduced denticle coverage except along fin edges, and lateral keels on the caudal peduncle, all contributing to minimized drag and efficient locomotion in open water. Its propulsion relied primarily on the large, crescent-shaped caudal fin featuring equal-sized upper and lower lobes, which generated powerful thrust for sustained swimming. The broad-based pectoral fins, positioned behind the gills, facilitated maneuvering and lift during agile turns, while short dorsal fin spines of porous bone likely streamlined flow over the back, reducing turbulence. These adaptations indicate Cladoselache was specialized for fast, agile swimming as a high-speed predator, akin to modern lamniform sharks, enabling effective open-water pursuits rather than benthic hovering.^[1]^[22] Sensory capabilities in Cladoselache were adapted for detecting prey in dimly lit Devonian marine environments, with large eyes supported by expansive orbits—occupying nearly half the braincase length—and a sclerotic ring of 24 small rectangular plates per eye, providing structural reinforcement for enhanced visual acuity in low-light conditions. The lateral line system, evidenced by foramina for anterodorsal rami and integration with cranial nerves like the trigeminal, detected hydrodynamic pressure waves and vibrations, aiding in prey localization and predator avoidance. Olfactory structures, inferred from the ethmoid region's narrow internasal plate and diverging tracts in compression fossils, suggest sensitivity to chemical cues, potentially emphasized as in related cladoselachians with laterally expanded nasal capsules for stereo-olfaction.^[23]^[25] Buoyancy control in Cladoselache, inferred from comparisons with modern chondrichthyans, likely involved a large liver filled with low-density oils to achieve near-neutral buoyancy, compensating for the lack of a swim bladder and supporting its active, pelagic lifestyle and predation in mid-water columns.

Diet and ecology

Cladoselache was primarily a piscivorous predator, with direct evidence from gut contents in well-preserved specimens from the Late Devonian Cleveland Shale indicating that it preyed on small ray-finned bony fishes, crustaceans such as the shrimp-like Concavicaris, and conodont animals.^[26] Of approximately 53 specimens with identifiable stomach contents examined from this deposit, about 65% contained remains of small actinopterygian fishes, 28% showed evidence of crustacean prey, and 9% included conodont elements, often with multiple prey types ingested simultaneously.^[26] Rare gut contents also reveal predation on other chondrichthyans, including shark teeth and scales, marking the earliest documented instance of shark-on-shark trophic interactions in the fossil record.^[27] These findings suggest that Cladoselache employed a strategy of grasping and swallowing smaller, soft-bodied prey whole, facilitated by its multi-cusped, smooth-edged teeth.^[26] As a mid-level predator in Famennian marine ecosystems approximately 370 million years ago, Cladoselache occupied a pelagic niche in open waters adjacent to reefs, where it likely pursued agile, schooling prey using its streamlined body for high-speed pursuits.^[1] It coexisted with larger placoderms such as the arthrodire Dunkleosteus terrelli, which attained lengths over 6 meters and probably preyed upon Cladoselache, positioning the latter below the apex in the food web while it targeted smaller ray-finned fishes and invertebrates.^[28] Possible competition existed with early actinopterygians for similar prey resources in these productive seas.^[9] The abundance of Cladoselache fossils in mass mortality assemblages from the black shales of the Cleveland Member of the Ohio Shale Formation implies a gregarious lifestyle, potentially involving schooling behavior to evade larger predators or coordinate hunts.^[1] These deposits formed in oxygen-poor, anoxic basins during episodes of marine deoxygenation, indicating that Cladoselache tolerated low-oxygen conditions as a nektonic species capable of inhabiting stratified waters above the hypoxic seafloor.^[29]

Soft tissues and scales

The body of Cladoselache was largely smooth, lacking denticles except for small multi-cusped scales along the fin edges, mouth, and around the eyes, which contributed to reduced drag and efficient locomotion. This lightweight integument, distinct from the heavy bony armor of contemporary placoderms, was suited to agile swimming, as evidenced by the sparse and fine-grained nature of its dermal covering. Exceptional preservation includes traces of internal organs and muscle tissues, revealing myomere patterns similar to those in modern sharks, providing insight into the animal's muscular organization. Possible traces of fin webbing are also noted in select fossils, suggesting a flexible integument that supported efficient propulsion.^[30] Exceptional preservation of these features results from rapid burial in the anoxic, fine-grained shales of the Cleveland Member of the Ohio Shale Formation, where oxygen-poor bottom waters minimized decay and scavenging. Specimens collected in the 1880s, such as those studied by early paleontologists, retain outlines of the skin and organic traces, highlighting the formation's role in conserving non-skeletal anatomy.^[31]

Fossil record

Geological distribution

Cladoselache fossils are predominantly known from the Late Devonian period, specifically the Famennian stage (approximately 372–359 million years ago), within the Appalachian Basin of eastern North America.^[32] The genus is represented by numerous well-preserved specimens embedded in black shales, reflecting deposition in marine environments during a time of widespread oceanic anoxia.^[33] The primary locality for Cladoselache is the Cleveland Member of the Ohio Shale, a bituminous black shale formation exposed in northeastern Ohio, particularly along the southern shore of Lake Erie.^[34] This stratigraphic unit, part of the broader Ohio Shale sequence in the Appalachian Basin, spans thicknesses of 20–60 feet and is characterized by fissile, organic-rich layers with carbonate concretions that facilitated exceptional fossil preservation.^[34] The formation's deposition is linked to Late Devonian anoxic events, including conditions akin to the post-Kellwasser oceanic stagnation, where low-oxygen bottom waters prevailed in a subsiding basin.^[35] Fossils of Cladoselache are abundant in the Cleveland Member, far outnumbering those from other sites and indicating the genus was a common component of local marine ecosystems.^[36] This high abundance underscores its endemism to the warm, equatorial epicontinental seas of the region, where shallow waters over the Laurentian craton supported a diverse nektonic fauna.^[33] Outside Ohio, Cladoselache remains are rare, with no confirmed occurrences beyond North American marine deposits, though related cladoselachians, such as Maghriboselache mohamezanei described in 2023, have been reported from Famennian strata in Morocco.^[5] The paleoenvironment consisted of tropical, shallow inland seas with stratified waters—oxygenated at the surface but dysoxic or anoxic at depth—fostering the accumulation of organic matter in the black shales.^[34]

Preservation and taphonomy

The taphonomy of Cladoselache fossils is characterized by rapid burial in dysaerobic, anoxic muds of the Cleveland Shale, which minimized postmortem decay and scavenging, resulting in commonly articulated skeletons with minimal disarticulation or scattering of elements.^[37] Low-energy depositional environments, marked by stratified water columns and absence of bioturbation or benthic activity, facilitated this preservation by allowing carcasses to settle intact before significant decomposition occurred.^[31] Fossils are frequently entombed within flattened carbonate concretions featuring organic cores, which further protected remains from oxidative degradation.^[30] The skeletal cartilage of Cladoselache, which was prismatic and lightly calcified during life, is preserved through permineralization, enhancing fine details against compaction.^[3] This process yielded high-contrast preservation between the calcified cartilage and pyritized enclosing sediment, preserving fine details of the neurocranium and fin radials.^[3] Exceptional cases include traces of soft tissues, such as muscle fibers and integument, preserved in many articulated specimens through anoxic "pickling" effects possibly aided by urea release from decomposing tissues.^[30] Taphonomic biases in the Cladoselache record stem from mass die-off events in the oxygen-depleted basin. Delicate prismatic cartilage is prone to loss outside concretions, and sampling favors conspicuous nodules, further skewing the record toward larger, more intact forms.^[30] Recent advancements, including micro-CT scanning in studies from the 2020s, have mitigated preservation issues by non-destructively revealing internal structures like the braincase and hyoid arch in both Cladoselache and closely related cladoselachians.^[38] These preservation dynamics provide a nearly complete ontogenetic series for Cladoselache, from juveniles to adults, which is exceptionally rare among Devonian chondrichthyans and enables detailed reconstructions of growth and anatomy.^[30]

References

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Two schools of thought prevailed: Newberry (1873, p. 155) and. Orton (1878, p. 635-636) felt that the concretions were syngenetic, whereas Daly (1900, p. 143 ...
[37]
Coincidence of photic zone euxinia and impoverishment of ...
Nov 18, 2019 · Evidence for shallow-water 'Upper Kellwasser' anoxia in the Frasnian-Famennian reefs of Alberta, Canada. ... Cleveland Shale of the Appalachian ...
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[PDF] OHIO f OSSILS - Ohio.gov
Devonian Cleveland Shale: Cleveland Mus. Nat. Hist. Sci. Pub. vol. 8, No. 10 ... Newberry, John S. (1873), Descriptions of Fossil Fishes: Ohio Geol ...
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shark taphonomy in the cleveland shale of northeastern ohio, usa: a ...
Oct 22, 2014 · Cladoselachian and ctenacanth sharks in the Cleveland Shale are known for being abundant and relatively well preserved. Can taphonomic condition ...Missing: Cladoselache | Show results with:Cladoselache
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https://doi.org/10.1186/s13358-023-00266-6