The largest placoderm was the 10 metres (33 ft) long Dunkleosteus.
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 m (33 ft) and weighing 3.6 t (4.0 short tons) 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 , 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. As such, 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.
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. 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. missouriensis is known from fragments from Frasnian Missouri. Dunkle and Lane regard them as being very similar to D. terrelli.
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 20 ft 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.
Dinichthys herzeri is an extinct, giant, marine arthrodire placoderm from the Late Devonian (Famennian stage) of Ohio and Tennessee. It was comparable in size, shape, and ecological role to the better-known Dunkleosteus. Originally described in 1868 by John Newberry on the basis of an incomplete skull roof and mandibles (AMNH 81), this species remains imperfectly known to this day.
For much of the 20th century, many unrelated large arthrodires were classified together within this genus, including species now assigned to Dunkleosteus, Eastmanosteus, and Titanichthys. Today, Dinichthys is considered a monotypic genus, containing only the type species, D. herzeri. Similarly, in a 2010 analysis, the family Dinichthyidae that once held a wide range of arthrodire genera was redefined as comprising only Dinichthys.
The type species of Dunkleosteus was originally described as Dinichthys terrelli by Newberry in 1873. After complete exoskeletons of this species were discovered in the early 20th century Din. terrelli served as the basis for life reconstructions of the genus as opposed to the fragmentary Din. herzeri, even long after terrelli was separated into Dunkleosteus by Jean Pierre Lehman in 1956. As a result, most illustrations captioned as Dinichthys are actually pictures of Dunkleosteus.
Its relative, Titanichthys, may have rivaled it in size.
These particular animals may have reached lengths of 10 m (33 ft) and are estimated to have weighed in at 3.6 tonnes. Titanichthys is a genus of giant, aberrant marine placoderm from shallow seas the Late Devonian of Morocco and Eastern North America. Many of the species approached Dunkleosteus in size and build. Unlike its relative, however, the various species of Titanichys had small, ineffective-looking mouth-plates that lacked a sharp cutting edge. It is assumed that Titanichthys was a filter feeder that used its capacious mouth to swallow or inhale schools of small, anchovy-like fish, or possibly krill-like zooplankton, and that the mouth-plates retained the prey while allowing the water to escape as it closed its mouth.
Antineosteus rufus from the upper Emsian of the Czech Republic is described based on two fragments of large dermal plates discovered in the Suchomasty Limestone. The original length of the animal is inferred to have exceeded that of Tityosteus rieversae - the largest Lower Devonian placoderm recorded so far. The occurrence of A. rufus in the Prague Basin is consistent with other giant homostiids in several areas. These animals were apparently adapted to plankton- feeding, although they appeared in the conditions of collapsed diversity of the planktic communities during the “Devonian Nekton Revolution”. This successful feeding strategy made them the first vertebrates occupying the nutrient-rich ecospace producing the largest animals up to the present.