Holocephali (sometimes spelled Holocephala; Greek for "complete head" in reference to the fusion of upper jaw with the skull) 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
left|thumb|252x252px|French naturalist [[Charles Lucien Bonaparte, who erected the order Holocephali to encompass living chimaeras]]
The first published use of Holocephali (then spelled "Holocephala") 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 (upper jaw) seen in chimaeras. As defined by Müller and Bonaparte, Holocephala encompassed the living genera Chimaera and Callorhinchus. 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 (then Cestracion) sharks, 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 Danish paleontologist Egil Nielsen and British paleontologist 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. a superorder,
Multiple classifications of Holocephali have been proposed by contemporary authors, which differ greatly from one another. 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 (2016), which differs from earlier editions by disbanding Paraselachimorpha.
{| class="wikitable mw-collapsible mw-collapsed collapsible"
! width="450px" |Taxonomy according to the Fifth Edition of Fishes of the World (2016)
- †Order Debeeriiformes
- †Order Eugeneodontiformes
- Superorder Holocephalimorpha (roughly equiv. to Holocephali sensu stricto)
- †Order Chondrenchelyiformes
- †Order Menaspiformes
- †Order Cochliodontiformes
- Order Chimaeriformes (chimaeras)
- Incertae sedis (uncertain placement)
- †Family Harpacanthidae
- †Family Gregoriidae
- Nomen dubium (dubious orders)
- †Order Desmiodontiformes
<small>† Extinct</small>
|-
|}
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 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.
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; Subclass Subterbranchialia
- †Order Iniopterygia (Iniopterygiformes)
- †Family Chondrenchelyidae (equiv. to Chondrenchelyiformes; alternatively placed within Holocephali by Stahl) Symmoriiformes are sometimes regarded as the sister group to Holocephali rather than members of the subclass itself due to their differing morphology. 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.]]
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; it 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 reproductive organs, lower jaw and vertebrae may form reinforced fibers interwoven with the cartilage termed fibrocartilage. 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 vertebral column of ossified, disk-shaped cartilaginous rings which are sometimes termed "pseudocentra" or "chordacentra", and which are different from vertebral centra in sharks and rays. 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 lack ribs.|left]]
The jaw suspension of modern chimaeras and many of their extinct relatives is holostylic (sometimes termed autostylic), 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. Holocephalans typically possess five gill arches, The gill arches of iniopterygians, petalodonts and holocephalimorphs are tightly packed and positioned beneath the skull. Living chimaeras and the extinct Helodus possess two otoliths (inner ear elements).
Teeth
The holocephalan fossil record consists almost entirely of isolated tooth plates, and these form the basis of study for extinct members. although this may not apply to all included members.