| Holocephalans | |
|---|---|
_(Bear_Gulch_Limestone%2c_Upper_Mississippian%3b_Montana%2c_USA)_(33784748875).jpg)  Chimaera opalescens (a modern chimaera) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Vertebrata |
| Infraphylum: | Gnathostomata |
| Clade: | Eugnathostomata |
| Class: | Chondrichthyes |
| Subclass: | Holocephali Bonaparte, 1832 |
| Included taxa (total group) | |
For other proposed members and classification schemes,see text | |
Holocephali (sometimes spelledHolocephala;Greek for "complete head" in reference to the fusion ofupper jaw with theskull) is asubclass ofcartilaginous fish. The only livingholocephalans are the three families ofchimaeras, but the group also includes many extinct members and was more diverse during thePaleozoic andMesozoiceras. The earliest knownfossils of holocephalans date to theMiddle Devonian, and the subclass likely reached its peak diversity during the followingCarboniferous Period.Molecular clock studies suggest that holocephalansdiverged from their closest relatives,elasmobranchs such assharks andrays, during theEarly Devonian or theSilurian Period.
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 inclusiveclade within the larger subclassesEuchondrocephali orSubterbranchialia and with its members spread into the now obsolete groupsParaselachimorpha orBradyodonti. Per these classification schemes, the name Holocephali is used only for chimaeras and their closest relatives. Recent research has suggested that the ordersCladoselachiformes andSymmoriiformes, which were historically considered relatives or ancestors of sharks, should instead be considered holocephalans. Information on the evolution and relationships of extinct holocephalans is limited, however, because most are known only from isolated teeth ordorsal finspines, which form much of the basis of their classification.
Chimaeras, the only surviving holocephalans, include mostlydeep-sea species which are foundworldwide. They all possess broad, wing-likepectoral fins, a single softcover over thegills,upper jaws which are fused to the skull, and six plate-like crushing teeth in the mouth. Males possess both two sets of paired sex organs around the pelvic fins and an unpaired, toothed structure termed a cephalicclasper on the head. Females reproduce by laying large, leatheryegg cases. The skin of living chimaeras lacksscales orarmor plates, with the exception of tooth-like scales termeddermal denticles on thesensory andsex organs. Chimaeras are unique amongvertebrates in that their tooth plates containorgans called tritors, which are made of the mineralwhitlockite. Fossils similar to living chimaeras are known as far back as theEarly Carboniferous.
While some resembled their living relatives, many extinct holocephalans had skulls and bodies which were unlike modern chimaeras. In members of extinct groups, the upper jaws were often not fused to the rest of the skull and the jaws supported rows of separate, shark-like teeth. The bodies of most extinct holocephalans were totally covered in dermal denticles, which in Paleozoic andMesozoic members were sometimes fused intoarmor plates. Many extinct holocephalans weresexually dimorphic, and the males of some species possessed large grasping organs on the head. In some groups the teeth were specialized into fused, curled structures termed "tooth whorls", or arranged into flattened, crushing surfaces termed "tooth pavements". The shape of the teeth in many extinct holocephalans suggests they had adiet of shelled prey, although other species instead likely hunted softer prey likecephalopods or smaller fish. Fossils of holocephalans are most abundant inshallowmarinedeposits, although an extinct species is known fromfreshwater environments as well.
The first published use of Holocephali (then spelled "Holocephala") was by Swiss naturalistJohannes Müller in 1835, and the group was formally defined and classified by French naturalistCharles Lucien Bonaparte between 1832 and 1841.[1][2][3] The name of the group comes from the Greek rootshólos meaning "whole" or "complete" andkephalos meaning head, and is in reference to the complete fusion of the braincase and thepalatoquadrates (upper jaw) seen in chimaeras.[4][5][6] As defined by Müller and Bonaparte, Holocephala encompassed the living generaChimaera andCallorhinchus.[2][3][7]: 43 Fossils of tooth plates andfin spines from theMesozoic era were later assigned to Holocephali throughout the 1830s and 1840s.[8][9][10] Many additionaltaxa were described and illustrated by the SwissnaturalistLouis Agassiz between 1833 and 1843, including a number ofPaleozoic era tooth andspine genera now considered to belong to Holocephali.[3][10][11] Both Agassiz and other influential researchers such as English biologistRichard Owen allied many Paleozoic representatives of the group with livingHeterodontus (thenCestracion) sharks,[3][10] rather than with chimaeras.[7]: 43 [8][11] By the late 1800s, researchers such as Irish zoologistFredrick McCoy and British naturalistJames William Davis questioned the relationship between these Paleozoic fossils andHeterodontus.[7]: 43 [10]
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During the late 19th and early 20th century, British paleontologistArthur Smith Woodward recognized many fossilchondrichthyans as forming a distinct taxonomic group, and in 1921 named this groupBradyodonti.[3][10][12] Woodward considered Bradyodonti anorder, although it was sometimes considered aclass or subclass by later publications.[5][13] He suggested that the bradyodonts were intermediate between sharks and chimaeras (then considered equivalent to Holocephali), and indicated that the latter had evolved from Paleozoic ancestors.[10][12][14]: 152 Later work by the Danish paleontologistEgil Nielsen and British paleontologistJames Alan Moy-Thomas expanded the Bradyodonti to include theEugeneodontiformes andOrodontiformes (then the familiesEdestidae andOrodontidae)[14]: 152 [15][16] as well as modern chimaeras, despite these taxa's differences from the group as defined by Woodward.[3][10][13] The broadest usage of Bradyodonti is roughly equivalent tototal-group Holocephali,[7]: 41–43 [13][17] and its composition remains similar to Holocephali as used by modern authors.[10]
Holocephali is treated as a subclass of the class Chondrichthyes by many modern authors (e.g.Joseph Nelson),[18]: 40–48 although the group has alternatively been ranked as an order,[2][19] a superorder,[5][17][20]: 46 or as a class.[4][5][13] When Charles Lucien Bonaparte first defined Holocephala, he considered it to be an order within the larger subclassElasmobranchii (different from modern usage; also contained the then-orderselachii).[2][3][19] Several authors during the 20th century regarded the Holocephali as its own class within the (now obsolete) superclassElasmobranchiomorphi, which also included the classes Selachii (or Elasmobranchii), the extinctArthrodira (orPlacodermi), and under some definitions the extinctAcanthodii.[3][7]: 43 [13] Holocephali is still sometimes considered a lower taxonomic unit within a larger subclass by some contemporary authors.[3][18]: 48–49
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 [18]: 49 The ordersOrodontiformes,Petalodontiformes,Iniopterygiformes,Debeeriiformes,Helodontiformes andEugeneodontiformes were formerly united under the superorder Paraselachimorpha by researcher Richard Lund.[5][21] The paraselachimorphs were defined as asister group to either the superorder Holocephalimorpha (chimaeras and their closest relatives; also coined by Lund) or, in earlier works, the similarly defined Bradyodonti. However, Paraselachimorpha is now regarded as eitherparaphyletic or a non-diagnosticwastebasket taxon, including by Lund himself, and the taxa which formerly made up Paraselachimorpha are now considered anevolutionary grade of early-diverging holocephalans.[22][18]: 48–49 Likewise, the historically significant order Bradyodonti, consisting variously of taxa now placed in Petalodontiformes, Orodontiformes, Eugeneodontiformes, Helodontiformes,Menaspiformes,Cochliodontiformes,Copodontiformes,Psammodontiformes,Chondrenchelyformes, andChimaeriformes,[5][10][13] has also been abandoned by recent authors and is considered a paraphyletic grade.[7]: 41–45 [10][23]
Multiple classifications of Holocephali have been proposed by contemporary authors, which differ greatly from one another.[18]: 49–50 [24] In a 1997 paper, researchers Richard Lund and Eileen Grogan coined the subclass Euchondrocephali to refer to thetotal group of holocephalans (fish more closely related to living chimaeras 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 or, in older works, paraselachians.[3][25][26] 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.[19][21][18]: 48–49 Joseph S. Nelson, in his reference textFishes of the World, opted to use the name Holocephali for aclade identical in composition to Euchondrocephali. Below is the taxonomy of total-group Holocephali as defined in the Fifth Edition ofFishes of the World (2016), which differs from earlier editions by disbanding Paraselachimorpha.[18]: 48–51 [27]
| Taxonomy according to the Fifth Edition ofFishes of the World (2016)[18]: 48–51 |
|---|
† 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][17][18]: 48–49 This group united the chimaera-like taxa, which were distinguished by their holostylic jaw suspension, with the entirely extinctiniopterygians and thePolysentoridae which possessed at least in some cases an unfused upper jaw.[17]: 23–45 [32]: 146 This classification scheme was followed in both Volume 3A of theHandbook 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 before.[7][20][25]: 25, 109 Later works have regarded Subterbranchialia as a potentially paraphyleticwastebasket taxon of chondrichthyans with poorly defined relationships,[20]: 41–42 [23] and others have re-included the orodonts, eugeneodonts and petalodonts within Holocephali.[18]: 48–49 [25]: 25–26 Zangerl's proposed classification is provided below, with differences between it and the classification used by Stahl (1999) noted.[7][17]
| Taxonomy according to Zangerl (1979; 1981)[17][20]: 49–50 |
|---|
Taxa classified within subclass Elasmobranchiisensu Zangerl (1981)[20]: 49–50 [25]: 109
† Extinct |
Some studies have found the shark-likesymmoriiformes to be early diverging members of the Holocephali,[33][34][35] although this group is more often considered either to be related to elasmobranchs or to bestem-group chondrichthyans.[18]: 45–46 [36][37] Alternatively, Symmoriiformes are sometimes regarded as the sister group to Holocephali rather than members of the subclass itself due to differing morphology.[32]: 136–141 The traditionally-recognized orderCladoselachiformes, which is sometimes included within Symmoriiformes, may also be considered holocephalan under this classification scheme.[33] While the anatomy of the jaws and teeth differs dramatically between Symmoriiformes and typical holocephalans, these show similarities in the internal anatomy of theircrania and both possess rings along their lateral lines, which may suggest close relation.[25]: 25 [34][32] French paleontologistPhilippe Janvier first suggested a connection between the Holocephali and the Symmoriiformes (then Symmoriida) in his 1996 textbookEarly Vertebrates,[25]: 25 [32]: 138–141 [37] and the subsequent descriptions of the cladoselachian and Symmoriida taxaMaghriboselache andFerromirum, as well as the redescription of the symmoriiformDwykaselachus have found additional support for the hypothesis.[38][33][34] The taxonomy presented inEarly Vertebrates is provided below, which considered several taxa otherwise considered holocephalan to form apolytomy with Holocephali and Elasmobranchii (iniopterygians), or sit outside of crown-group Chondrichthyes.[32]: 147–149
| Taxonomy according toEarly Vertebrates (1996)[32]: 148–149 |
|---|
Subclass Holocephali
Taxa classified asincertae sedis within crown-group Chondrichthyes, or alternatively forming a clade with Holocephali
Taxa classified as stem-group Chondrichthyes
Taxa considered too poorly known to place within Chondrichthyes[32]: 147–148
† Extinct |
_Fig._8.jpg)
All holocephalans possess an internal skeleton made up ofcartilage, which in some regions of the body ismineralized to provide additional strength. The mineralized tissues come in two forms in different regions of the skeleton; it may either form a network oftessellations or plates coating the outer surface of the underlying soft cartilage or, in certain regions such as thereproductive organs, lower jaw andvertebrae may form reinforced fibers interwoven with the cartilage termedfibrocartilage.[7]: 26 [41][40] 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 andhexagonal, and they appear more like those of sharks and rays than those of modern chimaeras.[40][41][42] Thespinal cord of holocephalans is supported by a flexiblenerve cord called anotochord. In many taxa close to and within Chimaeriformes this notochord is additionally covered by avertebral column of ossified, disk-shaped cartilaginous rings which are sometimes termed "pseudocentra" or "chordacentra",[3][43][44] and which are different from vertebralcentra in sharks and rays.[45][40] The pseudocentra directly behind to the skull (cervical vertebrae) may be fused into a single unit termed asynarcual in some groups.[7]: 31–32 [40][46] In many Paleozoic holocephalans the vertebral rings were either unmineralized or absent, and the notochord was completely unmineralized. Dorsal (upper) and ventral (lower)processes are present along the vertebral column of holocephalans, which were typically mineralized even in early taxa without preserved vertebral rings. Like other cartilaginous fish, holocephalans lackribs.[3][7]: 31–32 [40]
Thejaw suspension of modern chimaeras and many of their extinct relatives is holostylic (sometimes termed autostylic),[45][48]: 60 [28] meaning that the upper jaws (palatoquadrates) are entirely fused to the skull (neurocranium orchondrocranium) and only the lower jaws (Meckel's cartilages) are able toarticulate.[7]: 26 [18]: 41, 48 [48]: 60 Holostyly has been proposed to have evolved independently in several extinct holocephalan groups due to a similar lifestyle.[7]: 26 [43][47] The ancestral mode of jaw suspension among holocephalans has been termed autodiastyly (alternatively termed unfused holostyly),[3][43][18]: 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][43][49] and embryonic chimaeras show the condition at early stages of development.[49][50] Other forms of jaw suspension, termed hyostyly and amphistyly, are present in modern elasmobranchs and in some potential holocephalan groups.[48]: 60 [49][32]: 140–144 In hyostilic and amphistylic jaw suspension, the upper jaws are disconnected from the cranium. Hyostylic and amphistylic jaws are supported by soft tissue, as well as by a modifiedpharyngeal arch termed thehyoid arch orhyomandibula.[3][49][18]: 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.[51][49][18]: 41, 48 This soft operculum is considered a characteristic feature of the Holocephali,[18]: 48 [45][51] 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.[51][52]: 143–144, [167] Holocephalans typically possess five gill arches,[45][51][18]: 48 although eugeneodonts may have had a small,vestigial sixth gill arch.[53] Thegill arches of iniopterygians, petalodonts and holocephalimorphs are tightly packed and positioned beneath the skull.[17][45][54] Living chimaeras and the extinctHelodus possess twootoliths (inner ear elements).[55]
The fins of holocephalans may include paired pectoral and pelvic fins, either one or twodorsal 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 radialpterygiophores, and by thin rays calledceratotrichia. 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 chimaeras the first dorsal fin is retractable, and is additionally supported by a large fin spine and the synarcuum (cervical vertebrae). The paired fins are supported by thepectoral girdles (scapulocoracoids) andpelvic girdles, respectively. The pectoral girdles are fused along theirventral (lower) point of contact in modern chimaeras but unfused in earlier holocephalans.[3][7]: 32–38 [45] 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.[56][45][29] In the Chondrenchelyiformes and some orodonts all fins were very small and the body shape waseel-like (termedanguilliform).[56][24][20] Members of the Eugeneodontiformes lacked second dorsal fins and anal fins, as well as potentially pelvic fins, and hadfusiform, streamlined bodies.[20]: 79 [53][57]
The holocephalan fossil record consists almost entirely of isolated tooth plates, and these form the basis of study for extinct members.[5][14][45] The teeth of holocephalans are made up of acrown and a base (sometimes called aroot), the anatomies of which vary greatly depending on the specific order.[7]: 16–19 [25]: 109 The subclass is often characterized by teeth which grow slowly and are either shed infrequently or are retained throughout life and are never shed (sometimes termed statodonty),[58][59][60] although this may not apply to all included members.[3][10] In many holocephalans the teeth are stronglyheterodont, meaning that their morphology varies in different regions of the mouth and different groups of teeth (termedtooth families) are specialized for different purposes. In most members of the subclass tooth families are arranged into those at theanterior (front), middle andposterior (rear) of the jaws.[3][7]: 16–17 [61] When applicable the teeth may be further classified as paired, lateral teeth along the margins of the jaws, unpaired symphyseal teeth along the midline,[15][62][63] and in some cases paired, parasymphyseal teeth near the midline axis of the jaw.[28][55] In some groups the bases of some teeth are fused into connected structures called tooth whorls. The dentition may also consists of flat, unfused, plate-like teeth in tight-fitting rows, a configuration termed a "toothpavement" with specific elements termed "pavement teeth". Some derived members possessed only a tooth pavement made up of a few large, specialized plates,[3][25]: 109 [60] while others had pavements in the rear of the mouth and syphyseal tooth whorls at the front.[62][63][55]
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Holocephalan teeth are made up ofdentin,[64][65][66] which in holocephalans is divided into three main forms.[58][67] The anatomical terminology used to describehistology and arrangement of holocephalan dentin is inconsistent,[7]: 18–19 and the same forms have been given different names by different authors.[58][64][60] Most of the tooth consists of softer, vascularized trabecular dentin (in a form referred to by some authors as osteodentin due to its resemblance to bone),[60][64][61]: 480–481 with a thin outer layer of strongerenameloid (also called vitrodentin or pallial dentin)[7]: 19 that is typically missing due to wear or abrasion.[58][61][64] An organization of dentin called tubular dentin (alternatively tubate dentin) is present in the dentitions of most holocephalans, which is a form arranged in vertical tubules and reinforced by additional minerals. In chimaeras these tubules are made up of the unique,hypermineralized tissue whitlockin (also calledkosmin, cosmine, or pleromin) which is composed of the mineralwhitlockite rather thanapatite which makes up the rest of the tooth plate (and the entirety of the teeth in othervertebrates). This is the only known example of whitlockite being naturally used in animal teeth instead of apatite, and it gives these regions of the tooth plates extreme strength.[58][61] Earlier holocephalan teeth lack whitlockin, and their tubules instead consist of an enameloid-like tissue sometimes termed orthotrabeculine. The roots or bases of holocephalan teeth containlamellar tissues, and arevascularized and containblood vessels.[7]: 18–19 [25]: 15 [68]
Eugeneodonts and orodonts both possessed a symphyseal tooth row along the midline of the lower jaw and rows of pavement teeth lining the lateral regions of the mouth,[10][62][69] and some eugeneodonts also had an additional row of symphyseal teeth on the upper jaw.[15][63][70] The eugeneodonts are known primarily from their tooth whorls, which in some species were extremely large, had fused tooth roots that prevented teeth from shedding, and formedlogarithmic spirals.[25]: 117 [63][71] Orodont teeth were less specialized, and the pavement teeth were very similar to those of eugeneodonts, the teeth of early elasmobranchs such ashybodonts, and the tooth plates of cochliodonts and helodonts. Orodontiformes is sometimes considered apolyphyletic (unnatural) grouping of early holocephalans with similar tooth morphology, rather than a trueclade.[20]: 91–94 [25]: 110
The tooth structure of the petalodonts was extremely diverse, but few members are known from more than isolated teeth and the classification of many taxa is uncertain.[22][25]: 133–134 [54] In those with complete dentitions known, most are heterodont (tooth shape varies) while others are homodont (teeth are essentially identical). Petalodont teeth are generally thought to fall into four morphologies:Petalodus-type (incisor-like),Ctenoptychius-type (multi-cusped),Fissodus-type (bifurcated) andJanassa-type (molar-like), multiple of which may have been present in the mouth of a single species.[54][72][73] In the homodont taxonJanassa bituminosa there were many rows of teeth in the mouth which were retained throughout the animal's life and formed a "platform" for new teeth to grow onto.[25]: 134–135 [54] The teeth ofDebeeriiformes (and the dubiousDesmiodontiformes) were similar in morphology to Petalodontiformes and also displayed heterodonty, although they differed in histology and arrangement.[25]: 151–152 [43]
The Holocephalimorpha is a clade which unites the holostylic holocephalans and many taxa with similar tooth plates. Many Holocephalimorphs, such as theCochliodontiformes,Psammodontiformes andCopodontiformes are known primarily or exclusively from their flattened tooth plates,[13][21][74] which in cochliodonts such asCochliodus grew in a distinctive spiral.[10][55] Better known holocephalimorphs such asChondrenchelys had a set of large, crushing, flattened tooth plates attached to the jaws, as well as a set of extra-oral (separate from the jaw) petalodont-like tooth plates in the anterior region of the mouth which may have been attached to the labial (lip) cartilage.[24][75] The teeth of the genusHelodus, the sole member of the order Helodontiformes, are sometimes considered transitional between those of orodont-like (particularly eugeneodont) fishes and the holocephalimorphs, and consist of both rows of separate pavement teeth and teeth fused into fused tooth whorls. Historically the whorls ofHelodus were given the genus namePleuroplax, but they are now known in articulated specimens alongside the separate teeth. In isolation, the unfused teeth ofHelodus are similar to those seen in other groups of holocephalan, and this genus has historically been used as a wastebasket taxon for bead-like holocephalan teeth.[55][60][75]
Modern chimaeras and their closest fossil relatives have only three pairs of highly specialized tooth plates, which are derived from fused tooth families and consist of two pairs in the upper jaw and a single pair in the lower jaw.[45][59] The teeth of chimaeras have specialized structures called tritors which are composed ofwhitlockite, which take the shape of both tubules and rounded structures called ovids within thematrix of the tooth, and pads on the surface of the tooth.[45][58][61] The arrangement of the tritors is a distinguishing characteristic of different chimaera species.[65][76] The upper frontmost tooth plates are incisor-like and protrude from the mouth, giving the mouth a beak-like orrodent-like appearance.[45][61][77]: 142 In recent works, the frontmost upper teeth are referred to asvomerine plates, the rear upper crushing plates aspalatine (or palatal)[78] plates, and the single pair of lower teeth are referred to as themandibular plates.[45][61][64]
The tooth morphology of the iniopterygians differs wildly from that of any other proposed holocephalans, and more closely resembled the dentition of elasmobranchs in histology.[18]: 49 [25][60] Iniopterygian teeth consisted of multiple fused tooth whorls with sharpcusps, arranged symphyseally or parasymphyseally, which were movable and articulated. Some also possessed flattened plates within the mouth, termedbuccal plates, which were distinct from the tooth plates of other holocephalans.[28][79] The jaws of iniopterygians were also lined with small, sharp denticles.[28] The teeth of the possibly holocephalan Symmoriiformes (and the sometimes included Cladoselachiformes) werecladodont (three-cusped), and grew and were replaced in a manner similar to those of sharks,[33][55][60] albeit at a slower rate than in modern sharks.[57]
In adult modern chimaeras,scales are present along thelateral line and, in males, on the reproductive organs, while most of the body is covered in smooth, scaleless skin.[18]: 48 [45]Embryonic and juvenile chimaeras do possess additional scales along their backs, which also last into adulthood inCallorhinchus.[3][7]: 8 [45] Conversely, Paleozoic and Mesozoic chimaeriforms such asSqualoraja andEchinochimaera, as well as members of other extinct orders, have scales covering the entire body throughout life. The scales of holocephalans areplacoid (also termed dermal denticles), meaning they contain apulp cavity, are made up primarily oforthodentin and are coated in an outer layer of hardenameloid.[3][7]: 8–12 [18]: 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 Some holocephalans had armor plates made up of dentin, as well as dentinous spines which protruded from the top of the head, the lower jaw, or the first dorsal fin.[3][7]: 8–12 [13] Armor plating gradually reduced during the evolution of the Chimaeriformes,[13] and modern chimaeras lack any armor and retain only a dorsal fin spine, which in at least some species isvenomous.[23][45]
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 asMenaspis,Deltoptychius,Harpagofututor, and a number of extinct chimaeriforms. Some holocephalans display a distinctive arrangement of ring-shaped scales enclosing the lateral line, which is considered a unique feature of the group.[13][45][80]
Holocephalans are typicallysexually dimorphic. Males may possess up to three forms of external reproductive organs: paired pelvicclaspers used for the transfer ofsperm like those of other cartilaginous fish, paired prepelvic tenaculae, and paired or unpaired frontal or cephalic claspers.[7][18]: 48 [45] In certain Paleozoic species, additional paired spines are sometimes present on the heads of males, and while some authors in the past have considered these structures homologous to cephalic claspers,[13] they are now considered distinct due to their differing histology.[7][14][44] Unlike other cartilaginous fish, chimaeras lack acloaca and instead possess separateanal andurogenital openings.[18]: 48 [45]
In modern chimaera males, the cephalic clasper (also called the cephalic tenaculum) is a tooth-bearing,[81] unpaired cartilaginous structure on the top of the head that is used to grab females during mating.[45][74][77]: 142 The clasper has been hypothesized to be derived from the upper jaw, and is equipped with whorls of hook-like teeth that vary in morphology between species and which are similar to the teeth of sharks. It can be flexed and retracted into a pocket on the head. In some extinct holocephalans such asHelodus simplex,[81]myriacanthoids,Psammodus andTraquairius nudus the males also possessed extremely long cephalic claspers, which in some taxa are as long as the skull androstrum.[7]: 29–30 [44][74] Similar, albeit paired structures are present in the generaHarpagofututor andHarpacanthus, which likely served a similar grabbing purpose. 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.[24][44][82]
In chimaeras and some related groups the males also possess prepelvic tenaculae. These are paired, skeletally supported, retractable structures that protrude in front of the pelvic fins and are used during mating. In chimaeras these are covered in tooth-like denticles.[3][21][45] Similar, hook-like organs (termed tenacular hooks) are known in some iniopterygian males, but these are convergently evolved and nothomologous to those in chimaeras.[28][79]
All living chimaeras reproduce by egg-laying (oviparity). The egg cases of both living chimaeras and their close fossil relatives are proportionally large and composed ofcollagen, and in living chimaeras are laid two at a time.[23][83][84] Chimaera egg cases are characterized by an elongated, fusiform shape and astriated flap, termed aflange or collarette, that protrudes from their outer rim.[7]: 38–39 [84][76] The egg anatomy is unique in each family of chimaeras, allowing for isolated fossilized eggs to be identified to the family level.[45][84][85] Egg cases similar to those of living chimaeras, which are assigned to theoogeneraCrookallia andVetacapsula, are known from theLate Carboniferous (Pennsylvanian) and may have been laid byhelodonts.[84][85] Because of the rarity of egg capsules and presence of isolated fossilizedfetuses from the Early Carboniferous (Mississippian)Bear Gulch Limestone fossil site, it is possible that many early holocephalan groups may have been live-bearing (viviparous orovoviviparous) rather than egg-laying, although it is also that possible that egg cases from many species simply happen to not have been preserved.[56][83][86]
Young juvenile holocephalans have very weakly calcified skeletons and are poorly represented in the fossil record. Fossils of fetal or newbornDelphyodontos, which may have been an early holocephalan, are an exception, as these have uniquely calcified skulls and sharp, hook-like teeth. Based on its anatomy andcoprolites (fossilizedfeces),Delphyodontos may have engaged inintrauterine cannibalism and was live-born (viviparous).[7]: 38–39 [83][84] The chondrenchelyiformHarpagofututor gave birth to extremely large young, which besides their uncalcified skeletons were well-developed and likely matured quickly. FemaleHarpagofututor are known to have contained up to five fetuses from multiple litters, and unlikeDelphyodontos 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 theuterus (histophagy).[86]
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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.[45][23][87] The enigmatic, heavilysquamated fishesStensioella,Pseudopetalichthys andParaplesiobatis, all known from poorly-preserved body fossils from theEarly Devonian ofGermany, have been proposed by researcher Phillippe Janvier to be the earliest holocephalans,[32]: 147, 171 [77]: 76 [88]: 61–64 although they have alternatively been considered unrelatedplacoderms.[18]: 37 [31]: 58 [89] Taxa that are conventionally assumed to be stem-group chondrichthyans such asPucapampella andGladbachus from the Early-Middle Devonian have also occasionally been suggested to be the first holocephalans.[7]: 154 [24][32]: 148 Tooth fossils that are confidently considered to belong to the group first appear during the Middle Devonian (Givetian stage),[33][84][90] althoughmolecular clock andtip dating does suggest an earlier origin. Based on this data, it is proposed that the total-group Holocephali split from the Elasmobranchii between theSilurian and the Early Devonian, with estimates ranging from 421–401 million years ago depending on the methods employed.[35][91][92] By theFamennian stage of theLate Devonian early members of nearly all holocephalan orders had appeared,[33][93] although no skeletons or body fossils are known until the following Carboniferous.[33] The Chimaeriformes may have evolved during theMississippian subperiod of the Carboniferous,[21][30][78] although other estimates suggest a much laterTriassic orJurassic origin of this group.[21][35][77]: 77
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 postcranial structure they have similar tooth and skull anatomy.[24][59] The Chimaeriformes may have alternatively evolved from other fishes within the larger cladeCochliodontimorpha, 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 [23] 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.[44] A modified version of this cladogram was also utilized by Grogan, Lund & Greenfest-Allen (2012) which excludes the Iniopterygiformes from Holocephali (here Euchondrocephali).[21]
| Euchondrocephali (=Holocephali sensu lato) |
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While it is now accepted that Holocephali is the sister group to Elasmobranchii based on both morphology and genetics,[18]: 40–41 [21][92] 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 apolyphyletic (unnatural) grouping.[3][13][23] A particular group of placoderms called thePtyctodontiformes (or Ptyctodontida) were suggested by researchersTor Ørvig andErik Stensiö to be the direct ancestors of Holocephali due to their chimaera-like anatomy.[23][88]: 113 [94] Under this scheme, chimaeras are considered unrelated to any Paleozoic cartilaginous fish, and potentially the MesozoicSqualoraja andmyriacanthids.[14][59] 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 now believed to result fromconvergent evolution.[13][18]: 37 [23] An alternative hypothesis, advocated for by researcherColin Patterson, was that the holocephalans were neither descended from the elasmobranchs nor the ptyctodonts, and instead shared a distant common ancestor with both groups within the larger clade Elasmobranchiomorpha.[7]: 41 [13][23] In light of the description of holocephalantransitional fossils during the 1970s and 1980s an independent origin of Chondrichthyes has been widely discarded,[5][21][88]: 113 and Elasmobranchii and Holocephali are united by the shared anatomy of their pelvic claspers and the tesserae that reinforce their cartilage skeletons.[18]: 40–43 [21][95]: 197–200
Within Chondrichthyes, three contemporary hypotheses are proposed for the evolutionary relationship between Holocephali and groups traditionally considered elasmobranchs.[25]: 25 Richard Lund and Eileen Grogan have suggested a deep split between the elasmobranchs and the holocephalans, with the Holocephali descending from a distant chondrichthyan ancestor with an autodiastylic jaw.[19][21][25]: 25 Following Philippe Janvier's suggestion of close relation, some researchers have instead proposed that ancestral holocephalans were similar in anatomy to cladodonts like the Symmoriiformes andCladoselache and that those groups may be reflective of the ancestral holocephalan state.[25]: 25 [33] Researcher Michal Ginter and coauthors have alternatively suggested that the holocephalans are descended from anOrodus-like animal, and are close relatives of hybodonts, protacrodonts and crown-group elasmobranchs. Ginter's proposal is based on the similar tooth morphology between these four groups, particularly the anatomy of the tooth base or root. This analysis restricts the definition ofcrown-group Chondrichthyes and regards the iniopterygians, Symmoriiformes, and cladoselachians asstem-group Chondrichthyes.[21][25]: 25 [93]
The Bear Gulch Limestone, a unit of theHeath Formation located in the state ofMontana, has been recognized for preserving complete body fossils of fishes dating to theMississippian subperiod.[3][5][96] The majority of fish species known from the site are chondrichthyans, of which more than 40 are early holocephalans.[21][56][88]: 113 Many of the holocephalans known from Bear Gulch belong to lineages that are otherwise known only from teeth or are entirely unrecognized.[5][44][96] These fossils also preserve gut contents,[21][43] color patterns,[43][96] complete life histories,[86] and internal organs,[21][96] allowing for a detailed understanding of the animal's 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.[21][43][56] The environmental conditions and faunal composition of Bear Gulch are believed to be representative of other, less well-knownMississippian marine fossil formations elsewhere in the world.[21][56] The Bear Gulch limestone is designated as aKonservat-Lagerstätte by paleontologists, and forms much of the basis for our modern understanding of early holocephalan evolution and ecology.[3][21][88]: 113 Additional sites, such as theGlencartholm andManse Burn shales ofScotland have also yielded detailed holocephalan fossils from the early Carbonifeorus.[10][21][48]: 174
Both living and fossil holocephalans have aworldwide distribution.[7]: 147–151 [77]: 142 All chimaeras and nearly all extinct holocephalans are known from marine environments, although the helodontHelodus simplex is known from a freshwater deposit.[7]: 40 [31]: 78–83 Living chimaeras are specialized fordeep sea habitats,[77]: 71, 142 with onlyHydrolagus colliei and the three species ofCallorhinchus being regularly found inwaters shallower than 200 meters.[45][92][97] While some authors have suggested holocephalans inhabited deep-water environments since the Paleozoic or Mesozoic,[7]: 40 [21][77]: 77 ancestral chimaeras are alternatively thought to have been shallow-water fishes, and the radiation of the group into deepwater niches instead occurred only during the earlyCenozoic era.[35]
Adaptations for aduropagous diet such as flattened tooth plates and a fused, immobile skull are prevalent among extinct and living holocephalans,[7]: 40 but feeding styles are greatly variable. Modern chimaeras are generalist, opportunist feeders that regularly eat both soft-bodied and shelled prey.[45][61] The genusCallorhinchus is known to eatworms,crustaceans and hard-shelledmollusks, and other chimaeras are also known to prey onsmall fish. Smaller prey are often eaten whole via suction feeding, which is achieved using the muscles of the throat and flexible, cartilaginous lips. The bite forces of chimaeras are weaker than those of durophagous sharks, and chimaeras may rely on their vomerine tooth plates to split and crack shells rather than solely crushing them.[61] Mesozoic chimaeriforms likely had similar feeding strategies to their modern relatives.[7]: 40
During the late Paleozoic, many holocephalan lineages became specialized for feeding styles besides durophagy. Theedestoids, a lineage of Eugeneodontiformes, werepelagicmacropredators which fed on fish andcephalopods.[69][71] The genusEdestus has been proposed to have fed by processing prey between its paired tooth-whorls,[50] while the relatedHelicoprion may have been a specialist hunter ofbelemnoids andammonoids.[71] The poorly-known petalodontMegactenopetalus may have also been a macropredator based on its large, interlocking blade-like tooth plates.[98] The iniopterygianIniopera was a specialized suction feeder that fed in a similar manner to some livingbony fish and aquaticsalamanders.[47] Other iniopterygians have been considered specialists that shredded soft-bodied prey with their mobile tooth whorls.[28][79]
Modern holocephalans are vulnerable to a range of parasitic infections. Among these aretapeworms of the orderGyrocotylidea, which are found only in chimaeras and are thought to be aprimitive,relict group.[99][100] Fossilized tapeworms are also known in the symmoriiformCobelodus, which represent the earliest evidence of parasitism in the group if Symmoriiformes are considered members of Holocephali.[101][102]
Total-group Holocephali has seen a significant decline in diversity since the Paleozoic, and only a single, morphologically-conserved order survives today.[18]: 48–49 [21][35] The holocephalans peaked in diversity during the Mississippian, and they make up the majority of known chondrichthyan taxa from the time.[21][26][103] 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.[26] By the end of the Permian, most holocephalan groups had become extinct,[18]: 48–49 [21][92] although the Eugeneodontiformes remained widespread and diverse for a brief period during theEarly Triassic.[25][53][104] The order Chimaeriformes also continued throughout the Mesozoic, but the subordersMyriacanthoidei and the sometimes includedSqualorajoidei became extinct during theJurassic period,[30][105] leaving only three families in the suborder Chimaeroidea persisting through theCenozoic and into the present.[18]: 51–53 [77]: 76–77 [95]: 200 Today, chimaeras make up as few as 4% of named cartilaginous fish,[106]: 1–4 and consist of 60 known species.[107]
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