Holocephali

Holocephali (Sometimes spelled Holocephala; Greek for "complete head" in reference to the fusion of upper jaw with the rest of the skull) is a subclass of cartilaginous fish. While the only living holocephalans are three families within a single order which together are commonly known as chimaeras, the group includes many extinct orders and was far more diverse during the Paleozoic and Mesozoic eras. The earliest known fossils of holocephalans 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 subclass diverged from its closest relatives, elasmobranchs such as sharks and rays, during the Early Devonian or 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 in some works having many of its members are arranged in the now obsolete groups Paraselachimorpha and Bradyodonti. Some recent research has suggested that the orders Cladoselachiformes 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, which form much of the basis of their classificaion.

Many early holocephalans had skulls and bodies which were unlike modern chimaeras, with upper jaws that were not fused to the rest of the skull and separate, shark-like teeth. The bodies of most holocephalans were covered in tooth-like scales termed dermal denticles, which in many Paleozoic and Mesozoic members were sometimes fused into armor plates. Holocephalans are sexually dimorphic, with males possessing both claspers on the pelvic fins and additional specialized clasping organs on the head and before the pelvic fins. The teeth of most holocephalans consist of slow-growing plates which suggest a durophagous lifestyle, and in some groups these plates were specialized into fused structures termed "tooth whorls" or arranged into crushing surfaces termed "tooth pavements". Fossils of holocephalans are most abundant in shallow marine deposits, although certain extinct species are known from freshwater environments as well.

Chimaeras, the only surviving holocephalans, include mostly deep-sea species which are found worldwide. They all possess broad, wing-like pectoral fins, opercular covers over the gills, fused skulls and upper jaws, and six plate-like crushing teeth. 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 lacks scales or armor plates, with the exception of scales on the sensory and sex organs, and the 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 Carbonifeorus period.