Holocephali

Early research , 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. 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. 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. 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. == Anatomy ==

Internal skeleton (A–C), Sibyrhynchus (D), Edaphodon (E–F), and Helodus'' (G), displaying mineralized tessellations 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. Holocephalan teeth are made up of dentin, which in holocephalans is divided into three main forms. The anatomical terminology used to describe histology and arrangement of holocephalan dentin is inconsistent, Eugeneodonts, orodonts and petalodonts (formerly Campodus or Agassizodus), with a symphyseal tooth whorl in the anterior region of the jaw and a lateral tooth pavement in the posterior region 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. In the homodont taxon Janassa 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. 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 jaw. The arrangement of the tritors is a distinguishing characteristic of different chimaera species. The jaws of iniopterygians were also lined with small, sharp denticles. Reproduction Harpagofututor (below) possessed both paired pelvic claspers and paired, antler-like cephalic claspers, both of which are absent in females (above)|260x260px Holocephalans are typically sexually dimorphic. Males may possess up to three forms of external reproductive organs: paired pelvic claspers used for the transfer of sperm like those of other cartilaginous fish, paired prepelvic tenaculae, and paired or unpaired frontal or cephalic claspers. unpaired cartilaginous structure on the top of the head that is used to grab females during mating. 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. 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. Egg cases similar to those of living chimaeras, which are assigned to the oogenera Crookallia and Vetacapsula, are known from the Late Carboniferous (Pennsylvanian) and may have been laid by helodonts. 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 coprolites (fossilized feces), Delphyodontos may have engaged in intrauterine cannibalism and was live-born (viviparous). The chondrenchelyiform Harpagofututor gave birth to extremely large young, which besides their uncalcified skeletons were well-developed and likely matured quickly. Female Harpagofututor are known to have contained up to five fetuses from multiple litters, and unlike Delphyodontos 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 the uterus (histophagy). == Evolution ==

has sometimes been considered the earliest-known holocephalan. The enigmatic, scaled fishes Stensioella, Pseudopetalichthys and Paraplesiobatis, which are 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. They have alternatively been considered unrelated placoderms. 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. By the Famennian stage of the Late Devonian early members of many holocephalan orders had appeared, although no skeletons or body fossils are known until the following Carboniferous period. although other estimates suggest a much later Triassic or Jurassic origin of this group. 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. While the order Chimaeriformes continued throughout the Mesozoic, the suborders Myriacanthoidei and Squalorajoidei (sometimes order Squalorajiformes) became extinct during the Jurassic period, leaving only three families in the suborder Chimaeroidea persisting through the Cenozoic and into the present. and consist of 60 known species. Ancestry Several groups have been proposed as sister taxa or potential 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. Under this scheme, chimaeras are considered unrelated to any Paleozoic cartilaginous fish, and potentially also unrelated to the Mesozoic Squaloraja and myriacanthids. Within Chondrichthyes, three contemporary hypotheses are proposed for the evolutionary relationship between Holocephali and groups traditionally considered elasmobranchs. 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. 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 and Cladoselache and that those groups may be reflective of the ancestral holocephalan state. Researcher Michal Ginter and coauthors have alternatively suggested that the holocephalans are descended from an Orodus-like animal, and are close relatives of hybodonts, protacrodonts and crown-group elasmobranchs. Ginter and coauthor'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 of crown-group Chondrichthyes and regards the iniopterygians, Symmoriiformes, and cladoselachians as stem-group Chondrichthyes. == 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. The majority of fish species known from the site are chondrichthyans, of which more than 40 are early holocephalans. While some authors have suggested holocephalans inhabited deep-water environments since the Paleozoic or Mesozoic, Diet Adaptations for a duropagous diet such as flattened tooth plates and a fused, immobile skull are prevalent among extinct and living holocephalans, The genus Edestus has been proposed to have fed by processing prey between its paired tooth-whorls, The iniopterygian Iniopera was a specialized suction feeder that fed in a similar manner to some living bony fish and aquatic salamanders. 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. == Notes ==