Ghost sharks split from true sharks nearly 400 million years ago
Chimaeras—order Chimaeriformes—are cartilaginous cousins nicknamed ghost sharks, ratfish, or rabbitfish. Fossils show they were once diverse and abundant; living forms share a last common ancestor with sharks and rays nearly 400 million years ago. Many haunt depths of hundreds to thousands of metres.
Chimaeras are soft-bodied, with bulky heads and long tapering tails, and can reach 150 centimetres. Their skeletons are entirely cartilage, as in other Chondrichthyes, though some classifications rank them as subclass Holocephali and others as a class of their own. Large pectoral fins generate lift at a leisurely pace, so they seem to fly through the water. In many species the rounded snout stretches into a sensory organ that finds prey by electroreception, and the upper jaw is fused solidly to the skull.
Their teeth set them apart from sharks. Instead of rows of sharp, constantly replaced teeth, they have just six large permanent tooth plates, arranged in three pairs, that keep growing throughout life, ideal for crushing crustaceans, brittle stars and molluscs on the seabed. Carboniferous relatives were more varied, including suction feeders that lived up in open water. Males fertilise internally with claspers, as sharks do, but add retractable appendages called tenacula, including a knobbed rod on the forehead used to grip the female's pectoral fins, and females lay eggs in spindle-shaped leathery cases.
Most live on temperate sea floors, some below 2,000 metres, though Callorhinchus frequents shallower water. No species is rated Endangered, but four are Vulnerable and four Near Threatened, pressed by bycatch, targeted fisheries for pale and dark ghost sharks, trawling and deep-sea mining. A gill parasite, Chimaericola leptogaster, can grow to 50 millimetres. Deep-water surveys and fresh study of museum specimens set off a boom in newly named species during the first decade of this century.
Source: Chimaera
