| Onychodus | |
|---|---|
 | |
| Holotype skull ofOnychodus jandemarrai (Showing single tusk whorl, while actually there should be a pair) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Order: | †Onychodontiformes |
| Family: | †Onychodontidae |
| Genus: | †Onychodus Newberry,1857 |
| Type species | |
| †Onychodus sigmoides Newberry, 1857 | |
| Species | |
 | |
| Localities of knownOnychodus fossils | |
Onychodus (/ɒˈnɪkədəs/, fromGreek meaning "claw-tooth")[1] is agenus ofprehistoriclobe-finned fish which lived during theDevonianPeriod (Eifelian -Famennianstages, around 374 to 397million years ago). It is one of the best known of the group ofonychodontiform fishes.[2][3] Scatteredfossil teeth ofOnychodus were first described from Ohio in 1857 byJohn Strong Newberry.[3][4][5] Otherspecies were found in Australia, England, Norway and Germany showing that it had a widespread range.
Onychodus was about 2 to 4 meters in length and was apelagic animal.[6] Like other onychodontiformes, it had a pair of tooth spirals (parasymphysial tooth whorls) bearing tusk-like teeth.
The most well-preservedspecimen ofOnychodus has been found in theGogo Formation ofWestern Australia givingpalaeontologists more information about the structure of the fish. Other species ofOnychodus are known only from poor material based on isolated tusks, teeth and scales.[3]
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The most characteristic feature is a pair of[6] retractable, laterally compressed tusk whorls at the front of the lower jaw. These were not attached to any other bone, but fit into a pair of deep cavities on the palate and were free to move. The lower jaw was connected with the upper jaw in a way that made the tusk whorl thrust out as a dagger when the head was raised.[7] The upper jaw, containing 30 teeth which decrease in size posteriorly, is well preserved in many individuals. Juveniles have six tusks, while adults have three.
A relatively complete specimen ofOnychodus jandemarrai from Western Australia shows that its length was 47 cm long, the head being 10 cm in length with tusks 1.2 cm long.[6] This specimen is only about half the size of larger individuals, since skulls measuring 19 cm in length have been found. However, a single tusk 4 cm long was found, showing that this specimen belonged to an even larger individual.[6] Evidence found of the body reveals that a cross section of this fish would have been oval in shape. On the sides of the body,Onychodus had a series of pores which provided asensory system that enabled the fish to locate prey and to position itself in narrow spaces.[6] The tail fin is almost symmetrical around the vertebral column. It was rounded slightly and would have been very flexible with a broad sweep producing forward motion.[6] A long fin extends posteriorly, along half the tail fin, forming the second dorsal fin.[6] Evidence of the first dorsal fin is incomplete, but scientists believe that a fossil element found was its fin support. Ventrally, the large anal fin extends back beneath the anterior part of the tail fin. Scales that overlap anteriorly have been found, the smallest being only 5 mm across, and the largest 22 mm.
Onychodus is thetype genus of theorderOnychodontida and thefamilyOnychodontidae to which it belongs. The family name 'Onychodontidae' was created forOnychodus by theBritish palaeontologistArthur Smith Woodward in 1891.[9] The group of onychodontiformes, described in 1973 by the late Dr.Mahala Andrews, was characterized by a highly kinetic skull and tusk-like teeth.
Suggestions by palaeontologistJohn A. Long refer to a closephylogenetic relationship betweenOnychodus and the basal lobe-finned fishPsarolepis from China.[10][11] It is generally acknowledged thatOnychodus andPsarolepis are both basal bony fishes, because of the absence of major features that unite coelacanths, lungfishes andtetrapod-like lobe-finned fishes. The position ofOnychodus andPsarolepis in thecladogram is outside the major clade of sarcopterygians (lobe-finned fishes), but at a position more derived thanactinopterygians (ray-finned fishes).
Onychodus fossils were first described byJohn Strong Newberry in 1857.[3] The type species isOnychodus sigmoides Newberry, 1857, from either the Columbus Limestone and/or the Delaware Limestone (Middle Devonian) at Milford, Delaware, and Sandusky, Ohio.[3] Some of the specimens were illustrated inReport of the Geological Survey of Ohio, Volume I, Part II in 1873. One of his plates was drawn by geologistGrove Karl Gilbert. Most specimens that Newberry described belong toOnychodus sigmoides, but he also described another species,Onychodus hopkinsi.[3][5] The name of the speciesOnychodus sigmoides comes from the word "sigmoid," meaning "S shaped," referring to the shape of the teeth (or tusks) on the parasymphysial whorl.[12] Most species ofOnychodus have this basic form of teeth. In 1889, Neweberry described a third species ofOnychodus,O. ortoni.[3][5] Some of theOnychodus specimens examined by J. S. Newberry are in theAmerican Museum of Natural History, and others are in theOrton Geological Museum atThe Ohio State University.[3]
In 1995, Roy Wagner,[4] Operations Manager for American Aggregates Corp., inOstrander,Delaware County, Ohio, United States, found parts ofOnychodus sigmoides including a lower jaw, and skull and other fragments. All remains were found as isolated elements because the bones ofOnychodus were loosely joined and dispersed quickly after the death of the fish.[4] Studies show thatOnychodus sigmoides was about 3 m in length and was the largest known bony fish of the Middle Devonian Period.
Onychodus jaekeli was the first species ofOnychodus to be described in the 20th century. Fossils were found in Germany and described by palaeontologistWalter Gross in 1965.[13]Onychodus jaekeli has up to nine barbed tusks of equal length.[6]
O. yassensis was found inNew South Wales at theLate Devonian Canowindra site and described by palaeontologistDavid Lindley in 2002.[14]
Onychodus jandemarrai, found in the Gogo Formation (Devonian) atKimberley, Western Australia, is the best known species of the genus. It was described in 2006 by the late Dr.Mahala Andrews, with colleagues DrJohn Long, DrPer Ahlberg, Prof.Ken Campbell and Dr Richard Barwick from material collected by theBritish Museum of Natural History, theBureau of Mineral Resources and theAustralian National University inCanberra. Material gathered by Dr. Long from limestone in the Gogo Formation contributed much new informationabout the genus. Although much is known of this species, there are missing details such as the shape of the posterior end of the tail, thepectoral andpelvic fins and the firstdorsal fin.[6] The name of speciesO. jandemarrai comes from the Aboriginal warriorJandamarra who lived in that area.[6]
Less known species ofOnychodus are:Onychodus anglicus (Woodward 1888) which was found in England,[15]Onychodus arcticus (Woodward 1889) fromSpitsbergen,[16] Norway,Onychodus hopkinsi Newberry, 1857 andOnychodus ortoni Newberry, 1889 from Ohio, USA,[3][5]Onychodus obliquidentatus (Jessen 1966), fromRhenish Massif, Germany,[17] andO. firouzi (Janvier & Martin, 1979).[18]
In 2017, Mann et al. described a new species,Onychodus eriensis, from theDundee Formation ofPelee Island, Canada.[19] In 2025, Goodchild et al. described a new species,O. mikijuk, from the Nordstrand Point Formation ofDevon Island,Nunavut Territory, Canada.[20]
Many features of onychodont anatomy are known only fromOnychodus itself. As other onychodontiformes, it has a highly kinetic and flexible skull.[21] This unusual characteristic was due to the loose attachment of the skull bones, which sometimes overlapped, and were connected only by soft tissue andcartilage.[5] Even the braincase was only partially made of bone.
Other distinctive traits are related to the tusk whorls on the lower jaw. Because they could rotate, this was a different method of jaw articulation which did not compare with primitive ray-finned fish. The lower jaw was entirely articulated with cartilage, without an intermediate structure between the opposite sides, allowing the separation of the bones when prey was struck. Moreover, the loose articulation caused lateral movement, making the tusk walls move out of alignment. The parts of the lower jaw would have rotated inwards upon closing in order for the tusk whorls to fit exactly into the hollow spaces in the upper jaw. It is suggested that a ligament attachment and retractor mechanism existed in a pit under the tusk whorl, a unique condition in vertebrates. The capacity of the teeth on the lower jaw to fit with the tooth rows in the upper jaw, upon closure, kept the tusk whorls in place.[6]
Through studies scientists believe thatOnychodus was an ambush predator. John A. Long has suggested thatOnychodus probably hid in the Devonian reefs, lunging at its prey as it swam by.[4] A fossil specimen ofOnychodus from Western Australia was found with aplacoderm fish half its length logged in its throat.[22][23] This interesting find was described and illustrated by Dr. John A. Long.[4][21][23] The pectoral fins were strong enough for the animal to "walk" around the sea floor in search of a hiding place between coral colonies. The posterior dorsal fin was quite powerful, providing quick speed for capturing prey.
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