Holocephali
Holocephali is a subclass of cartilaginous fish. The only living holocephalans are the three families of chimaeras, but the group also includes many extinct members and was more diverse during the Paleozoic and Mesozoic eras. The earliest known fossils of holocephalans date to the Middle Devonian, and the subclass likely reached its peak diversity during the following Carboniferous Period. Molecular clock studies suggest that holocephalans diverged from their closest relatives, elasmobranchs such as sharks and rays, during the Early Devonian or the Silurian Period.
Extinct holocephalans are typically divided into a number of orders, although the relationships between 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 Cladoselachiformes 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 species which are found worldwide. They all possess broad, wing-like pectoral fins, a single soft cover over the gills, 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 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 and sex organs. Chimaeras are unique among vertebrates 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 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 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 diet of shelled prey, although other species instead likely hunted softer prey like cephalopods or smaller fish. Fossils of holocephalans are most abundant in shallow marine deposits, although an extinct species is known from freshwater environments as well.
Research history and taxonomy
Early research
The first published use of Holocephali was by Swiss naturalist Johannes Müller in 1835, and the group was formally defined and classified by French naturalist Charles Lucien Bonaparte between 1832 and 1841. 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 palatoquadrates seen in chimaeras. As defined by Müller and Bonaparte, Holocephala encompassed the living genera Chimaera and Callorhinchus. Fossils of tooth plates and fin spines from the Mesozoic era were later assigned to Holocephali throughout the 1830s and 1840s. Many additional taxa were described and illustrated by the Swiss naturalist 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 English biologist Richard Owen allied many Paleozoic representatives of the group with living Heterodontus sharks, rather than with chimaeras. By the late 1800s, researchers such as Irish zoologist Fredrick McCoy and British naturalist James William Davis questioned the relationship between these Paleozoic fossils and Heterodontus.File:Dr. A. S. Woodward of the British Museum.jpg|thumb|296x296px|British paleontologist Arthur Smith Woodward, who allied plate-like Paleozoic fish teeth with chimaeras and who erected the order Bradyodonti
During the late 19th and early 20th century, British paleontologist Arthur Smith Woodward recognized many fossil chondrichthyans 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, and indicated that the latter had evolved from Paleozoic ancestors. Later work by the Danish paleontologist Egil Nielsen and British paleontologist James Alan Moy-Thomas expanded the Bradyodonti to include the Eugeneodontiformes and Orodontiformes 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 total-group Holocephali, and its composition remains similar to Holocephali as used by modern authors.
Holocephali is treated as a subclass of the class Chondrichthyes by many modern authors, although the group has alternatively been ranked as an order, a superorder, or as a class. When Charles Lucien Bonaparte first defined Holocephala, he considered it to be an order within the larger subclass Elasmobranchii. Several authors during the 20th century regarded the Holocephali as its own class within the superclass Elasmobranchiomorphi, which also included the classes Selachii, the extinct Arthrodira, and under some definitions the extinct Acanthodii. Holocephali is still sometimes considered a lower taxonomic unit within a larger subclass by some contemporary authors.
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, 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 or, in earlier works, the similarly defined Bradyodonti. However, Paraselachimorpha is now regarded as either paraphyletic or a non-diagnostic wastebasket taxon, including by Lund himself, and the taxa which formerly made up Paraselachimorpha are now considered an evolutionary grade of early-diverging holocephalans. 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.Multiple classifications of Holocephali have been proposed by contemporary authors, which differ greatly from one another. In a 1997 paper, researchers Richard Lund and Eileen Grogan coined the subclass Euchondrocephali to refer to the total group of holocephalans. 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. 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. Joseph S. Nelson, in his reference text Fishes of the World, opted to use the name Holocephali for a clade identical in composition to Euchondrocephali. Below is the taxonomy of total-group Holocephali as defined in the Fifth Edition of Fishes of the World, which differs from earlier editions by disbanding Paraselachimorpha.
| Taxonomy according to the Fifth Edition of Fishes of the World |
; Subclass Holocephali sensu lato
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An alternative classification was proposed by paleontologist Rainer Zangerl in 1979, who considered Holocephali to be a superorder within the newly-erected subclass Subterbranchialia. This group united the chimaera-like taxa, which were distinguished by their holostylic jaw suspension, with the entirely extinct iniopterygians and the Polysentoridae 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 noted.
| Taxonomy according to Zangerl ' |
; Subclass Subterbranchialia
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Some studies have found the shark-like symmoriiformes to be early diverging members of or relatives of Holocephali, although members of this group are traditionally considered to be either relatives of elasmobranchs or to be stem-group chondrichthyans. Symmoriiformes are sometimes regarded as the sister group to Holocephali rather than members of the subclass itself due to their differing morphology. The traditionally-recognized order Cladoselachiformes, 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, they show similarities in the internal anatomy of their crania and both possess rings along their lateral lines, which may suggest a close relationship. French paleontologist Philippe Janvier first suggested a connection between the Holocephali and the Symmoriiformes in his 1996 textbook Early Vertebrates, and the subsequent descriptions of the cladoselachian and symmoriid taxa Maghriboselache and Ferromirum, as well as the redescription of the symmoriiform Dwykaselachus have found additional 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, or sit outside of crown-group Chondrichthyes.
| Taxonomy according to Early Vertebrates |
Subclass Holocephali
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