Dunkleosteus

D. terrelli. Credit: Entelognathus on the Wikimedia Commons.

*Dunkleosteus* is an extinct genus of dunkleosteid arthrodiran placoderm that lived in the United States, Canada, Poland, Morocco and possibly Belgium during the Late Devonian.

## History

*D. terrelli*, the type and largest species being about 8.79 meters (28.8 feet) long, was named after the former curator of the Cleveland Museum of Natural History, David Dunkle, and the specific epithet after the discoverer, Jay Terrell. This species is often found in the Upper Frasnian-Upper Famennian Huron and Cleveland Shale (Ohio), the Conneaut (Pennsylvania), Chattanooga Shale (Tennessee), Lost Burro Formation (California) and possibly Ives Breccia (Texas).

*D. belgicus* was named from a specimen from Belgium, first named as *Dinichthys belgicus*. Denison (1978) suggested that the remains should be recombined as *D. belgicus*, while Lelièvre (1982) suggests that some of these remains should rather be attributed to *Ardennosteus*. However, this species needs reevaluation and further preparation. *D. denisoni* (after Denison) is known from a small median dorsal plate similar in structure to *D. marsaisi*. It is very small, and from the Famennian of the Holy Cross Mountain, Poland[*citation needed*].

*D. marsaisi* is from the Lower Famennian of Atlas Mountain, Morocco. Adult skulls are often 35 centimeters long, but it varies. The snout is narrower and more postpineal fenestrae are present. It is often synonymized with *D. terrelli* and Schultze classifies it as *Eastmanosteus*. D. magnificus was originally named Dinichthys magnificus by Hussakof and Bryant (1919), then as "D. mirabilis" by Heintz (1932) and finally D. magnificus by Dunkle and Lane (1971). It is from the Frasnian Rhinestreet Shale of New York. *D. missouriensis* is from the Frasnian of Missouri, being very similar to *D. terrelli* by Dunkle and Lane. *D. newberryi* (after Newberry) was described from an infragnathal plate 28 centimeters long. It bears a prominent anterior cusp and is from the Genessee Group of New York.

*D. amblydoratus* ("blunt spear", after the way the nuchal and paranuchal plates shape into a spearhead) was described in 2010 by Carr and Hlavin from the Late Devonian Kettle Point, Canada. It was named from fragments that may represent a fish 6 meters long. *D. raveri* was named in the same paper, sometimes estimated to be only 1 meter long but is likely to be around 2.5 meters long. The holotype is from a carbonate concretion to the bottom of Huron Shale of the Famennian of Ohio Shale. It bears large eyes and is named after Clarence Raver, who discovered the concretion and thus the holotype.

## Description

    Only *Dunkleosteus*' head and shoulders were covered by a protective shield. The rest of *Dunkleosteus*' body had no protective armour or scales. *Dunkleosteus* was a placoderm. *Dunkleosteus* pulled back its jaws, revealing sharpened bony plates, and the animal began to suck in water to pull in food. *Dunkleosteus* is mostly known from the fossilized armour on the animal's face, and the massive slates of serrated bone the placoderm bore in replacement of teeth. The mandible supports a gap in between plates, and forms a sort of locking mechanism. *Dunkleosteus* may have been one of the first vertebrates to internalize egg fertilization.
The proportions of *Dunkleosteus* are largely unknown. Only about 5% of fossils preserve a quarter of the skeleton. Because of this, most reconstructions use *Coccosteus* as a reference, and was closely related as well. However, one specimen preserves the pectoral fin, and shows that placoderm fin morphology was more variable than once thought, based off a variety of characteristics. In 2017, new research gave *D. terelli* a more shark-like profile, based on ecological niches. This coupled with the strong anterior lobe on *Dunkleosteus*' tail contrasts with other placoderms shapes. *Dunkleosteus* likely possessed sclerotic rings, bony attachments that sit in the eyes of deep-diving animals to keep them from combusting under the pressure. Most reconstructions of *Dunkleosteus* feature shrink-wrapped pectoral fins, usually constricted down to the bone.

 File:Raveri skeletal.png  Engelman (2023) used new allometric body size estimation techniques to recover a maximum body length of 4.1 meters (13.5 feet) for the largest specimen (CMNH 5936). It was suggested that no Devonian vertebrates could have grew over 5 meters (16.4 feet). Other species were recovered as much smaller, with some acquiring a length of 25 centimeters (9.8 inches).

Tranverse articular facets are seen in *D. terrellii* and *D. raveri* and a thickened anterolateral contact face are seen in *D. amblydoratus*, both on the parasphenoid. D. raveri was named from MCZ 13277, an incomplete but uncrushed skull roof and parasphenoid in the ventral, lacking the nuchal and paranuchal plates. D. amblydoratus was named from UM 101105, an isolate nuchal plate with a transverse thickened left paranuchal plate and an incomplete parasphenoid. CMNH 8982 is a D. terrelli specimen preserving pterygiophores and ceratotrichia of the caudal fin, with the former being preserved as a carbonaceous film. This shows that the caudal extended upwards and was aplesodic. This has been used to speculate that Dunkleosteus were much more like sharks in appearance, based on CMNH 8982 and their niche,

## Classification

Newberry (1873) coined *Dinichthys terrelli* in Dinichthyidae. However, it was moved into the genus *Dunkleosteus* with the type *D. terrelli* in 1956, with Lehman (1956) and Stensiö (1963) placing into Dunkleosteidae (suggested by Anderson (2009) andresurrecting the family to hold it and it's closest relatives by Carr and Hlavin in 2010), and Dunkleostoidea as the namesake. Species mentioned as "*D.* n. sp." 1 and 2 by Carr (1995) were mentioned to be in reference unpublished species by Carr and Hlavin. These would be named *D. raveri* and *D. amblydoratus*. Zhu and Zhu (2013), in the naming of *Kiangyousteus*, recover the following (though they do not test all species):

### Evolution

 File:Raveri.png
*D. raveri* is a metaspecies, since it retains archaic traits, the most important being the retention of dermal ornamentation, which allows the stratigraphic placement of *Dunkleosteus* specimens. Archaic traits combined with placement lower than *D. terrelli* has allowed Carr and Hlavin (2010) to speculate that it may have evolved into the *D. terrelli* found later in time. *D. amblydoratus* may also be a metaspecies, though being fragmentary. It is definitely *Dunkleosteus*, however, based on a contacting parasphenoid and anterior superognathals. *D. amblydoratus* lacks unambiguous diagnostic features, which may suggest that it is a metaspecies or potential ancestor of *D. terrelli* or *D. raveri*. In *D. amblydoratus*, anterior articulation may be a transitional form of the derived transverse articular facets.

## Species

* *D. terrelli*
* *D. belgicus*?
* *D. denisoni*
* *D. marsaisi*
* *D. magnificus*
* *D. missouriensis*
* *D. newberryi*
* *D. amblyodoratus*
* *D. raveri*
* *D. tuderensis*

### Reassigned Species

* *D. intermedius* (=*D. terrelli*[*citation needed*])
* *D. curtus*[*citation needed*]
* "*D." yunnanensis*

## Synonyms

* *Dinichthys curtus*
* *Dinichthys magnificus* (=*D. magnificus*)
* *Dinichthys mirabilis* (=*D. magnificus*)
* *Dinichthys terrelli* (=*D. terrelli*)
* *Dinichthys newberryi* (=*D. newberryi*)

## Paleobiology

### Ontogeny

Studies on juvenile specimens of *Dunkleosteus* show that the jaws were as robust as adults. On a smaller scale, juvenile were able to bite down just as hard as adults, being able to shear into resistant prey. This adaptation is unlike tetrapods, which generally have more gracile mechanisms until adulthood.

### Diet

*Dunkleosteus* possessed a four-bar linkage mechanism used for opening the jaws; incorporating connections on the skull, the thoracic shield and the mandible and its muscles. This not only gave *Dunkleosteus* the ability to open their jaws in about 20 milliseconds, but complete the whole process from 50-60 milliseconds, producing massive bite force when the jaws were snapped shut. These forces are estimated at 6000 newtons (612-1349 pounds of force) at the tip of the blades in the fishes mouth in the largest recorded individuals. These forces are enough to crack open cuticle or dermal armour. This suggests that *Dunkleosteus* was well adapted to hunting nektonic ammonites and placoderms. Fossils of *Dunkleosteus* have been found with fish-bone-boluses and semi-digested and half-eaten fish carcasses. This suggests that *Dunkleosteus* likely regurgitated bones, rather than passing them. Mature *Dunkleosteus* probably hunted in deep waters, and juveniles hunted in the shallows.

## Paleoecology

In the Cleveland Member, Late Famennian, the fish fauna is one of the most speciose and well-collected known from the Late Devonian. Most of the published animals are obligated benthic livers or are tied to the seafloor. However, *D. terrelli* is one of the few pelagic organisms, based on analysis of the water column and the fauna and is supported by Chi-squared statistical analyses of the distribution of material. Dry and wet weights suggest that it was not able to rest on the seafloor due to physical proportions and their relation to sediments, and a static buoyancy mechanism being present. This environment would have been 30-100 meters deep. Arthrodire remains from this member with shark spines embedded in their oral components indicate predation on sharks, and inversely shark and arthrodire remains in the stomach contents in a palaeoniscid fish; though these are indeterminate and probably not belonging to *D. terrelli*. Individuals here are, at smallest, 25 centimeters long.

Murphy (1979) described a nearly complete *D. terrelli* anterodorsal plate that was uncovered on Route 1-79 in 1969 during construction, hailing from the Riceville Shale with the black shale determined to be of an eastern extension of the Cleveland Shale. It was discovered in thin, light gray siltstone that was interbedded with near black micaceous shale. Marine invertebrates were found in the siltstone, but not in the dark shale. Bioturbation mixed the dark and light shale in certain places. Due to being an extension, the environment was likely very similar. Here indicates an analogue to deltaic Bedford/Berea, prodelta lower Bedford/Cleveland transition zones and the Chagrin Formation. However, t was deposited slightly earlier and over a smaller area. This specimen was therefore of considerable stratigraphic use.

Dunkle and Wilson (1952) describe the phosphoric unit of the Huoy Formation, noting a fauna including cf. *D. terrelli* and conodonts. These beds indicate a transition of the Devonian and Mississippian (Carboniferous) due to the fauna changing into animals that are strictly from the Lower Carboniferous. Specifically, the Ives Breccia Member of the Huoy consists of lag deposits of detrital chert of varied age and source, but is mainly of the Late Devonian. The Chattanooga Shale is suggested to be an extension of the Marcellus Shale. This formation often produced conodonts that date to the Devonian. The Lost Burro Formation ranges from the Lower/Middle to the Late Devonian. Fossil outcrops in Death Valley National Park produce *D. terrelli*, small cladodont sharks, cochliodonts and *Blieckaspis*.

At Kettle Point in the Canadian First Nations is an Upper Devonin shale known as the Kettle Point Formation that is exposed at the tip of the point near-to the shores of Lake Huron. Here, *D. amblydoratus* is found in spherical or ovoid concretions locally known as "kettles". The Anishinabek considered these rare formations the eggs of thunderbirds, and are protected, though often found on local properties. *D. newberryi* is from the New York Genesee group, where plant fossils and a diverse fauna of the Devonian-Mississippian transition may be found.From the aforementioned Kettle Point, *D. raveri* is known from the Huron Shale Member, and does not overlap with *D. terrelli.* Thus, *D. terrelli*, *D. amblydoratus* and *D. raveri* are defined as North American *Dunkleosteus* taxa.

## Gallery

*Dunkleosteus*, as reconstructed with soft tissue lips covering its "teeth".
**Credit: Andrey Atuchin.**The anatomy of *Dunkleosteus'* jaw including major anatomy, organs and joints.Juvenile *Dunkleosteus* specimen housed in the Cleveland Museum of National History.A beached *Dunkleosteus*.
**Credit: kos1604 on DeviantArt.**A compressed "basal" cf. *D. terrelli*, CMNH 50237, about 10 centimeters long (based on scalebars).ROMVP65785, a young *D.* sp. from the Atlas Mountains of Morocco at the Royal Ontario Museum.
**Credit: CLumsystiggy on DeviantArt.***D. amblydoratus* at the Royal Ontario Museum.
**Credit: Clumsystiggy on DeviantArt.**
## References

1. ↑ Dorling Kindersley Encyclopedia of Dinosaurs and Prehistoric Life, p. 41
2. ↑ 2.0 2.1 2.2 2.3 2.4 https://www.google.ca/books/edition/Placodermi/syALAQAAIAAJ?hl=en&gbpv=0&bsq=isbn:9780895740274
3. ↑ https://www.sciencedirect.com/science/article/abs/pii/S0016699595801324
4. ↑ https://doi.org/10.1046%2Fj.1467-2979.2000.00015.x
5. ↑ https://doi.org/10.5962%2Fbhl.title.5270
6. ↑ https://www.deviantart.com/clumsystiggy/art/Dunkleosteus-raveri-909667817
7. ↑ 7.0 7.1 7.2 7.3 https://academic.oup.com/zoolinnean/article/159/1/195/2608083?login=true
8. ↑ 8.0 8.1 8.2 8.3 https://en.wikipedia.org/wiki/Dunkleosteus
9. ↑ https://twitter.com/Carnoferox/status/1323678447776968706
10. ↑ https://www.newscientist.com/article/2361629-ancient-fish-thought-to-be-larger-than-sharks-was-actually-quite-short/
11. ↑ https://incertaesedisblog.wordpress.com/2020/11/03/ceratotrichia-and-dorsal-fin-shape-in-arthrodires/
12. ↑ https://www.researchgate.net/profile/Robert-Carr-9/publication/235924324\_Placoderm\_diversity\_and\_evolution/links/00463514388def2cea000000/Placoderm-diversity-and-evolution.pdf
13. ↑ https://academic.oup.com/zoolinnean/article/169/4/798/3797003?login=false
14. ↑ https://books.google.ca/books?id=mR\_cxcxO8h8C&pg=PA511&lpg=PA511&dq=dunkleosteus+curtus&source=bl&ots=wvbvR7uyfO&sig=ACfU3U1u7fqBI1yzrwTJ5QI30nw1tk9nvw&hl=en&sa=X&ved=2ahUKEwilvcasucb2AhURB50JHXOlD10Q6AF6BAhNEAM#v=onepage&q=dunkleosteus%20curtus&f=false
15. ↑ https://tumblr-amnh-org.cdn.ampproject.org/c/s/tumblr.amnh.org/post/140467924529/wow-its-friday-have-a-great-weekend/amp
16. ↑ https://www.researchgate.net/profile/Robert-Carr-9/publication/235924093\_The\_Cleveland\_Museum\_of\_Natural\_History\_PALEOECOLOGY\_OF\_DUNKLEOSTEUS\_TERRELLI\_PLACODERMI\_ARTHRODIRA/links/546229a40cf2c0c6aec1a831/The-Cleveland-Museum-of-Natural-History-PALEOECOLOGY-OF-DUNKLEOSTEUS-TERRELLI-PLACODERMI-ARTHRODIRA.pdf
17. ↑ https://kb.osu.edu/bitstream/handle/1811/22642/1/V079N5\_233.pdf
18. ↑ http://www.thefossilforum.com/index.php?/topic/36602-devonian-placoderms-of-morocco/
19. ↑ https://pubs.geoscienceworld.org/gsa/gsabulletin/article-abstract/68/7/807/4920/DEVONIAN-MISSISSIPPIAN-TRANSITION-IN-CENTRAL-TEXAS?redirectedFrom=fulltext
20. ↑ https://en.wikipedia.org/wiki/Chattanooga\_Shale
21. ↑ https://en.wikipedia.org/wiki/Lost\_Burro\_Formation
22. ↑ https://en.wikipedia.org/wiki/Chippewas\_of\_Kettle\_and\_Stony\_Point\_First\_Nation#Geology
23. ↑ https://paleobiodb.org/classic/displaySearchStrataResults?group\_formation\_member=Genesee
24. ↑ https://www.jstor.org/stable/1299812