Dunkleosteus

Dunkleosteus (meaning "Dunkle's Bone") is a prehistoric fish, and one of the largest arthrodire placoderms ever to have lived. This carnivorous predator lived during the Late Devonian period, about 380-360 million years ago.

No other placoderms rival its size: it is considered to have been one of the fiercest marine predators. This hunter, measuring 6 m (20 ft) and one ton, was a hypercarnivorous apex predator. Few other placoderms, save, perhaps, its contemporary, Titanichthys, rivaled Dunkleosteus in size.

Dunkleosteus is member of the pachyosteomorph arthrodires, and is more specifically usually placed in the family Dinichthyidae, a family composed mostly of large, carnivorous arthrodires like Gorgonichthys. Anderson (2009)[1] suggests that because of its primitive jaw structure Dunkleosteus should be placed outside the family Dinichthyidae, perhaps close to the base of the clade Pachyosteomorpha, near Eastmanosteus, but this idea has yet to be tested.

New studies have revealed several features in both its food and biomechanics as its ecology and physiology. The Placodermi first appeared in the Silurian, and the group became extinct during the transition from the Devonian to the Carboniferous, leaving no descendants.

In recent decades, the Dunkleosteus has achieved recognition in popular culture, with a large number of specimens on display, and notable appearances in entertainment media.

Numerous fossils of some species have been found in North America, Poland, Belgium, and Morocco.

Description
Due to its heavily armoured nature, Dunkleosteus was likely a relatively slow (albeit powerful) swimmer. It is presumed to have dwelled in diverse zones of inshore waters, although it is unknown whether or not it was also somewhat pelagic, that is, swimming freely in open ocean. Fossilization tends to have preserved only the especially armoured frontal sections of specimens, and thus it is uncertain what exactly the hind sections of this ancient fish were like. As such, the reconstructions of the hindquarters are often based on related, albeit smaller, arthrodires.

The most famous specimens of Dunkleosteus are displayed at the Cleveland Museum of Natural History. Others are displayed at the New York Museum of Natural History and in the Queensland Museum in Brisbane, Queensland.

Instead of teeth, Dunkleosteus possessed two pairs of sharp gnathal plates which formed a beak. After studying a biomechanical model of the fish's jaws, scientists at the Field Museum of Natural History and the University of Chicago concluded that Dunkleosteus had the most powerful bite of any fish, well ahead of sharks, including the great white shark. Dunkleosteus could concentrate a pressure of up to 8,000 pounds-force per square inch (55 megapascals) at the tip of its mouth, effectively placing Dunkleosteus in the league of Tyrannosaurus rex and modern crocodiles as having the most powerful known bite. A 2007 study of the biomechanics of D.terreli, interpreted fossil jaws to build a computer model of the fish's bones and muscles and determined that it could tear apart its food with a force of 4400 newtons at the tip of the jaws, and had a maximum bite force of more than 5300N at the back of the dental plates.[2]Dunkleosteus could also open its mouth in one-fiftieth of a second, which would have caused a powerful suction that pulled the prey into its mouth, a food-capture technique reinvented by many of the most advanced teleost fishes today.

The discovery of Dunkleosteus armor with unhealed bite marks strongly suggest that they cannibalized each other when the opportunity arose. Frequently, fossils of Dunkleosteus are found with boluses of fish bones, semi-digested and partially eaten remains of other fish. As a result, the fossil record indicates that it may have routinely regurgitated prey bones rather than digesting them.

Some have suggested that placoderms such as Dunkleosteus were outcompeted by the smaller, swifter fishes, such as the early shark Cladoselache. However, others have pointed out that this assessment fails to take into account that predatory placoderms would have inhabited different ecological niches than the early sharks during the Devonian period.

Dunkleosteus, like most other placoderms, may have also been among the first vertebrates to internalize egg fertilization, akin to the modern process of vivipary as seen in some modern sharks. The first recorded sexual dimorphism appeared in the middle-Devonian ptyctodontid placoderm Rhamphodopsis, which had external clasping organs on males that are strong indicators of intromittent sexual intercourse and internal fertilization. Claspers are modified structures at the base of the pelvic fins which transmit sperm directly inside the female. Females had wide pelvic basal plates. See Placoderm and Ptyctodontida.

Morphological studies done on the "jaw bones" (inferognathals) of juvenile and adult Dunkleosteus suggest that Dunkleosteus went through a change in jaw morphology and diet as it aged. Juveniles had stiffer jaws more similar to Coccosteus, and appear to have fed on various soft-bodied aquatic animals. The jaws of adults were more flexible to hold struggling prey, and were well equipped to bite through the bony armor of hard-bodied animals like other placoderms.