Dunkleosteus

Dunkleosteus fossils were first discovered in 1867 by Jay Terrell, a hotel owner and amateur paleontologist who collected fossils in the cliffs along Lake Erie near his home of Sheffield Lake, Ohio (due west of Cleveland), United States. Terrell donated his fossils to John Strong Newberry and the Ohio Geological Survey, who in 1873 described all the material as belonging to a single new genus and species: Dinichthys herzeri. However, with later fossil discoveries, by 1875 it became apparent multiple large fish species were present in the Ohio Shale. Dinichthys herzeri came from the lowermost layer, the Huron Shale, whereas most of the fossils were coming from the younger Cleveland Shale and represented a distinct species. Most of Terrell's original collection does not survive, having been destroyed by a fire in Elyria, Ohio, in 1873. The largest collection of Dunkleosteus fossils in the world is housed at the Cleveland Museum of Natural History, with smaller collections (in descending order of size) held at the American Museum of Natural History, Smithsonian National Museum of Natural History, Yale Peabody Museum, the Natural History Museum in London, and the Cincinnati Museum Center. Specimens of Dunkleosteus are on display in many museums throughout the world (see table below), most of which are casts of the same specimen: CMNH 5768, the largest well-preserved individual of D. terrelli. The original CMNH 5768 is on display in the Cleveland Museum of Natural History. ==Taxonomy==

Dunkleosteus was named by Jean-Pierre Lehman in 1956 to honour David Dunkle (1911–1984), former curator of vertebrate paleontology at the Cleveland Museum of Natural History. The genus name Dunkleosteus combines David Dunkle's surname with the Greek word ( 'bone'), literally meaning "Dunkle's bone". Originally thought to be a member of the genus Dinichthys, Dunkleosteus was later recognized as belonging to its own genus in 1956. It was thought to be closely related to Dinichthys, and they were grouped together in the family Dinichthyidae. However, in the phylogenetic analysis of Carr and Hlavin (2010), Dunkleosteus and Dinichthys were found to belong to separate clades of arthrodires: Dunkleosteus belonged to a group called the Dunkleosteoidea while Dinichthys belonged to the distantly related Aspinothoracidi. Carr & Hlavin resurrected the family Dunkleosteidae and placed Dunkleosteus, Eastmanosteus, and a few other genera from Dinichthyidae within it. Dinichthyidae, in turn, is left a monospecific family, though closely related to arthrodires like Gorgonichthys and Heintzichthys. The cladogram below from the study of Zhu & Zhu (2013) shows the placement of Dunkleosteus within Dunkleosteidae and Dinichthys within the separate clade Aspinothoracidi: of three species Alternatively, the subsequent study by Zhu et al. (2016) using a larger morphological dataset recovered Panxiosteidae well outside of Dunkleosteoidea, leaving the status of Dunkleosteidae as a clade grouping separate from Dunkleosteoidea in doubt, as shown in the cladogram below: Species At least ten different species of Dunkleosteus have been described so far. However, many of them are poorly characterized and may be synonyms of previously named species or not pertain to Dunkleosteus. D. denisoni is known from a small median dorsal plate, typical in appearance for Dunkleosteus, but much smaller than normal. It is comparable in skull structure to D. marsaisi. 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 (1971) moved it to Dunkleosteus, This species has a skull length of and a total estimated length of approximately . D. newberryi 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 newberryi. In total, of the ten or so species listed above only four are agreed upon as valid species of Dunkleosteus by all researchers: D. terrelli (which may or may not include Dunkleosteus material from Morocco), D. raveri, D. tuderensis, and possibly D. amblyodoratus (which is known from limited material that appears distinct but is difficult to compare with other dunkleosteids). The taxonomy of early late Devonian (Frasnian) species is poorly established, whereas latest Devonian (Famennian) species are easily referable to this genus. This is not counting additional material assigned to Dunkleosteus sp. from the Famennian of California, Texas, Tennessee, and Poland. ==Description==

Size and anatomy Dunkleosteus was covered in dermal bone forming armor plates across its skull and front half of its trunk. This armor is often described as being over thick, Across the rest of the body the armor is generally much thinner, only about in thickness. The plates of Dunkleosteus had both a hard cortical and a marrow-filled cancellous layer, unlike most teleost fishes and more similar to tetrapod bones. 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 fossils of smaller arthrodires, such as Coccosteus, which have preserved hind sections, However, most cited length estimates are speculative and lack quantitative or statistical backing, and lengths of or more are poorly supported. The most extensive analyses of body size and shape in Dunkleosteus terrelli produce length estimates of ~ for typical adults of this species, with very rare and exceptional individuals potentially reaching lengths of . Upper jaw perimeter overestimates the size of complete arthrodires like Coccosteus and the estimates of Ferrón et al. (2017) result in Dunkleosteus having an extremely small head and hyper-elongate trunk relative to the known dimensions of the fossils. Engelman (2023), using an ellipsoid volumetric method, estimated weights of for typical ( long) adult Dunkleosteus, and weights of for the largest ( in this study) individual. The only vertebral remains known for Dunkleosteus are a small series of 16 vertebrae within the trunk armor of the specimen CMNH 50322. Most of these vertebrae are highly fused, and have very prominent, laterally-projecting articular facets compared to other arthrodires. Although many arthrodires show the incorporation of anterior vertebrae into a synarcual, in these species the fused region is small whereas the fused region of Dunkleosteus extends almost to the end of the trunk armor, which would make its spine very stiff. This, along with a ridge on the inside of the trunk armor suggesting an unusually well-developed attachment for the horizontal septum, suggests Dunkleosteus may have had an anteriorly stiffened spine and specialized connective tissues to transmit force generated by the anterior trunk muscles to the tail fin, similar to thunniform vertebrates like lamnids and tunas. The pelvic girdle of Dunkleosteus is relatively small relative to the overall size of the armor. Several specimens preserve associated pelvic girdles, but their original position was not recorded during preservation. However, because these specimens were excavated from cliff faces, they were probably found in close to the armor, suggesting these fins were associated with the end of the ventral shield as in other arthrodires. One specimen may preserve pelvic fin basals near the end of the trunk armor. Length estimations of D. terrelli ==Paleobiology==

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 by movable joints. Based on biomechanical modelling, Anderson and Westneat suggested that 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 As a result, the fossil record indicates it may have routinely regurgitated prey bones rather than digest them. Mature individuals probably inhabited deep sea locations, like other placoderms, living in shallow waters during adolescence. In addition, a specimen of Dunkleosteus (CMNH 5302), and Titanichthys (CMNH 9889), show damage said to be puncture damage from the bony fangs of other Dunkleosteus. Some other placoderms have been found with evidence that they may have been viviparous, including what appears to have been an umbilical cord. Growth 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. ==See also==