Chimaera

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. In modern chimaeras the mineralized tessellations are irregularly shaped, smaller and less defined than in other cartilaginous fish, which has historically resulted in confusion as to whether these structures were present. In many extinct holocephalans the tessellations are large and hexagonal, and they appear more like those of sharks and rays than those of modern chimaeras.

Both living and fossil holocephalans have a worldwide distribution. All chimaeras and nearly all extinct holocephalans are known from marine environments, although the helodont Helodus simplex is known from a freshwater deposit. Living chimaeras are specialized for deep sea habitats, with only Hydrolagus colliei and the three species of Callorhinchus being regularly found in waters shallower than 200 meters. While some authors have suggested holocephalans inhabited deep-water environments since the Paleozoic or Mesozoic, ancestral chimaeras are alternatively thought to have been shallow-water fishes, and the radiation of the group into deepwater niches instead occurred only during the early Cenozoic era.

Adaptations for a duropagous diet such as flattened tooth plates and a fused, immobile skull are prevalent among extinct and living holocephalans, but feeding styles are greatly variable. Modern chimaeras are generalist, opportunist feeders that regularly eat both soft-bodied and shelled prey. The genus Callorhinchus is known to eat worms, crustaceans and hard-shelled mollusks, and other chimaeras are also known to prey on small fish. Smaller prey are often eaten whole via suction feeding, which is achieved using the muscles of the throat and flexible, cartilaginous lips. The bite forces of chimaeras are weaker than those of durophagous sharks, and chimaeras may rely on their vomerine tooth plates to split and crack shells rather than solely crushing them. Mesozoic chimaeriforms likely had similar feeding strategies to their modern relatives.

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. Many of the holocephalans known from Bear Gulch belong to lineages that are otherwise known only from teeth or are entirely unrecognized. These fossils also preserve gut contents, color patterns, complete life histories, and internal organs, allowing for a detailed understanding of the animal's ecology and behavior. The site preserves an exceptional diversity of species, and is considered the most completely preserved Paleozoic cartilaginous fish fauna known.

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. In certain Paleozoic species, additional paired spines are sometimes present on the heads of males, and while some authors in the past have considered these structures homologous to cephalic claspers, they are now considered distinct due to their differing histology. Unlike other cartilaginous fish, chimaeras lack a cloaca and instead possess separate anal and urogenital openings.