Temporal range: Frasnian-Famennian, 382–358 Ma
|Reconstructed skull, Vienna Natural History Museum|
D. terrelli (Newberry, 1873 [originally Dinichthys])
Dunkleosteus is an extinct genus of arthrodire placoderm fish that existed during the Late Devonian period, about 358–382 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 6 m (20 ft) long and 1 t (1.1 short tons) in weight, 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 exterior, 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, Dunkleosteus possessed 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 armor 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.
The type species, D. terrelli, is the largest, best-known species of the genus. It has a rounded snout. D. terrelli's fossil remains are found in Upper Frasnian to Upper Famennian Late Devonian strata of the United States (Huron and Cleveland Shales of Ohio, the Conneaut of Pennsylvania, Chattanooga Shale of Tennessee, Lost Burro Formation, California, and possibly Ives breccia of Texas) and Europe.
D. belgicus (?) is known from fragments described from the Famennian of Belgium. The median dorsal plate is characteristic of the genus, but, a plate that was described as a suborbital is apparently an anteriolateral plate.
D. denisoni is known from a small median dorsal plate, typical in appearance for Dunkleosteus, but much smaller than normal.
D. marsaisi refers to the Dunkleosteus fossils from the Lower Famennian Late Devonian strata of the Atlas Mountains in Morocco. It differs in size, the known skulls averaging a length of 35 cm and in form to D. terrelli. In D. marsaisi, the snout is narrower, and a postpineal fenestra may be present. Many researchers and authorities consider it a synonym of D. terrelli. H. Schultze regards D. marsaisi as a member of Eastmanosteus.
D. magnificus is a large placoderm from the Frasnian Rhinestreet Shale of New York. It was originally described as "Dinichthys magnificus" by Hussakof and Bryant in 1919, then as "Dinichthys mirabilis" by Heintz in 1932. Dunkle and Lane moved it to Dunkleosteus in 1971.
D. newberryi is known primarily from a 28-cm-long infragnathal with a prominent anterior cusp, found in the Frasnian portion of the Genesee group of New York, and originally described as "Dinichthys newberryi".
D. amblyodoratus is known from some fragmentary remains from Late Devonian strata of Kettle Point, Canada. The species name means "blunt spear" and refers to the way the nuchal and paranuchal plates in the back of the head form the shape of a blunted spearhead. Although it is known only from fragments, it is estimated to have been about 6 m long in life.
D. raveri is a small, possibly 1-m-long species known from an uncrushed skull roof, found in a carbonate concretion from near the bottom of the Huron Shale, of the Famennian Ohio Shale strata. Besides its small size, it had comparatively large eyes. Because D. raveri was found in the strata directly below the strata where the remains of D. terrelli are found, D. raveri may have given rise to D. terrelli. The species name commemorates Clarence Raver of Wakeman, Ohio, who discovered the concretion where the holotype was found.
Dunkleosteus was named in 1956 to honour David Dunkle, then curator of Vertebrate Paleontology at the Cleveland Museum of Natural History. The type species (D. terrelli) was originally described in 1873 as a species of Dinichthys.
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