| Dunkleosteus | |
|---|---|
 | |
| Information | |
| Range | N. America, Poland, Morocco, Belgium |
| Estimated Population | 0 |
| Scientific Classification | |
| Kingdom | Animalia |
| Phylum | Chordata |
| Class | Placodermi |
| Order | Arthrodira |
| Family | Dunkleosteidae |
| Genus | Dunkleosteus |
| Conservation Status | |
 Extinct | |
Dunkleosteus is an extinct genus of arthrodire placoderm fish that existed during the Late Devonian period, about 380–360 million years ago. Some of the species, such as D. terrelli, D. marsaisi, and D. magnificus, are among the largest arthrodire placoderms ever to have lived.
The largest species, D. terrelli, measuring up to 10 meters (33 feet) and weighing 3.6 tonnes, was a hypercarnivorous apex predator. Few other placoderms, save, perhaps, its contemporary Titanichthys, rivaled Dunkleosteus in size.
Dunkleosteus is a pachyosteomorph arthrodire originally placed in the family Dinichthyidae, a family composed mostly of large, carnivorous arthrodires like Gorgonichthys. Anderson (2009) suggests, 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. Carr and Hlavin (2010) resurrect Dunkleosteidae and place Dunkleosteus, Eastmanosteus, and a few other genera from Dinichthyidae within it. (Dinichthyidae, in turn, is made into a monospecific family).
New studies have revealed several features in both its food and biomechanics, as well as its ecology and physiology. Placodermi first appeared in the Silurian, and the group became extinct during the transition from the Devonian to the Carboniferous, leaving no descendants. The class persisted in the fossil record for at least 70 million years, in comparison to the 400-million-year-long history of sharks.
In recent decades, Dunkleosteus has achieved recognition in popular culture, with a large number of specimens on display, and notable appearances in entertainment media like Sea Monsters - A Walking with Dinosaurs Trilogy and River Monsters. Numerous fossils of some species have been found in North America, Poland, Belgium, and Morocco. The name Dunkleosteus combines the Greek osteus (οστεος), meaning "bone", and Dunkle, in honor of David Dunkle of the Cleveland Museum of Natural History.
Due to its heavily armoured nature, Dunkleosteus was probably a relatively slow but powerful swimmer. It is thought to have dwelt in diverse zones of inshore waters. Fossilization tends to have preserved only the especially armoured frontal sections of specimens, thus it is uncertain what exactly the hind sections of this ancient fish were like. Therefore, the reconstructions of the hindquarters are often based on smaller arthrodires, such as Coccosteus, that had hind sections preserved. The most famous specimens of Dunkleosteus are displayed at the Cleveland Museum of Natural History. Others are displayed at the American Museum of Natural History and in the Queensland Museum in Brisbane, Queensland. Instead of teeth, Dunkleosteuspossessed two pairs of sharp bony plates which formed a beak-like structure. Dunkleosteus, together with most other placoderms, may have also been among the first vertebrates to internalize egg fertilization, as seen in some modern sharks.
Dunkleosteus terrelli possessed a four-bar linkage mechanism for jaw opening that incorporated connections between the skull, the thoracic shield, the lower jaw and the jaw muscles joined together by movable joints. This mechanism allowed D. terrelli to both achieve a high speed of jaw opening, opening their jaws in 20 milliseconds and completing the whole process in 50–60 milliseconds, comparable to modern fishes that use suction feeding to assist in prey capture; and produce high bite forces when closing the jaw, estimated at 6,000 N (1,350 lbf) at the tip and 7,400 N (1,660 lbf) at the blade edge in the largest individuals. The pressures generated in those regions were high enough to puncture or cut through cuticle or dermal armour suggesting that D. terrelli was perfectly adapted to prey on free-swimming, armored prey like arthropods, ammonites, and other placoderms. Frequently, fossils of Dunkleosteus are found with boluses of fish bones, semidigested and partially eaten remains of other fish. As a result, the fossil record indicates it may have routinely regurgitated prey bones rather than digest them.
Morphological studies on the lower jaws of juveniles of D. terrelli reveal they were proportionally as robust as those of adults, indicating they already had the ability to produce high bite forces and likely were able to shear into resistant prey tissue similar to adults, albeit on a smaller scale. This pattern is in direct contrast to the condition common in tetrapods in which the jaws of juveniles are more gracile than in adults.