Dunkleosteus is an extinctgenus of arthrodireplacodermfish that existed during the Late Devonian period, about 358–382 million years ago. The name Dunkleosteus combines the Greek ὀστέον, osteon, meaning "bone", and Dunkle, in honor of David Dunkle of the Cleveland Museum of Natural History. It consists of ten species: D. terrelli, D. belgicus, D. denisoni, D. marsaisi, D. magnificus, D. missouriensis, D. newberryi, D. amblyodoratus, and D. raveri; some of which are among the largest placoderms to have ever lived. The largest species, D. terrelli grew up to long and in weight. Dunkleosteus could quickly open and close its jaw, like modern daysuction feeders, and had a bite force of at the tip and at the blade edge. Numerous fossils of the various species have been found in North America, Poland, Belgium, and Morocco.
Taxonomy
Dunkleosteus was named in 1956 to honour David Dunkle, then curator of vertebrate paleontology at the Cleveland Museum of Natural History. The type speciesD. terrelli was originally described in 1873 as a species of Dinichthys. Dunkleosteus is an arthrodire originally placed in the family Dinichthyidae, which is composed mostly of large, carnivorous fish like Gorgonichthys. Anderson 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 resurrect Dunkleosteidae and place Dunkleosteus, Eastmanosteus, and a few other genera from Dinichthyidae within it. Dinichthyidae, in turn, is made into a monospecific family.
Species
At least 10 different species of Dunkleosteus have been described so far. The type species, D. terrelli, is the largest, best-known species of the genus, measuring in length. It has a rounded snout. D. terrellifossil remains are found in Upper Frasnian to Upper Famennian Late Devonian strata of the United States 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 anterolateral. D. Denison 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 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. missouriensis is known from fragments from Frasnian Missouri. Dunkle and Lane regard them as being very similar to D. terrelli. D. Newberry is known primarily from a long infragnathal with a prominent anterior cusp, found in the Frasnian portion of the Genesee Group of New York, and originally described as " Dinichthys Newberry". 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 long in life. D. raveri is a small species, possibly 1 meter long, 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.
Description
The largest species, D. terrelli, is estimated to have grown up to in length and in weight, making it one of the largest placoderms to have existed. Like other placoderms, Dunkleosteus had a two-part bony, armoured exterior, which may have made it a relatively slow but powerful swimmer. 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.
Mainly the armored frontal sections of specimens have been fossilized, and consequently, the appearance of the other portions of the fish is mostly unknown. In fact, only about 5% of Dunkleosteus specimens have more than a quarter of their skeleton preserved. Because of this, many reconstructions of the hindquarters are often based on smaller arthrodires, such as Coccosteus, which had preserved hind sections. However, an exceptionally preserved specimen of D. terrelli preserves ceratotrichia in a pectoral fin, implying that the fin morphology of placoderms was much more variable than previously thought, and was heavily influenced by locomotory requirements. This knowledge, coupled with the knowledge that fish morphology is more heavily influenced by feeding niche than phylogeny, allowed a 2017 study to infer the body shape of D. terrelli. This new reconstruction gives D. terrelli a much more shark-like profile, including a strong anterior lobe on its tail, in contrast to reconstructions based on other placoderms. The most famous specimens of Dunkleosteus are displayed at the Cleveland Museum of Natural History, and others are displayed at the American Museum of Natural History, National Museum of Natural History, State Museum of Pennsylvania, Harrisburg and in the Queensland Museum in Brisbane, Queensland.
Diet
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 and producing high bite forces when closing the jaw, estimated at at the tip and at the blade edge in the largest individuals. The pressures generated in those regions were high enough to puncture or cut throughcuticle or dermal armor suggesting that D. terrelli was adapted to prey on free-swimming, armored prey such as ammonites and other placoderms. Fossils of Dunkleosteus are frequently 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. It probably inhabited inshore waters.
Juveniles
Morphological studies on the lower jaws of juveniles of D. terrelli reveal they were proportionally as robust as those of adults, indicating they already could 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.
Media Depictions
Dunkleosteus has occasionally been depicted in various media and games. These include the 2003 BBC TV series Chased By Sea Monsters and the Animal Planet series River Monsters for the episode 'Prehistoric Terror' amongst a handful of other depictions.
