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Holocephali
Temporal range: Middle Devonian-Holocene 393.3–0 Ma Molecular clock suggests first appearance during the Late Silurian
Helicoprion bessonowi
(a eugeneodont)
Orodus greggi
(an orodont)
Belantsea montana
(a petalodont)
Deltoptychius armigerus
(a menaspiform)
Echinochimaera meltoni
(an early chimaeriform)
Chimaera opalescens
(a modern chimaeriform)
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Class: Chondrichthyes
Subclass: Holocephali
Bonaparte, 1832
Included taxa

See text

Holocephali (Sometimes spelled Holocephala; Greek for "complete head" in reference to the fusion of upper jaw with the neurocranium) is a subclass of mostly extinct cartilaginous fish. While Holocephali is today represented by only 56 species belonging to three families and a single order which together are commonly known as chimaeras, the group was far more diverse throughout the Paleozoic and Mesozoic eras. Their earliest known fossils date to the Middle Devonian period, and the group likely reached its peak diversity during the following Carboniferous period. Molecular clock studies suggest that the group diverged from its closest relatives, elasmobranchs such as sharks and rays, as early as the Silurian period. While holocephalans similar to modern chimaeras are known from the Early Carboniferous, many extinct members of the group were radically different in appearance and ecology compared to their living relatives.

Extinct holocephalans are typically divided into a number of orders, although the interrelationships of these groups are poorly understood. Several different definitions of Holocephali exist, with the group sometimes considered a less inclusive clade within the larger subclasses Euchondrocephali or Subterbranchialia, and in some works seeing many of its members moved to the now obsolete groups Paraselachimorpha or Bradyodonti. Some recent research has suggested that the orders Cladoselachiformes and Symmoriiformes, historically considered relatives or ancestors of sharks, should also be included in Holocephali. Information on the evolution and relationships of extinct holocephalans is limited, however, because most are known only from isolated teeth or dorsal fin spines, and many of the better-understood taxa were not closely comparable to chimaeras.

Early holocephalans such as the orodonts, eugeneodonts, and some iniopterygians had skulls and bodies which were unlike modern chimaeras, with upper jaws (called palatoquadrates) that were not fused to the rest of the skull and a streamline, fusiform body shape. The bodies of most holocephalans were covered in tooth-like scales termed dermal denticles, which in many Paleozoic and Mesozoic members were fused into armor plates. Holocephalans may be sexually dimorphic, with males possessing both claspers on the pelvic fins and additional specialized clasping organs on the head and before the pelvic fins. The teeth of most holocephalans consist of slow-growing plates which suggest a durophagous lifestyle, and in some groups these plates were specialized into fused structures termed "tooth whorls". Fossils of holocephalans are most abundant in shallow marine deposits, although certain species are known from estuarine and freshwater environments as well.

Living chimaeras, which are also known as ratfish, rabbitfish, spookfish or ghostsharks, include mostly deep-sea species which are found worldwide in temperate-tropical oceans. They all possess large eyes, broad pectoral fins, opercular covers over the gills, fused skulls and upper jaws, and six plate-like crushing teeth. Like their extinct relatives they are sexually dimorphic, and males possess both two sets of paired sex organs around the pelvic fins and an unpaired clasper on the head. Females reproduce by laying large, leathery egg cases. Unlike their extinct relatives, the skin of living chimaeras is devoid of scales or armor plates, with the exception of scales on the sensory and sex organs. Because chimaeras have changed relatively little throughout their evolutionary history, they are often termed living fossils and are considered important for understanding early vertebrate evolution.

Research history and classification

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Early research

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French naturalist Charles Lucien Bonaparte, who erected the order Holocephali to encompass living chimaeras

Holocephali was first proposed as "Holocephala" by Johannes Müller, and was formally described by naturalist Charles Lucien Bonaparte between 1832 and 1841.[1][2][3] The name of the group comes from the Greek roots hólos meaning "whole" or "complete" and kephalos meaning head,[4] and is in reference to the complete fusion of the braincase and the palatoquadrates (upper jaw) seen in chimaeras.[5][6] As defined by Müller and Bonaparte, Holocephala encompassed the living genera Chimaera and Callorhinchus.[2][3][7]: 43  Fossil taxa, consisting primarily of tooth-plates and fin spines from the Mesozoic, were assigned to Holocephali throughout the 1830s and 1840s.[8][9][10] Many additional taxa were described and illustrated by the naturalist Louis Agassiz between 1833 and 1843 in Researches sur Les Poissons Fossiles, including a number of Paleozoic-age tooth and spine genera now considered to belong to Holocephali.[3][10][11] Both Agassiz and other influential researchers such as Richard Owen allied many Paleozoic representatives of the group with living Heterodontus (or Cestracion) sharks,[3][10] rather than with chimaeras.[7]: 43 [8][11] During the late 1800s, researchers such as Fredrick McCoy and James William Davis questioned the association between Paleozoic taxa and Heterodontus.[7]: 43 [10]

British paleontologist Arthur Smith Woodward, who was the first to ally plate-like Paleozoic fish teeth with chimaeras and who erected the order Bradyodonti[10][12]

Throughout the late 19th and early 20th century British paleontologist Arthur Smith Woodward recognized many fragmentary fossil fishes as Paleozoic holocephalans, and in 1921 united them under the newly coined order Bradyodonti.[3][10][12] This order, sometimes considered a class or subclass by later publications,[5][13] linked the living chimaeras with Paleozoic taxa known from teeth.[10][12][14]: 152  Later work by the paleontologists Egil Nielsen and James Alan Moy-Thomas expanded the Bradyodonti to include the Eugeneodontiformes and Orodontiformes (then the families Edestidae and Orodontidae)[14]: 152 [15] as well as the Chimaeriformes, despite these taxa's differences from the group as defined by Woodward.[3][10][13] The broadest usage of Bradyodonti encompassed an assemblage of fishes roughly equivalent to total-group Holocephali,[7]: 41–43 [13][16] and its composition remains similar to Holocephali as used by recent authors.[10]

While treated as a subclass of the class Chondrichthyes by modern authors (e.g. Joseph Nelson),[17]: 40–48  Holocephali has previously been ranked as an order,[2] a superorder,[5][16][18]: 46  or a class.[5][13] When Charles Lucien Bonaparte first coined Holocephala, he considered it to be an order within the larger subclass Elasmobranchii (different from modern usage; also contained the then-order selachii).[2][3][7]: 43  Several authors during the 20th century regarded the Holocephali as its own class within the (now obsolete) superclass Elasmobranchiomorphi, which also included the classes Selachii (or Elasmobranchii), Arthrodira (or Placodermi), and under some definitions the Acanthodii.[3][7]: 43 [13] Holocephali is still sometimes considered a lower taxonomic unit within a larger subclass by some contemporary authors, due to the name being a misnomer if taxa with unfused crania and upper jaws are included.[3][17]: 48–49 

Recent classifications

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The interrelationships of extinct holocephalan orders have been characterized as difficult to define and subject to change, due in part to limited data.[3][7]: 43 [17]: 49  The orders Orodontiformes, Petalodontiformes, Iniopterygiformes, Debeeriiformes, Helodontiformes and Eugeneodontiformes were formerly united under the superorder Paraselachimorpha by researcher Richard Lund.[5][19] The paraselachimorphs were defined as a sister group to either the superorder Holocephalimorpha (chimaeras and their closest relatives) or, in earlier works, the similarly defined Bradyodonti. However, Paraselachimorpha is now regarded as either paraphyletic or a non-diagnostic wastebasket taxon, including by Lund himself, and the taxa which formerly made up Paraselachimorpha are now considered an evolutionary grade of early-diverging holocephalans.[20][17]: 48–49  Likewise, the historically significant order Bradyodonti, consisting variously of taxa now placed in Petalodontiformes, Orodontiformes, Eugeneodontiformes, Helodontiformes, Menaspiformes, Cochliodontiformes, Copodontiformes, Psammodontiformes, Chondrenchelyformes, and Chimaeriformes,[5][10][13] has also been abandoned by recent authors and is considered a paraphyletic grade.[7]: 41–45 [10][21]

In a 1997 paper, Richard Lund and Eileen Grogan coined the subclass Euchondrocephali to refer to the total group of holocephalans (fish more closely related to living holocephalans than to living elasmobranchs).[3] Under this classification scheme, Holocephali has a much more restricted definition and excludes the orodonts, eugeneodonts, and petalodonts, which are considered more basal euchondrocephalans.[3][22][23] Other authors have used Holocephali to include all fishes more closely related to living chimaeras than to elasmobranchs, a definition equivalent to Lund and Grogan's Euchondrocephali.[3][19][17]: 48–49  Below is the taxonomy of total-group Holocephali as defined in the Fifth Edition of Joseph Nelson's Fishes of the World (2016), which follows Lund and Grogan's taxonomy but opts to use the name Holocephali rather than Euchondrocephali.[17]: 48–51 

Taxonomy according to the Fifth Edition of Fishes of the World (2016)[17]: 48–51  based on the work of Lund & Grogan (1997; 2004; 2012)[3][19]
Subclass Holocephali sensu lato (equiv. to Euchondrocephali)

† Extinct

An alternative classification was proposed by paleontologist Rainer Zangerl in 1979, who considered Holocephali to be a superorder within the newly-erected subclass Subterbranchialia (named in reference to the position of the gills relative to the skull).[3][16][17]: 48–49  This group united the chimaera-like taxa, which were distinguished by their holostylic jaw suspension, with the iniopterygians and the Polysentoridae which possessed at least in some cases an unfused upper jaw.[16]: 23–45 [18][28]: 146  This classification scheme was followed in both Volume 3A of the Handbook of Paleoichthyology, authored by Zangerl, and Volume 4, authored by Barbara J. Stahl. Both of these authors considered the traditionally "bradyodont" orodonts, petalodonts, eugeneodonts and desmiodontiforms to be elasmobranchs, rather than holocephalan as generally assumed.[7][18][22]: 25  Later works have regarded Subterbranchialia as a potentially paraphyletic wastebasket taxon of chondrichthyans with poorly defined relationships,[18]: 41–42 [21] and others have re-included the orodonts, eugeneodonts and petalodonts within Holocephali.[17]: 48–49 [22] Zangerl's proposed classification is provided below, with differences between it and the classification used by Stahl (1999) noted.[7][16]

Taxonomy proposed by Zangerl (1979)[16]: 458–459  and Zangerl (1981).[18]: 49–50  Utilized by Stahl (1999)[7]: 44–45 [17]: 48–49 
Subclass Subterbranchialia

Taxa classified within subclass Elasmobranchii sensu Zangerl (1981)[18]: 49–50 [22]: 109 

† Extinct

While often considered to either be closely related to elasmobranchs or to be stem-group chondrichthyans,[17]: 45–46 [29][30] some studies have found the shark-like symmoriiformes to be early diverging members of the Holocephali.[29][31][32] Alternatively, Symmoriiformes are sometimes regarded as the sister-group to Holocephali, but are not considered members of the subclass themselves due to differing morphology.[28]: 136–141  The traditionally-recognized order Cladoselachiformes, which is sometimes included within Symmoriiformes, may also be considered holocephalan under this classification scheme.[29] While the anatomy of the jaws and teeth differs dramatically between Symmoriiformes and typical holocephalans, these show similarities in the internal anatomy of their crania and both possess rings along their lateral lines, which may suggest close relation.[22]: 25 [31][28] Paleontologist Philippe Janvier first suggested a connection between the Holocephali and the Symmoriiformes (then Symmoriida) in his 1996 textbook Early Vertebrates,[22]: 25 [28]: 138–141  and the subsequent descriptions of the cladoselachian and Symmoriida taxa Maghriboselache and Ferromirum, as well as the redescription of the symmoriiform Dwykaselachus have continued to find support for the hypothesis.[33][29][31] The taxonomy presented in Early Vertebrates is provided below, which considered several taxa otherwise considered holocephalan to form a polytomy with Holocephali and Elasmobranchii (iniopterygians), or sit outside of crown-group Chondrichthyes.[28]: 147–149 

Taxonomy proposed by Janvier (1996)[28]: 148–149 
Unranked clade within crown-group Chondrichthyes

Taxa classified as incertae sedis within crown-group Chondrichthyes, and potentially forming a clade with Holocephali

Taxa classified as stem-group Chondrichthyes

Taxa considered too poorly known to place within Chondrichthyes[28]: 147–148 

  • †Order Orodontida (Orodontiformes)
  • †Genus Polysentor (Polysentoridae)
  • †Genus Zamponipteron (considered tentatively holocephalan and potentially associated with Pucapampella by Janvier)
  • †Genus Pucapampella (considered tentatively holocephalan and potentially associated with Zamponipteron by Janvier)
  • †Order Stensioellida (considered tentatively holocephalan by Janvier; alternatively considered a placoderm)
  • †Order Pseudopetalichthyida (considered tentatively holocephalan by Janvier; alternatively considered a placoderm)

† Extinct

Anatomy

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Main article: Fish anatomy
A generalized male Chimaera (fig. 1), displaying the dorsal fin spine (fig. 2-5), the cephalic clasper (fig. 6-7), pelvic claspers (fig. 9), prepelvic tenacula (fig. 10) and tooth-plates (fig. 11-12)
The cartilaginous skeleton of a female Chimaera, with key anatomical details labeled

Internal skeleton

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Fossilized cartilages of Cladoselache (A-C), Sibyrhinchus (D), Edaphodon (E-F), and Helodus (G), displaying mineralized tessellations[35]

All holocephalans possess an internal skeleton made up of cartilage, which in some regions of the body is ossified to provide additional strength. The mineralized tissues may form either as a network of hexagonal tessellations coating the outer surface of the underlying flexible cartilage, or in certain regions (e.g. the reproductive organs, lower jaw and vertebrae) dense, reinforced fibers interwoven with the cartilage termed fibrocartilage.[7]: 26 [36][35] In modern chimaeras the mineralized tessellations are irregularly shaped, smaller and less defined than in other cartilaginous fish, which has historically resulted in confusion as to whether these structures were present. In many extinct holocephalans the tessellations are large and hexagonal, and they appear morphologically more like those of sharks and rays than those of modern chimaeras.[35][36] The spinal cord of holocephalans is supported by a flexible nerve cord called a notochord, and in many taxa close to or within Chimaeriformes this notochord is sheathed by a vertebral column of ossified, disc-shaped cartilaginous rings (sometimes termed "pseudocentra" or "chordacentra";[3][37][38] different from vertebral centra in sharks and rays).[39][35] The vertebral rings directly behind to the skull (cervical vertebrae) typically fused into a single unit termed a synarcual.[7]: 31–32 [35][40] In many Paleozoic holocephalans, however, the vertebral rings were unmineralized or absent and the notochord was not ossified. Dorsal (upper) and ventral (lower) processes are present along the vertebral column of holocephalans, which were typically ossified even in early taxa without preserved vertebral rings.[3][7]: 31–32 [35] Like other cartilaginous fish, holocephalans lack ribs.[7]: 31–32 [17]: 48 

Skull, jaw and gills

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Main articles: Fish jaw and Fish gill
Reconstructed skull, gill and pectoral musculature of the extinct iniopterygian Iniopera (A-C, E, G, I) compared with that of the living Callorhinchus (D, F, H, J). Both genera have holostylic jaws[41]

The jaw suspension of modern chimaeras and many of their extinct relatives is holostylic (sometimes termed autostylic)[39][42]: 60 [43], meaning that the upper jaws (palatoquadrates) are entirely fused to the skull (neurocranium or chondrocranium) and only the lower jaws (Meckel's cartilages) are able to articulate.[7]: 26 [17]: 41, 48 [42]: 60  Holostyly has been proposed to have evolved independently in several extinct holocephalan groups due to a similar lifestyle.[7]: 26 [37][41] The ancestral mode of jaw suspension among holocephalans has been termed autodiastyly (sometimes termed unfused holostyly),[3][37][17]: 41  meaning that the upper jaws are not fully fused to the cranium and instead articulate at two points, rendering them inflexible but still separated from the cranium. A number of early holocephalan groups exhibit autodiastyly,[3][37][44] and embryonic chimaeras show the condition at early stages of development.[44][45] Other forms of jaw suspension, termed hyostyly and amphistyly, are present in modern elasmobranchs and in some potential holocephalan groups.[42]: 60 [44][28]: 140–144  In hyostilic and amphistylic jaw suspension, the upper jaws are disconnected from the cranium. Hyostilic and amphistylic jaws are supported by soft tissue, as well as by a modified pharyngeal arch termed the hyoid arch or hyomandibula.[3][44][17]: 41 

In holostylic and autodiastylic holocephalans, the hyoid arch is retained but is not utilized in jaw suspension. Instead, the arch is positioned behind the skull and supports a soft, fleshy gill cover (operculum) which is reinforced by cartilaginous rays.[46][44][17]: 41, 48  This soft operculum is considered a characteristic feature of the Holocephali,[17]: 48 [39][46] although it is debated whether it was present in some early members of the subclass (e.g. Eugeneodontiformes) or if they had separate gill slits like elasmobranchs.[46][47]: 143–144, [167]  Holocephalans typically possess four to five gill arches,[39][46][17]: 48  although eugeneodonts may have had a small, vestigial sixth gill arch.[48] The gill arches of Iniopterygiformes and derived holocephalans are tightly packed and positioned beneath the skull.[16][39]

Fins

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Main article: Fish fin

The fins of holocephalans may include paired pectoral and pelvic fins, either one or two dorsal fins, a caudal (tail) fin, and in certain members a small anal fin. The fins are skeletally supported by cartilaginous blocks and rods called basal and radial pterygiophores and thin rays called ceratotrichia.[3][7]: 32–38 [39] The caudal fin of many holocephalans is heterocercal with a long upper lobe, although in some groups it is leptocercal (also called diphycercal) meaning it is symmetrical and elongated, and in modern chimaeras may also end in a long, whip-like filament. In living chimaeras the first dorsal fin is retractable, and is additionally supported by a large fin spine and the synarcuum (cervical vertebrae).[7]: 36–37 [39] The paired fins are supported by the pectoral girdles (scapulocoracoids) and pelvic girdles, respectively. The pectoral girdles are fused along their ventral (lower) point of contact in modern chimaeras but unfused in earlier holocephalans.[3][7]: 33–35  Some fins may be reduced or absent in specific holocephalan groups, or extremely large and specialized in others. Groups such as the iniopterygians, petalodonts and chimaeras have small, underdeveloped caudal fins and very large, wing-like pectoral fins.[49][39][25] In the Chondrenchelyiformes and some orodonts all fins were very small and the body shape is eel-like (termed anguilliform).[49][50][18] Members of the Eugeneodontiformes lacked second dorsal fins and anal fins, as well as potentially pelvic fins.[48][18]

Teeth

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The jaws and tooth-whorls of Edestus, a member of Eugeneodontiformes
A comb-like tooth from Polyrhizodus, a member of Petalodontiformes
The lower tooth-plates and mandible of Poecilodus, a member of Cochliodontiformes
The upper and lower tooth-plates of Edaphodon, a member of Chimaeriformes
Reconstructed lower jaw of a eugeneodontiform, compared to that of the unrelated modern Heterodontus shark. Eugeneodonts possessed a symphyseal tooth whorl in the anterior region of the jaw, and a lateral tooth-pavement in the posterior region

Holocephalans are characterized by teeth which grow slowly and are either shed infrequently or are retained throughout life and are never shed. The dentitions of some holocephalans are strongly heterodont, meaning that teeth are specialized for different purposes in different regions of the mouth. The tooth families can be arranged into those at the anterior (front) and posterior (rear) of the jaws,[51] and additionally into paired, lateral teeth along the margins of the jaws and unpaired, symphyseal teeth along the midline. In most holocephalan groups the lateral dentition consists of flat, plate-like teeth in tight-fitting rows, a configuration termed "pavement dentition" with specific elements termed "pavement teeth". The teeth of holocephalans consist of a crown and a base (sometimes called a root). In some extinct groups the bases of the teeth are fused into connected structures called tooth whorls, which may form smaller half-whorls or full, looping spirals.

Cross-section of a fossilized tooth-plate identified as Helodus. Members of this genus are noted to possess tubular dentin
Cross-section of the tooth-plate of the modern Chimaera phantasma, displaying osteodontin (OD) and whitlockite-based pleromin tissues (VP, CP)

Holocephalan teeth are composed primarily of a form of dentin termed osteodentin, and in some groups this is capped with an outer coating of stronger enameloid.

Chimaeras

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Main article: Chimaera

Modern chimaeras have only three pairs of highly specialized tooth-plates, with two pairs in the upper jaw and a single pair in the lower jaw. The morphology of the tooth plates varies depending on the species; in some the foremost plates are adapted for cutting prey and the rear plates for crushing, while in others all plates serve a crushing function.[51]

Skin and external skeleton

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Main article: Fish scale

In adult modern chimaeras, scales are present along the lateral line and, in males, on the reproductive organs, while most of the body is covered in smooth, scaleless skin.[17]: 48 [39] Embryonic and juvenile chimaeras do possess additional scales along their backs, which are lost during development in most species.[7]: 8 [39] Conversely, Paleozoic and Mesozoic chimaeriforms such as Squaloraja and Echinochimaera, as well as members of other extinct orders exhibit scales covering the entire body throughout life. The scales of holocephalans are placoid (also termed dermal denticles), meaning they contain a pulp cavity, are made up primarily of orthodentin and are coated in an outer layer of hard enameloid.[3][7]: 8–12 [17]: 48  In extinct holocephalans the scales may be either single-cusped (termed lepidomoria) or multi-cusped (termed polyodontode scales), the latter meaning the scales have multiple crowns growing from a single base.[3][7]: 8–9 [14]: 399–412 

Sensory organs

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Both modern and fossil holocephalans possess sensory canals on their heads and down the length of the body. The precise arrangement of these canals in extinct members of the group is difficult to determine, although they are well-documented in taxa such as Menaspis, Deltoptychius, Harpagofututor, and a number of extinct chimaeriforms.[7]: 38 [13][52] Some holocephalans display a distinctive arrangement of ring-shaped scales enclosing the lateral line, which is considered a unique feature of the group.[13][52]

Spines and armor

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Some holocephalans had armor plates made up of fused polyodontode scales, and many also possessed dentin spines which protruded from the top of the head, the mandible, or the first dorsal fin.[3][7]: 8–12  Modern chimaeras lack armor and retain only a dorsal fin spine, which in at least some species is venomous.

Reproduction

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Main article: Clasper
Males of the chondrenchelyiform Harpagofututor (below) possessed both paired pelvic claspers and paired, antler-like cephalic claspers, both of which are absent in females (above)

Many holocephalans are highly sexually dimorphic. Males may possess up to three sets of external reproductive organs: paired pelvic claspers like those of other cartilaginous fish, paired prepelvic tenacula, and paired or unpaired frontal or cephalic claspers.[7][17]: 48 [39] In certain Paleozoic species, additional dimorphic paired spines are sometimes present on the heads of males. While some authors in the past have considered these structures potentially homologous to cephalic claspers,[7][13] they are now considered distinct due to their differing histology.[7][14][38] Unlike other cartilaginous fish, chimaeras lack a cloaca and instead possess separate anal and urogenital openings.[17]: 48 [39]

Cephalic claspers

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Holocephalans such as the myriacanthoids, Psammodus and Traquairius nudus possess extremely long, unpaired cephalic claspers, which in some taxa are as long as the skull and rostrum.[7][38][53] The presence or absence of these structures varies even among closely related taxa, and it is thought that cephalic claspers have appeared separately in multiple holocephalan groups.[38]

Prepelvic tenacula

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Prepelvic tenaculae are paired, retractable structures that protrude in front of the pelvic fins of certain holocephalan groups.[3]

Eggs and development

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All living chimaeras reproduce by egg-laying (oviparity). The egg cases of both living chimaeras and their close fossil relatives are proportionally large, composed of collagen, and in living chimaeras are laid two at a time.[21][54][55] Chimaera egg cases are characterized by an elongated, fusiform shape and a membranous, striated flap (termed a flange or collarette) protruding from their outer rim.[7]: 38–39 [55][56] The egg anatomy is unique in each family of chimaeras, allowing for isolated fossilized eggs to be identified to the family level.[39][55][57] Egg cases similar to those of chimaeras, assigned to the oogenera Crookallia and Vetacapsula, are known from the Late Carboniferous (Pennsylvanian) and may have been laid by helodonts.[55][57] Because of the rarity of egg capsules and presence of isolated fossilized fetuses from the Early Carboniferous (Mississippian) Bear Gulch Limestone fossil site, it is possible that many early holocephalan groups may have been live-bearing (viviparous or ovoviviparous), although it is also that possible that egg cases from many species simply happen to not have been preserved.[49][54][58]

Young juvenile holocephalans have very weakly calcified skeletons and are often poorly preserved. Fossils of fetal or newborn Delphyodontos, which may be an early holocephalan, are an exception, as these have uniquely calcified skulls and sharp, hook-like teeth. Based on its anatomy and coprolites (fossilized feces), Delphyodontos may have engaged in intrauterine cannibalism and was thus likely live-born (viviparous).[7]: 38–39 [54][55] The chondrenchelyiform Harpagofututor gave birth to extremely large young, which besides their uncalcified skeletons were well-developed and likely matured quickly. Female Harpagofututor are known to have contained up to five fetuses from multiple litters, and unlike Delphyodontos it is considered unlikely the fetuses engaged in cannibalism. Instead, it is probable fetal Harpagofututor were fed either by unfertilized eggs (oophagy) or mucus within the uterus (histophagy).[58]

Evolution

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Main article: Evolution of fish
Stensioella hertzi has sometimes been considered the earliest-known holocephalan.[59]: 76  It is alternatively believed to be an early placoderm of indeterminate placement[17]: 37 [27]: 58 

While the holocephalan fossil record is extensive, most of these fossils consist only of teeth or isolated fin spines, and the few complete specimens that are known are often poorly preserved and difficult to interpret.[39][21][60] The enigmatic, heavily squamated fishes Stensioella, Pseudopetalichthys and Paraplesiobatis, all known from poorly preserved body fossils from the Early Devonian of Germany, have been proposed by researcher Phillippe Janvier to be the earliest holocephalans,[28]: 147, 171 [59]: 76 [61]: 61–64  although they have alternatively been considered unrelated placoderms.[17]: 37 [27]: 58 [62] Fragmentary fossils that are confidently considered holocephalan first appear during the Middle Devonian,[7]: 153-154 [55] although molecular clock and tip dating does suggest an earlier origin. Based on genetic data, it is proposed that the total-group Holocephali split from the Elasmobranchii between the Silurian and the Early Devonian, with estimates ranging from 421-401 million years ago depending on the methods employed.[32][63][64] The earliest known fossils of Holocephali sensu stricto (Holocephalimorpha) date to the Famennian stage of the Late Devonian, and consist of rare isolated tooth-plates assigned to the Cochliodontiformes.[7]: 80 [65] The Chimaeriformes may have evolved as early as the Mississippian subperiod of the Carboniferous,[26][66] although other estimates suggest a much later Triassic origin of the group.[32]

Several groups have been proposed as sister clades or ancestors of the Chimaeriformes. Some authors have favored a close relationship between the Chondrenchelyiformes and the chimaeras, as despite their wildly different skeletal structure they have similar tooth anatomy. The Chimaeriformes may have alternatively evolved from within the larger clade Cochliodontimorpha, as while the tooth plates of adult cochliodonts and chimaeriforms differ in their morphology, the tooth-plates of juvenile cochliodonts and modern chimaeras are very similar.[3][7]: 41 [21] Below is a cladogram proposed by Grogan and Lund (2004) for one possible phylogeny of Holocephali (considered by them Euchondrocephali), which nests Chimaeriformes within a poorly-resolved clade also containing the cochliodonts.[38] A modified version of this cladogram was also utilized by Grogan, Lund & Greenfest-Allen (2012) which excludes the Iniopterygiformes from Holocephali (Euchondrocephali).[19]

Euchondrocephali (=Holocephali sensu lato)

†ElWeir (="El Weirdo")

†Debeeriiformes

†Iniopterygia (=Iniopterygiformes)

†L2SP

†Eugeneodontida (=Eugeneodontiformes)

Holocephalimorpha

†Harpacanthus

Holocephali sensu stricto

†Harpagofututor (Chondrenchelyiformes)

†Squaloraja (Squalorajiformes?)

Chimaeriformes

†Acanthorhina (Myriacanthiformes?)

†cf. Physonemus

†Cochlliodont 1 (="Coch1")

†Echinochimaera

†Menaspiformes

Ancestry

[edit]
Historically, debate arose as to whether placoderms such as Ctenurella (above) or shark-like chondrichthyans such as Cladoselache (below) were the ancestors of Holocephali

While it is now accepted that Holocephali is the sister group to Elasmobranchii based on both morphology and genetics,[17]: 40–41 [19][64] this was historically a matter of debate. Two competing hypotheses were proposed for the evolution of the holocephalans: either they were descended from a shark-like ancestor, making the class Chondrichthyes a true, monophyletic (natural) group, or they were descended from some unrelated lineage of placoderms, making Chondrichthyes a polyphyletic (unnatural) grouping.[3][13][21] A particular group of placoderms called the Ptyctodontiformes (or Ptyctodontida) were suggested by researchers Tor Ørvig and Erik Stensiö to be the direct ancestors of Holocephali due to their chimaera-like anatomy.[21][61]: 113 [67] While the ptyctodonts do share many holocephalan-like features, such as a synarcual formed from the frontmost vertebrae, a fin spine, an operculum, and specialized pelvic and prepelvic claspers, these are believed to result from convergent evolution.[13][17]: 37 [21] An alternative hypothesis, advocated for by researcher Colin Patterson, was that the holocephalans were related to neither the elasmobranchs nor the ptyctodonts, and instead shared a distant common ancestor with both groups within the clade Elasmobranchiomorpha.[7]: 41 [13][21] In light of the description of holocephalan transitional fossils during the 1970s and 1980s an independent origin of Elasmobranchii and Holocephali has been discredited,[5][61]: 113  and the two subclasses are united by the shared anatomy of their pelvic claspers and the tesserae that reinforce their cartilage skeletons.[17]: 40–43 [19][68]: 197–200 

Within Chondrichthyes, three contemporary hypotheses are proposed for the evolutionary relationship between the two subclasses.[22]

Ecology

[edit]

Bear Gulch Limestone

[edit]
Main article: Bear Gulch Limestone

The Bear Gulch Limestone (a unit of the Health Formation located in the state of Montana) has been recognized for preserving detailed, complete body fossils of fishes dating to the Mississippian subperiod of the Carboniferous.[3][5][69] The majority of fish species known from the site are chondrichthyans, of which more than 40 are early holocephalans.[19][49][61]: 113  Many of the holocephalans known from Bear Gulch belong to lineages that are otherwise known only from teeth or are entirely unrecognized.[5][38][69] These fossils also preserve gut contents,[19][37] color patterns,[37][70] complete life histories,[58] and internal organs,[19][70] allowing for a detailed understanding of their ecology and behavior. The site preserves an exceptional diversity of species, and is considered the best studied and most completely preserved Paleozoic fish fauna known.[19][37][49] The environmental conditions and faunal composition of Bear Gulch are believed to be representative of other, less well-known Mississippian marine fossil formations elsewhere in the world.[19][49] The Bear Gulch limestone is designated as a Konservat-Lagerstätte by paleontologists, and forms much of the basis for our modern understanding of early holocephalan evolution and ecology.[3][19][61]: 113  Additional sites, such as the Glencartholm and Manse Burn shales of Scotland, have also yielded detailed holocephalan fossils from the early Carbonifeorus.[10][19][42]: 174 

Habitats

[edit]

All living holocephalans and nearly all extinct taxa are known from marine environments, although the helodont Helodus simplex is uniquely known from a freshwater deposit.[7]: 40 [27]: 78–83  Almost all living chimaeras are specialized for deep-sea habitats, with only Hydrolagus colliei and the three species of Callorhinchus being regularly found in water shallower than 200 meters.[7]: 40 [39][64] Ancestral chimaeras were likely shallow-water fishes, and the radiation of the group into deepwater niches occurred during the early Cenozoic era.[32]

Diet

[edit]

Adaptations for a duropagous diet such as flattened tooth plates and a fused, immobile skull are prevalent among holocephalans,[7] but feeding styles are greatly variable. Modern chimaeras are generalist, opportunist feeders that regularly eat both soft-bodied and shelled prey.[39][51] The genus Callorhinchus is known to eat worms, crustaceans and hard-shelled mollusks, and other chimaeras are also known to prey on small fish. Smaller prey items are often eaten whole via suction feeding rather than being crushed or bitten, which is likely achieved using the muscles of the throat and flexible, cartilaginous lips. The bite forces of chimaeras are weaker than those of living shell-crushing sharks, and chimaeras may rely on their incisor-like anterior teeth to split and crack shells rather than solely crushing them.[51]

During the late Paleozoic, many holocephalan lineages became specialized for feeding styles besides durophagy. The edestoids, a lineage of Eugeneodontiformes, were pelagic macropredators which fed on fish and cephalopods.[71][72] The genus Edestus has been proposed to have fed by processing prey between its paired tooth-whorls,[73] while the related Helicoprion may have been a specialist hunter of belemnoids and ammonoids.[72] The poorly-known petalodont Megactenopetalus may have also been a macropredator based on its large, interlocking blade-like tooth plates.[74] The sibyrhinchid iniopterygian Iniopera was a suction feeder that fed in a similar manner to some living bony fish and aquatic salamanders.[41]

Parasites

[edit]

Modern holocephalans are vulnerable to a wide range of parasitic infections. Among these are tapeworms of the order Gyrocotylidea, which are found only in chimaeras and are thought to be a primitive, relict group themselves.[75][76] Fossilized tapeworms are also known in the symmoriiform Cobelodus, which represent the earliest evidence of parasitism in the group if symmoriiformes are considered members of Holocephali.[77][78]

Decline

[edit]

Total-group Holocephali has seen a significant decline in diversity since the Paleozoic, and only a single, morphologically-conserved order survives today.[17]: 48–49 [32][64] The holocephalans peaked in diversity during the Mississippian subperiod of the Carboniferous and in certain regions make up the majority of known chondrichthyan taxa from that time.[23][79] Diversity remained relatively high throughout the later Carboniferous (Pennsylvanian subperiod), but the group saw a significant decline in diversity at the Carboniferous-Permian boundary which continued through the rest of the Permian period.[23] By the end of the Permian, most holocephalan groups had become extinct,[17]: 48–49 [19][64] although the Eugeneodontiformes remained widespread and diverse for a brief period during the Early Triassic.[22][48][80] The order Chimaeriformes also continued throughout the Mesozoic, but two of the three named Mesozoic suborders became extinct during the Jurassic period,[81][82] leaving only three families in the suborder Chimaeroidea persisting through the Cenozoic and into the present.[17]: 51–53 [59]: 76-77 [68]: 200  Today, chimaeras make up as little as 4% of named cartilaginous fish species.[83]: 1-4 

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