Holocephali (sometimes spelled Holocephala; Greek for "complete head" in reference to the fusion of Palatoquadrate with the rest of the skull) is a subclass of Chondrichthyes. The only living holocephalans are three families which together are known commonly as , but the group also includes many extinct members and was more diverse during the Paleozoic and Mesozoic eras. The earliest known of holocephalans date to the Middle Devonian Geochronology, and the subclass likely reached its peak diversity during the following Carboniferous Period. Molecular clock studies suggest that holocephalans Speciation from their closest relatives, Elasmobranchii such as and Batomorphi, during the Early Devonian or the Silurian 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 inclusive clade within the larger subclasses Euchondrocephali or Subterbranchialia and with its members spread into the now obsolete groups Paraselachimorpha or Bradyodonti. Per these classification schemes, the name Holocephali is used only for chimaeras and their closest relatives. Recent research has suggested that the orders Cladoselachidae and Symmoriiformes, 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 or dorsal fin spines, which form much of the basis of their classification.
Chimaeras, the only surviving holocephalans, include mostly Deep-sea fish species which are found worldwide. They all possess broad, wing-like pectoral fins, a single soft cover over the Gill, Palatoquadrate 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 cephalic clasper on the head. Females reproduce by laying large, leathery egg cases. The skin of living chimaeras lacks scales or armor plates, with the exception of tooth-like scales termed dermal denticles on the Sensory organ and . Chimaeras are unique among Vertebrate in that their tooth plates contain organs called tritors, which are made of the mineral whitlockite. Fossils similar to living chimaeras are known as far back as the Early CarboniferousGeochronology
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 and Mesozoic members were sometimes fused into armor plates. Many extinct holocephalans were sexually 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 a Durophagy, although other species instead likely hunted softer prey like Cephalopod or smaller fish. Fossils of holocephalans are most abundant in Photic zone Marine life deposits, although an extinct species is known from Freshwater fish environments as well.
Research history and classification
Early research
Holocephali was first proposed as "Holocephala" by Johannes Müller, and was formally described by naturalist Charles Lucien Bonaparte.
The name of the group comes from the Greek roots
hólos meaning "whole" or "complete" and
kephalos meaning head, and is in reference to the complete fusion of the braincase and the
(upper jaw) seen in chimaeras.
As defined by Müller and Bonaparte, Holocephala encompassed the living genera
Chimaera and
Callorhinchus.
Fossils of tooth plates and
Fin spine from the
Mesozoic era were later assigned to Holocephali throughout the 1830s and 1840s.
Many additional
Taxon were described and illustrated by the
Natural history Louis Agassiz between 1833 and 1843
, including a number of
Paleozoic era tooth and spine genera now considered to belong to Holocephali
. Both Agassiz and other influential researchers such as
Richard Owen allied many Paleozoic representatives of the group with living
Bullhead shark (or
Cestracion) sharks,
rather than with chimaeras.
By the late 1800s, researchers such as
Frederick McCoy and James William Davis questioned the relationship between these Paleozoic fossils and
Heterodontus.
During the late 19th and early 20th century, British paleontologist Arthur Smith Woodward recognized many fossil
Chondrichthyes as forming a distinct taxonomic group, and in 1921 named this group
Bradyodonti.
Woodward considered Bradyodonti an order, although it was sometimes considered a class or subclass by later publications.
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.
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)
as well as modern chimaeras, despite these taxa's differences from the group as defined by Woodward.
The broadest usage of Bradyodonti is roughly equivalent to
Stem group Holocephali,
and its composition remains similar to Holocephali as used by modern authors.
While treated as a subclass of the class Chondrichthyes by some authors (e.g. Joseph Nelson),
Recent classifications
The interrelationships of extinct holocephalan orders have been characterized as difficult to define and subject to change, due in part to limited data.
The orders
Orodontiformes, Petalodontiformes, Iniopterygiformes,
Debeeriiformes,
Helodus and Eugeneodontiformes were formerly united under the superorder Paraselachimorpha by researcher Richard Lund.
The paraselachimorphs were defined as a
sister 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 either
Paraphyly 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.
Likewise, the historically significant order Bradyodonti, consisting variously of taxa now placed in Petalodontiformes, Orodontiformes, Eugeneodontiformes, Helodontiformes,
Menaspiformes, Cochliodontiformes,
Copodontiformes,
Psammodontidae, Chondrenchelyformes, and
Chimaera,
has also been abandoned by recent authors and is considered a paraphyletic grade.
Multiple classifications of Holocephali have been proposed by contemporary authors, which differ greatly from one another.
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- 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).
This group united the chimaera-like taxa, which were distinguished by their holostylic jaw suspension, with the entirely extinct iniopterygians and the
which possessed at least in some cases an unfused upper jaw.
' 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 before.
Later works have regarded Subterbranchialia as a potentially paraphyletic wastebasket taxon of chondrichthyans with poorly defined relationships
, and others have re-included the orodonts, eugeneodonts and petalodonts within Holocephali.
Zangerl's proposed classification is provided below, with differences between it and the classification used by Stahl (1999) noted.
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- Subclass Subterbranchialia
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†Order Iniopterygia (Iniopterygiformes)
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†Family Chondrenchelyidae (equiv. to Chondrenchelyiformes; alternatively placed within Holocephali by Stahl)
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†Family
-
Superorder Holocephali
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†Suborder Helodus (equiv. to Helodontiformes)
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Order sensu Zangerl, 1981
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†Family Psammodontidae (equiv. to Psammodontiformes)
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†Family Copodontidae (equiv. to Copodontiformes)
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†Family Menaspiformes (equiv. to Menaspiformes)
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†Suborder Cochliodontoidei (equiv. to Cochliodontiformes)
-
Suborder Chimaera (equiv. to Chimaeriformes; chimaeras)
-
Bradyodontida incertae sedis
Taxa classified within subclass Elasmobranchii sensu Zangerl (1981)
-
†Order Eugeneodontida or Edestiformes (Eugeneodontiformes)
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†Order Orodontiformes (Orodontiformes)
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†Order Petalodontida (Petalodontiformes)
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†Order Desmiodus (Desmiodontiformes)
† Extinct |
|
Some studies have found the shark-like
symmoriiformes to be early diverging members of the Holocephali,
although this group is more often considered either to be related to elasmobranchs or to be
stem-group chondrichthyans.
Alternatively, Symmoriiformes are sometimes regarded as the sister group to Holocephali rather than members of the subclass itself due to differing morphology.
The traditionally-recognized order
Cladoselachidae, which is sometimes included within Symmoriiformes, may also be considered holocephalan under this classification scheme.
While the anatomy of the jaws and teeth differs dramatically between Symmoriiformes and typical holocephalans, these show similarities in the internal anatomy of their
Skull and both possess rings along their lateral lines, which may suggest close relation.
Paleontologist
Philippe Janvier first suggested a connection between the Holocephali and the Symmoriiformes (then Symmoriida) in his 1996 textbook
Early Vertebrates,
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.
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.
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- Unranked clade within crown-group Chondrichthyes
-
†Order Symmoriiformes (Symmoriiformes; considered part of total-group Holocephali by later authors)
-
Subclass Holocephali
-
Family Helodus (Helodontiformes)
-
Family Chondrenchelyidae (Chondrenchelyiformes)
-
Family Menaspiformes (Menaspiformes)
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Family Cochliodontidae (Cochliodontiformes)
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Family Echinochimaera (Chimaeriformes)
-
Suborder Chimaera (Chimaeriformes)
Taxa classified as incertae sedis within crown-group Chondrichthyes, or alternatively forming a clade with Holocephali
-
†Order Iniopterygia (Iniopterygiformes)
Taxa classified as stem-group Chondrichthyes
-
†Order Petalodontida (Petalodontiformes)
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†Order Eugeneodontida (Eugeneodontiformes; also termed Edestidae sensu lato by Janvier)
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†Order Cladoselachidae (Cladoselachiformes; included within Symmoriiformes by later authors)
Taxa considered too poorly known to place within Chondrichthyes
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†Order Orodontiformes (Orodontiformes)
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†Genus Polysentor (Polysentoridae)
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†Genus Zamponipteron (considered tentatively holocephalan and potentially associated with Pucapampella by Janvier)
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†Genus Pucapampella (considered tentatively holocephalan and potentially associated with Zamponipteron by Janvier)
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†Order Stensioella (considered tentatively holocephalan by Janvier; alternatively considered a placoderm)
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†Order Pseudopetalichthyida (considered tentatively holocephalan by Janvier; alternatively considered a placoderm)
† Extinct |
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Anatomy
Internal skeleton
All holocephalans possess an internal skeleton made up of
cartilage, which in some regions of the body is mineralized to provide additional strength. The mineralized tissues come in two forms in different regions of the skeleton. They may either form a network of tessellations or plates coating the outer surface of the underlying soft cartilage or, in certain regions such as the
Clasper, lower jaw and
may form reinforced fibers interwoven with the cartilage termed
fibrocartilage.
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
, and they appear morphologically more like those of sharks and rays than those of modern chimaeras.
The
spinal cord of holocephalans is supported by a flexible
nerve cord called a
notochord. In many taxa close to and within Chimaeriformes this notochord is additionally covered by a
Spinal column of ossified, disk-shaped cartilaginous rings which are sometimes termed "pseudocentra" or "chordacentra",
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and which are different from vertebral
Vertebral centra in sharks and rays.
The pseudocentra directly behind to the skull (cervical vertebrae) may be fused into a single unit termed a
synarcual in some groups.
In many Paleozoic holocephalans the vertebral rings were either unmineralized or absent, and the notochord was not mineralized. 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 lack
Rib cage.
Skull, jaw and gills
The
jaw suspension of modern chimaeras and many of their extinct relatives is holostylic (sometimes termed autostylic)
, meaning that the upper jaws (
) are entirely fused to the skull (
neurocranium or
chondrocranium) and only the lower jaws (Meckel's cartilages) are able to
Joint.
Holostyly has been proposed to have evolved independently in several extinct holocephalan groups due to a similar lifestyle.
The ancestral mode of jaw suspension among holocephalans has been termed autodiastyly (alternatively termed unfused holostyly),
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,
and embryonic chimaeras show the condition at early stages of development.
Other forms of jaw suspension, termed hyostyly and amphistyly, are present in modern elasmobranchs and in some potential holocephalan groups.
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 modified
pharyngeal arch termed the
Pharyngeal arch or
hyomandibula.
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.
This soft operculum is considered a characteristic feature of the Holocephali,
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.
Holocephalans typically possess five gill arches,
although eugeneodonts may have had a small,
Vestigiality sixth gill arch.
The
Branchial arch of iniopterygians, petalodonts and holocephalimorphs are tightly packed and positioned beneath the skull.
Living chimaeras and the extinct
Helodus possess two
(
inner ear elements).
Fins
The fins of holocephalans may include paired pectoral and pelvic fins, either one or two
, 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
Dorsal fin, and by thin rays called
ceratotrichia. 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 the
Shoulder girdle (
) and
pelvic girdles, respectively. The pectoral girdles are fused along their
ventral (lower) point of contact in modern chimaeras but unfused in earlier holocephalans.
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.
In the Chondrenchelyiformes and some orodonts all fins were very small and the body shape was
eel-like (termed
Fish locomotion).
Members of the Eugeneodontiformes lacked second dorsal fins and anal fins, as well as potentially pelvic fins, and had
fusiform, streamlined bodies.
Teeth
The holocephalan fossil record consists almost entirely of isolated tooth plates, and these form the basis of study for extinct members.
The teeth of holocephalans are made up of a crown and a base (sometimes called a
Tooth root), the anatomies of which vary greatly depending on the specific order.
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),
although this may not apply to all included members.
In many holocephalans the teeth are strongly
heterodont, meaning that their morphology varies in different regions of the mouth and different groups of teeth (termed
Dental anatomy) are specialized for different purposes. In most members of the subclass tooth families are arranged into those at the
anterior (front), middle and posterior (rear) of the jaws.
When applicable the teeth may be further classified as paired, lateral teeth along the margins of the jaws, unpaired symphyseal teeth along the midline,
and in some cases paired, parasymphyseal teeth near the midline axis of the jaw.
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 "tooth pavement" with specific elements termed "pavement teeth". Some derived members possessed only a tooth pavement made up of a few large, specialized plates,
while others had pavements in the rear of the mouth and syphyseal tooth whorls at the front.
Holocephalan teeth are made up of dentin,
Eugeneodonts, orodonts and petalodonts
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,
and some eugeneodonts also had an additional row of symphyseal teeth on the upper jaw.
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 formed logarithmic spirals.
Orodont teeth were less specialized, and the pavement teeth were very similar to those of eugeneodonts, the teeth of early elasmobranchs such as
Hybodontiformes, and the tooth plates of cochliodonts and helodonts. Orodontiformes is sometimes considered a
Polyphyly (unnatural) grouping of early holocephalans with similar tooth morphology, rather than a true
clade.
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.
Holocephalimorphs and Helodus
The Holocephalimorpha is a clade which unites the holostylic holocephalans and many taxa with similar tooth plates. Many Holocephalimorphs, such as the Cochliodontiformes,
Psammodontidae and
Copodontidae are known primarily or exclusively from their flattened tooth plates,
which in cochliodonts such as
Cochliodus grew in a distinctive spiral pattern.
Better known holocephalimorphs such as
Chondrenchelys 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.
The teeth of the genus
Helodus, 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 of
Helodus were given the genus name
Pleuroplax, but they are now known in articulated specimens alongside the separate teeth. In isolation, the pavement-teeth of Helodus 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.
Chimaeras
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.
The teeth of chimaeras have specialized whitlockin-composed structures called tritors, which variously take the shape of tubules and rounded structures (called ovids) within the matrix of the tooth, and pads on the surface of the tooth.
The arrangement of the tritors is a distinguishing characteristic of different chimaera species.
The upper frontmost tooth plates are incisor-like and protrude from the mouth, giving the mouth a beak-like or
rodent-like appearance.
In recent works, the frontmost upper teeth are referred to as
plates, the rear upper crushing plates as
Palate (or palatal)
plates, and the single pair of lower teeth are referred to as the
Mandible plates.
Iniopterygians and Symmoriiformes
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.
Iniopterygian teeth consisted of multiple fused tooth whorls with sharp cusps, arranged symphyseally or parasymphyseally, which were movable and articulated. Some also possessed flattened plates within the mouth, termed
Cheek plates, which were distinct from the tooth plates of other holocephalans.
The jaws of iniopterygians were also lined with small, sharp denticles.
The teeth of the possibly holocephalan Symmoriiformes (and the sometimes included Cladoselachiformes) were
cladodont (three-cusped), and grew and were replaced in a manner similar to those of sharks.
However, the rate of replacement was significantly slower than in sharks.
Skin and external skeleton
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.
and juvenile chimaeras do possess additional scales along their backs, which only last into adulthood in
Callorhinchus.
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
Fish scale (also termed dermal denticles), meaning they contain a pulp cavity, are made up primarily of
dentin and are coated in an outer layer of hard
enameloid.
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.
Some holocephalans had armor plates made up of dentin and spines which protruded from the top of the head, the lower jaw, or the first dorsal fin.
Armor plating gradually reduced during the evolution of the Chimaeriformes,
and modern chimaeras lack any armor and retain only a dorsal fin spine, which in at least some species is
and extremely painful.
Sensory organs
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. Some holocephalans display a distinctive arrangement of ring-shaped scales enclosing the lateral line, which is considered a unique feature of the group.
Reproduction
Holocephalans are typically sexually dimorphic. Males may possess up to three sets of external reproductive organs: paired pelvic
like those of other cartilaginous fish, paired prepelvic
Tenaculum, and paired or unpaired frontal or cephalic claspers.
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,
they are now considered distinct due to their differing histology.
Unlike other cartilaginous fish, chimaeras lack a
cloaca and instead possess separate
Anus and urogenital openings.
Clasping organs
In modern chimaera males, the cephalic clasper (also called the cephalic tenaculum) is a tooth-bearing,
unpaired cartilaginous structure on the top of the head that is used to grab females during mating.
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 as
Helodus,
,
Psammodus and
Traquairius the males also possessed extremely long cephalic claspers, which in some taxa are as long as the skull and rostrum.
Similar, albeit paired structures are present in the genera
Harpagofututor and
Harpacanthus, 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.
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.
Eggs and development
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 of
collagen, and in living chimaeras are laid two at a time.
Chimaera egg cases are characterized by an elongated, fusiform shape and a
striated flap, termed a
flange or collarette, that protrudes from their outer rim.
The egg anatomy is unique in each family of chimaeras, allowing for isolated fossilized eggs to be identified to the family level.
Egg cases similar to those of living chimaeras, which are assigned to the
Egg fossil Crookallia and
Vetacapsula, are known from the
Carboniferous (Pennsylvanian) and may have been laid by
Helodus.
Because of the rarity of egg capsules and presence of isolated fossilized
from the Early Carboniferous (Mississippian) Bear Gulch Limestone fossil site, it is possible that many early holocephalan groups may have been live-bearing (
Viviparity or
Ovoviviparity) rather than egg-laying, although it is also that possible that egg cases from many species simply happen to not have been preserved.
Young juvenile holocephalans have very weakly calcified skeletons and are poorly represented in the fossil record. Fossils of fetal or newborn Delphyodontos, 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 and (fossilized feces), Delphyodontos may have engaged in intrauterine cannibalism and was live-born (Viviparity).
Evolution
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.
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,
although they have alternatively been considered unrelated
.
Taxa that are conventionally assumed to be stem-group chondrichthyans such as
Pucapampella and
Gladbachus from the Early-
Middle Devonian have also occasionally been suggested to be the first holocephalans.
Tooth fossils that are confidently considered to belong to the group first appear during the Middle Devonian (
Givetian),
although
molecular clock and
tip dating does suggest an earlier origin. Based on this 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.
By the
Famennian of the
Devonian early members of nearly all holocephalan orders had appeared,
although no skeletons or body fossils are known until the following Carboniferous.
The Chimaeriformes may have evolved during the Mississippian subperiod of the Carboniferous,
although other estimates suggest a much later
Triassic or
Jurassic origin of this group.
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.
The Chimaeriformes may have alternatively evolved from other fishes 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.
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.
A modified version of this cladogram was also utilized by Grogan, Lund & Greenfest-Allen (2012) which excludes the Iniopterygiformes from Holocephali (here Euchondrocephali).
Ancestry
While it is now accepted that Holocephali is the sister group to Elasmobranchii based on both morphology and genetics,
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,
Monophyly (natural) group, or they were descended from some unrelated lineage of placoderms, making Chondrichthyes a
Polyphyly (unnatural) grouping.
A particular group of placoderms called the
Ptyctodontida (or Ptyctodontida) were suggested by researchers Tor Ørvig and Erik Stensiö to be the direct ancestors of Holocephali due to their chimaera-like anatomy.
Under this scheme, chimaeras are considered unrelated to any Paleozoic cartilaginous fish, and potentially the Mesozoic
Squaloraja and
Myriacanthidae.
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 from convergent evolution.
An alternative hypothesis, advocated for by researcher Colin 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.
In light of the description of holocephalan transitional fossils during the 1970s and 1980s an independent origin of Chondrichthyes has been widely discarded,
and Elasmobranchii and Holocephali are united by the shared anatomy of their pelvic claspers and the tesserae that reinforce their cartilage skeletons.
Within Chondrichthyes, three contemporary hypotheses are proposed for the evolutionary relationship between Holocephali and groups traditionally considered elasmobranchs.
Ecology
Bear Gulch Limestone
The Bear Gulch Limestone, a unit of the
Heath Formation located in the state of
Montana, has been recognized for preserving complete body fossils of fishes dating to the Mississippian subperiod of the
Carboniferous.
The majority of fish species known from the site are chondrichthyans, of which more than 40 are early holocephalans.
Many of the holocephalans known from Bear Gulch belong to lineages that are otherwise known only from teeth or are entirely unrecognized.
These fossils also preserve gut contents,
color patterns,
complete life histories,
and internal organs,
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.
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.
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.
Additional sites, such as the
Glencartholm and Manse Burn shales of
Scotland have also yielded detailed holocephalan fossils from the early Carbonifeorus.
Habitats
Both living and fossil holocephalans have a worldwide distribution.
All chimaeras and nearly all extinct holocephalans are known from marine environments, although the helodont
Helodus is known from a freshwater deposit.
Living chimaeras are specialized for
deep sea habitats,
with only
Hydrolagus colliei and the three species of
Callorhinchus being regularly found in
Epipelagic zone.
While some authors have suggested holocephalans inhabited deep-water environments since the Paleozoic or Mesozoic,
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 early
Cenozoic era.
Diet
Adaptations for a
Durophagy diet such as flattened tooth plates and a fused, immobile skull are prevalent among extinct and living holocephalans,
but feeding styles are greatly variable. Modern chimaeras are generalist, opportunist feeders that regularly eat both soft-bodied and shelled prey.
The genus
Callorhinchus is known to eat
,
and hard-shelled
Mollusca, and other chimaeras are also known to prey on
Forage 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.
Mesozoic chimaeriforms likely had similar feeding strategies to their modern relatives.
During the late Paleozoic, many holocephalan lineages became specialized for feeding styles besides durophagy. The edestoids, a lineage of Eugeneodontiformes, were Pelagic fish Apex predator which fed on fish and .
Parasites
Modern holocephalans are vulnerable to a range of parasitic infections. Among these are
Cestoda of the order
Gyrocotylidea, which are found only in chimaeras and are thought to be a primitive, relict group.
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.
Decline
Total-group Holocephali has seen a significant decline in diversity since the Paleozoic, and only a single, morphologically-conserved order survives today.
The holocephalans peaked in diversity during the Mississippian, and they make up the majority of known chondrichthyan taxa from the time.
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.
By the end of the Permian, most holocephalan groups had become extinct,
although the Eugeneodontiformes remained widespread and diverse for a brief period during the
Early Triassic.
The order Chimaeriformes also continued throughout the Mesozoic, but the suborders
Myriacanthidae and the sometimes included
Squaloraja became extinct during the
Jurassic period,
leaving only three families in the suborder Chimaeroidea persisting through the
Cenozoic and into the present.
Today, chimaeras make up as little as 4% of named cartilaginous fish,
and consist of 56 known species.
See also
-
Evolution of fish
-
List of chimaeras
-
List of prehistoric cartilaginous fish
External links
