Dunkleosteus

Dunkleosteus is an extinct genus of large arthrodire ("jointed-neck") fish that existed during the Late Devonian period, about 382–358 million years ago. It was a pelagic fish inhabiting open waters, and one of the first vertebrate apex predators of any ecosystem. Fossils of Dunkleosteus have been found in the United States, Canada, Poland, Belgium, and Morocco.

Dunkleosteus consists of ten species, some of which are among the largest placoderms ("plate-skinned") to have ever lived: D. terrelli, D. belgicus, D. denisoni, D. marsaisi, D. magnificus, D. missouriensis, D. newberryi, D. amblyodoratus, D. raveri, and D. tuderensis. However, the validity of several of these species is unclear. The largest and best known species is D. terrelli, the type species.

Since body shape is not known, various methods of estimation put the living total length of the largest known specimen of D. terrelli between long and its weight around .

Discovery

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.

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.

thumb|right|Front view of D. terrelli skull

The cladogram below from the study of Zhu & Zhu (2013) shows the placement of Dunkleosteus within Dunkleosteidae and Dinichthys within the separate clade Aspinothoracidi:

thumb|right|[[Life restorations 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

left|thumb|Comparison of multiple historical size estimates for D. terrelli , with the smallest size estimate (bottom) likely the most realistic

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.

| Newberry || 1889 || || Unstated (implied extrapolation from Coccosteus) ||

| Dean || 1895 || || Methods, measurements, and specimens unstated ||

| Hussakof || 1905 || (AMNH FF 195) <br /> (extrapolated to CMNH 5768 by Engelman 2023

| Anonymous|| 1923 || || Methods, measurements, and specimens used not stated ||

| Hyde || 1926 || || Methods, measurements, and specimens used not stated ||

| Romer || 1966 || || Methods, measurements, and specimens used not stated ||

| Colbert || 1969 || || Methods, measurements, and specimens used not stated ||

| Denison || 1978 || || Methods, measurements, and specimens used not stated ||

| Janvier || 2003 || || Methods, measurements, and specimens used not stated ||

| Young || 2003 || || Methods, measurements, and specimens used not stated ||

| Anderson and Westneat || 2007 || || Methods, measurements, and specimens used not stated ||

| Sallan and Galimberti || 2015 || || Methods, measurements, and specimens used not stated ||

| Ferrón et al. || 2017 || (average adult, CMNH 5768) <br /> (largest individual, CMNH 5936) || Upper jaw perimeter ||

| Johanson et al. || 2019 || (CMNH 50322) <br /> (extrapolated to CMNH 5768 by Engelman 2023 assuming similar head-trunk proportions) || Methods and measurements not stated ||

| Engelman || 2023 || (average adult, CMNH 5768) <br /> (largest individual, CMNH 5936) || Orbit-opercular length (head length minus snout) || 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.

References

External links

Introduction to the Placodermi: Extinct Armored Fishes with Jaws. Waggoner, Ben (2000). Retrieved Aug 1, 2005
MSNBC: Prehistoric fish packed a mean bite
BBC: Ancient 'Jaws' had monster bite