Species

At least ten different species^[13][17] 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.^[18] Dunkleosteus as currently defined is a wastebasket taxon for large dunkleosteoid arthrodires that are more evolutionarily derived than Eastmanosteus.^[18]

Labelled skull diagram of D. terrelli

The type species, D. terrelli, is the largest, best-known species of the genus. Size estimates for this species range from 4.1–10 m (13–33 ft) in length, though estimates greater than 4.5 m are poorly supported.^[3][2] Skulls of this species can be up to 60–70 cm (24–28 in) in length.^[2] D. terrelli's fossil remains are found in Upper Frasnian to Upper Famennian Late Devonian strata of the United States (Huron, Chagrin, and Cleveland Shales of Ohio, the Conneaut and Chadakoin Formations of Pennsylvania, the Chattanooga Shale of Tennessee, the Lost Burro Formation of California, and possibly the Ives breccia of Texas^[17]) 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 an anterolateral.^[17] Lelièvre (1982) considers this taxon a nomen dubium ("doubtful name") and suggests the material may actually pertain to Ardennosteus.^[19]

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.^[17]

D. marsaisi skull

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 centimetres (1.15 ft) 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.^[20] H. Schultze regards D. marsaisi as a member of Eastmanosteus.^[17][21]

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,^[17] whereas Dennis-Bryan (1987) considered it to belong to the genus Eastmanosteus^[22] This species has a skull length of 55 cm (22 in) and a total estimated length of approximately 3 m (9.8 ft).^[18]

D. missouriensis is known from fragments from Frasnian Missouri. Dunkle and Lane regard them as being very similar to D. terrelli.^[17]

D. newberryi is known primarily from a 28 centimetres (11 in) 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.^[17] Lebedev et al. (2023) noted D. newberryi has an unusually long marginal tooth row compared to other species of Dunkleosteus and lacks the accessory odontoids typical of this genus, suggesting it might not belong to Dunkleosteus or even Dunkleosteoidea^[18]

D. amblyodoratus is known from some fragmentary remains from Late Devonian strata of Kettle Point Formation, Ontario. 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.^[13]

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 containing the holotype.^[13]

D. tuderensis is known from an infragnathal found in the lower-middle Famennian-aged Bilovo Formation of the Tver Region in northwest Russia. The specific name refers to the Maliy Tuder River as the holotype was found on its bank.^[18]

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.^[18][23]

Size and anatomy

Comparison of multiple size estimates for D. terrelli

Mainly the armored frontal sections of specimens have been fossilized, and consequently, the appearance of the other portions of the fish is mostly unknown.^[24] In fact, only about 5% of Dunkleosteus specimens have more than a quarter of their skeleton preserved.^[25] Because of this, many reconstructions of the hindquarters are often based on fossils of smaller arthrodires, such as Coccosteus, which have preserved hind sections,^[2] leading to widely varying size estimates.^[2]

Dunkleosteus is one of the largest known placoderms, with its maximum size being variably estimated as anywhere from 4.1–10 metres (13–33 ft) by different researchers.^{[26][27][11][28][2]} However, most cited length estimates are speculative and lack quantitative or statistical backing, and lengths of 5 m (16 ft) or more are poorly supported.^[11][2] Most studies that estimate the length of Dunkleosteus do not provide information as to how these estimates were calculated, the measurements used to scale them, or which specimens were examined. Most are implied to be based on either CMNH 5768 (the largest complete armor of Dunkleosteus) or CMNH 5936 (the largest known jaw fragment).

Life reconstruction of D. terrelli, as presented by Ferrón et al. 2017^[11]

Life reconstruction of D. terrelli, as presented by Engelman 2023^[2]

Most of the studies with well-defined methods produce lengths of 5 metres (16 ft) or less for Dunkleosteus terrelli,^[2] with the exception of Ferrón et al. (2017), which produces larger estimates of 6.88–8.79 metres (22.6–28.8 ft) based on upper jaw perimeter of modern sharks.^[11] However, arthrodires have proportionally larger mouths than modern sharks, making the lengths estimated by Ferrón et al. (2017) unreliable.^[3] 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.^[3] If the reconstruction presented in Ferrón et al. (2017) is scaled to the known dimensions of CMNH 5768, it produces a length of 3.77 metres (12.4 ft).^[3]

Carr (2010) estimated a 4.6 metres (15 ft) long adult individual of Dunkleosteus terrelli to have weighed 665 kilograms (1,466 lb), assuming a shark-like body plan and a similar length-weight relationship.^[29] Engelman (2023), using an ellipsoid volumetric method, estimated weights of 950–1,200 kilograms (2,090–2,650 lb) for typical (3.41 metres (11.2 ft) long) adult Dunkleosteus, and weights of 1,494–1,764 kilograms (3,294–3,889 lb) for the largest (4.1 metres (13.5 ft) in this study) individual.^[2] The higher weights by Engelman (2023) are mostly a result of the fact that arthrodires tend to have relatively deeper and wider bodies compared to sharks.^[2]

An exceptionally preserved specimen of D. terrelli preserves a pectoral fin outline with ceratotrichia, 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 caudal fin shape of D. terrelli, reconstructing this fin with a strong ventral lobe, a high aspect ratio, narrow caudal peduncle, in contrast to previous reconstructions based on the anguilliform caudal fin of coccosteomorph placoderms.^[11]

Diet

Partial lower jaw of CMNH 5936, the largest known individual of Dunkleosteus terrelli. Scale = 10 cm.

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.^[27][26] 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^[26]) and producing high bite forces when closing the jaw, estimated at 4,414 N (450 kg_f; 992 lb_f) at the tip and 5,363 N (547 kg_f; 1,206 lb_f) at the blade edge,^[26] or even up to 6,170 N (629 kg_f; 1,387 lb_f) and 7,495 N (764 kg_f; 1,685 lb_f) respectively.^[27] The bite force is considered the highest of any living or fossil fish, and among the highest of any animal.^[26] The pressures generated in those regions were high enough to puncture or cut through cuticle or dermal armor,^[26] suggesting that D. terrelli was adapted to prey on free-swimming, armored prey such as ammonites and other placoderms.^[27]

In addition, teeth of a chondrichthyan thought to belong to Orodus (Orodus spp.) were found in association with Dunkleosteus remains, suggesting that these were probably stomach contents regurgitated from the animal. Orodus is thought to be tachypelagic, or a fast-swimming pelagic fish. Thus, Dunkleosteus might have been fast enough to catch these fast organisms, and not a slow swimmer like originally thought.^[11] Fossils of Dunkleosteus are frequently found with boluses of fish bones, semidigested and partially eaten remains of other fish.^[44] 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.^[45]

A specimen of Dunkleosteus (CMNH 5302), and Titanichthys (CMNH 9889), show damage said to be puncture damage from the bony fangs of other Dunkleosteus.^[27]

Reproduction

Dunkleosteus, together with most other placoderms, may have also been among the first vertebrates to internalize egg fertilization, as seen in some modern sharks.^[46] Some other placoderms have been found with evidence that they may have been viviparous, including what appears to have been an umbilical cord.^[47]

Growth

D. terrelli juvenile specimen CMNH 7424

D. terrelli adult specimen CMNH 5768

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.^[48]