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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)
Janassa bituminosa
(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. The 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, the elasmobranchs (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 subclass Euchondrocephali (Greek for "true cartilage head") or the obsolete subclass Subterbranchialia (lit. "under gills" in reference to the position of the branchial arches), both of which change the clade's composition. Some recent research has suggested that Cladoselachidae 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 more similar looking to modern sharks than to chimaeras, with upper jaws (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. Holocephali were often highly sexually dimorphic, with males possessing both claspers on the pelvic fins and additional specialized clasping organs on the head. The teeth of most holocephalans consisted 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, spookfish or ghost sharks, 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, holostylic jaws, and six plate-like crushing teeth which are used to feed on benthic invertebrates. 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 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.

Etymology

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The name of the group comes from the Greek roots hólos meaning "whole" and kephalos meaning head.[1] The name is in reference to the complete fusion of the braincase and the palatoquadrates (upper jaw) in chimaeras, a condition termed "holostyly".[2]

Research history

[edit]

Early research

[edit]
Charles Lucien Bonaparte, who erected the order Holocephali to encompass living chimaeras

Holocephali was originally named as the order "Holocephala" by naturalist Charles Lucien Bonaparte in 1832, and was intended to encompass the then-known species of living chimaera (Chimaera monstrosa and Callorhinchus).[3] Bonaparte considered it an order in the subclass Elasmobranchii (also containing the selachii),[3][4]: 43  within the now-obslolete group Plagiostomi (roughly equivalent to the modern class Chondrichthyes).[3][5][6] Fossil taxa, consisting primarily of tooth-plates and fin spines from the Mesozoic, were assigned to Holocephali throughout the 1830s and 1840s.[6][7] 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 tooth and spine genera.[8] Both Agassiz and other influential researchers such as Richard Owen allied many Paleozoic representatives of the group with living Heterodontus (or Cestracion) sharks, rather than with chimaeras.[4]: 43 [6][8] During the late 1800s, researchers such as Fredrick McCoy and James William Davis questioned the association between Paleozoic taxa such as cochliodonts and Heterodontus.[4]: 43 

In 1921, British paleontologist Arthur Smith Woodward recognized many fragmentary fossil fishes as Paleozoic holocephalans, and united them under the newly coined Bradyodonti.[9][10][11] This order, sometimes considered a class or subclass by later publications,[2][11] linked the living chimaeras with Paleozoic taxa known from teeth. 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 the Chimaeriformes, despite these taxa's differences from the group as defined by Woodward. The broadest usage of Woodward's Bradyodonti encompassed an assemblage of fishes roughly equivalent to total-group Holocephali.[2][4]: 41–43 [11]

Bear Gulch Limestone

[edit]
Main article: Bear Gulch Limestone

During the late 20th century, the Bear Gulch Limestone unit of the Health Formation was recognized for preserving detailed, complete body fossils of cartilaginous fishes dating to the Mississippian subperiod of the Carboniferous. Many of these fishes represent early holocephalans belonging to lineages that were previously known only from teeth or were otherwise entirely unrecognized.[2] Holocephalans recovered from Bear Gulch are known to preserve gut contents, color patterns and reproductive behaviors, allowing for a more detailed understanding of their ecology and behavior. This site has significantly reshaped the modern understanding of holocephalan evolution.[12]: 136 

Modern taxa and genetics

[edit]

Description

[edit]

Skull, jaw and gills

[edit]
The skull of a modern chimaera. The palatoquadrate (P.Pt) is entirely fused to to the neurocranium, and only the Meckel's cartilage (M) articulates. In many extinct holocephalans both the palatoquadrate and the Meckel's cartilage articulate

The jaw suspension of modern chimaeriformes and many of their extinct relatives is holostylic, meaning that the upper jaws (palatoquadrates) are entirely fused to the chondrocranium.[13] Holostyly has been proposed to have evolved independently in several extinct holocephalan groups, and is thought to be an adaptation for high bite force. The ancestral mode of jaw suspension among holocephalans has been termed autodiastyly, meaning that the palatoquadrates are not fused to the cranium and instead articulate at two points. Autodiastylic jaws are inflexible, but the palatoquadrates are still fully separated from the cranium. While a number of early holocephalan groups exhibit autodiastyly, and some embryonic chimaeras show the condition at early stages of development, no living fishes have autodiastylic jaws. A form of jaw articulation termed amphistyly is present in some modern elasmobranchs, and a similar condition occurs in some potential holocephalan groups.

Teeth

[edit]

Many holocephalan taxa are defined solely by their teeth, which are highly distinctive to each group. In most holocephalans the teeth consist of an arrangement of broad, flattened plates.

The teeth of several extinct holocephalans lineages (most prominently the eugeneodonts, cochliodonts and potentially iniopterygians) formed "tooth-whorls" where multiple crowns protrude from a single fused base or root. In the eugeneodont Helicoprion these whorls could consist of up to 180 tooth crowns forming a spiral, while in other taxa they consisted of less-specialized "half-whorls".

The tooth histology of the chimaeras consists of three main forms of dentin.

Skin and scales

[edit]

Reproductive organs

[edit]

Eggs and embryos

[edit]
The egg case of a living Cape elephantfish (Callorhinchus capensis)

The egg cases of both living chimaeras and their fossil relatives are large and leathery, and the anatomy of the egg-cases is variable across different families.[2]

Classification

[edit]

While traditionally treated as a subclass, Holocephali has alternatively been ranked as an order,[3] a superorder,[2][14][15] or a class.[2][11][16]

The orders Orodontiformes, Petalodontiformes, Iniopterygiformes, Debeeriiformes, Helodontiformes 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) or, in earlier works, the similarly defined Bradyodonti. However, Paraselachimorpha is today widely 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.[17][13]: 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, has also been abandoned by recent authors and is considered a paraphyletic grade of stem-group holocephalans.[4]: 41–45, Editor's Preface [12]: 136, 144  Some authors continue to use "bradyodont" as an informal descriptor.[18]

Lund & Grogan (1997) coined the subclass Euchondrocephali to refer to the total group of holocephalans (fish more closely related to living holocephalans than to living elasmobranchs). Under this classification scheme, Holocephali would have a much more restricted definition.[19] Other authors have used Holocephali to include all fish more closely related to living chimaeras than to elasmobranchs, a definition equivalent to Euchondrocephali.[13]: 48–49  Below is the taxonomy of total-group Holocephali as defined in the Fifth Edition of Fishes of the World (2016), which follows Lund and Grogan's taxonomy but opts to use the name Holocephali rather than Euchondrocephali.[13]: 48–51 

Taxonomy according to the Fifth Edition of Fishes of the World (2016)[13]: 48–51 
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).[14] 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 autodiastylic jaws.[14]: 23–45 [15][12]: 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.[4][15][23] Later works have regarded Subterbranchialia as a junior synonym of Holocephali sensu lato, and have included the orodonts, eugeneodonts and petalodonts within Holocephali.[13]: 48–49 [23] Zangerl's proposed classification is provided below, with differences between it and the classification used by Stahl (1999) noted.[4][14]

Taxonomy proposed by Zangerl (1979)[14]: 458–459  and Zangerl (1981).[15]: 49–50  Utilized by Stahl (1999)[4]: 44–45 [13]: 48–49 
Subclass Subterbranchialia

Taxa classified within subclass Elasmobranchii sensu Zangerl (1981)[15]: 49–50 [23]: 109 

† Extinct

Symmoriiformes and Cladoselachiformes, such as the Devonian genus Maghriboselache (pictured), have been suggested to belong to Holocephali by some paleontologists[24]

While often considered to either be closely related to elasmobranchs or to be stem-group chondrichthyans,[13]: 45–46 [24][25] some studies have found the shark-like symmoriiformes to be early diverging members of the Holocephali.[26][27][28] Alternatively, Symmoriiformes are sometimes regarded as the sister-group to Holocephali, but are not considered members of the subclass themselves.[12]: 136–141  The traditionally-recognized order Cladoselachiformes, which is sometimes included within Symmoriiformes, is also considered holocephalan under this classification scheme.[24] 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 cartilaginous rings along their lateral lines, which may suggest close relation.[23]: 25 [28][12] Paleontologist Philippe Janvier first suggested a connection between the Holocephali and the Symmoriiformes (then Symmoriida) in his 1996 textbook Early Vertebrates,[23][12]: 138–141  and the subsequent description of Maghriboselache and the redescription of Dwykaselachus have continued to find support for the hypothesis.[24][28] 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),[12] or sit outside of crown-group Chondrichthyes.[12]

Taxonomy proposed by Janvier (1996)[12]: 148–149 
Unranked clade within crown-group Chondrichthyes

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

Taxa classified as stem-group Chondrichthyes

Taxa considered too poorly known to classify[12]: 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)
  • †Order Pseudopetalichthyida (considered tentatively holocephalan by Janvier)

† Extinct

Evolution

[edit]
Stensioella hertzi has sometimes been considered the earliest-known holocephalan. It is alternatively believed to be an early placoderm of indeterminate placement[13]: 37 

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.[30][29] The fragmentary cartilaginous fish Pucapampella and Zamponiopteron from the Early-Middle Devonian of Bolivia have been described by paleontologist Philippe Janvier as potentially being the earliest holocephalans,[12]: 147–148  although later work instead suggests these represent earlier diverging stem-chondrichthyans.[29][31] The enigmatic, heavily squamated fishes Stensioella, Pseudopetalichthys and Paraplesiobatis, all known from poorly preserved body fossils from the Early Devonian of Germany, have also been proposed by Janvier to be early holocephalans due to their superficial similarities to members of Menaspiformes.[12]: 147, 171 [32][33]: 76  They have alternatively been suggested to be unrelated placoderms.[13]: 37 [32]

Fragmentary fossils that are confidently considered holocephalan are known from the Middle Devonian,[4]: 153-154  although molecular clock and tip dating does suggest an earlier origin. Based on these molecular findings, it has been proposed that the total-group Holocephali split from the Elasmobranchii between the Late Silurian and the Early Devonian, with estimates ranging from 421-401 million years ago depending on the methods employed.[34][27] 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.[4]: 80 [18] The Chimaeriformes had evolved by the Mississippian subperiod of the Carboniferous, likely from cochliodont-like ancestors.[35] While the tooth plates of adult chochliodonts and chimaeriforms differ in their morphology, the tooth-plates of juvenile cochliodonts and modern chimaeras are very similar, and researcher Richard Lund has suggested that chimaera tooth-plates evolved via neoteny.[4]: 41 

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

Historically, debate arose as to whether placoderms such as Ctenurella (above) or "selachian"-grade chondrichthyans such as Cladoselache (below) were the ancestors of Holocephali

While it is widely accepted that Holocephali is the sister group to Elasmobranchii,[13]: 43 [4][20] it 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 "selachian" ancestor, making the class Chondrichthyes a true, monophyletic group, or they were descended from some unrelated lineage of placoderms, making Chondrichthyes polyphyletic.

Three contemporary hypotheses have been proposed for the evolutionary relationship between the two chondrichthyan subclasses.[23]

The crown-group holocephalans are thought to have diverged from the Myriacanthidae during the Early Triassic, and the last common ancestor of the living chimaera families diverged during the following Jurassic period.[27]

Ecology

[edit]

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.[4]: 40 [22] Almost all living chimaeras are specialized for deep-sea habitats, with only Hydrolagus colliei and members of the genus Callorhinchus being regularly found in comparatively shallow waters.[4]: 40 [27] Ancestral chimaeras are thought to have been shallow-water fishes, and the radiation of the group into deepwater niches occurred during the early Cenozoic era.[27]

Diet

[edit]

Most holocephalans show adaptations for a duropagous diet.[4]

Predators

[edit]

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.[36][37] Fossilized tapeworms are also known in the symmoriiform Cobelodus, which represent the earliest evidence of parasitism in the group if symmoriiformes are included in Holocephali.[38][39]

References

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  1. ^ "FishBase Glossary". fishbase.mnhn.fr. Retrieved 2025-02-01.
  2. ^ a b c d e f g Lund, Richard; Lund, Richard (1977). New information on the evolution of the Bradyodont Chondrichthyes. Vol. 33. Chicago: Field Museum of Natural History. pp. 522–537. doi:10.5962/bhl.title.5311.
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