Eusthenodon
Eusthenodon Temporal range:
Late Devonian | |
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Skull | |
Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Clade: | Sarcopterygii |
Clade: | Tetrapodomorpha |
Clade: | Eotetrapodiformes |
Family: | †Tristichopteridae |
Genus: | †Eusthenodon Jarvik, 1952 |
Species | |
Eusthenodon (Greek for “strong-tooth” – eusthenes- meaning “strong”, -odon meaning “tooth”) is an extinct
The large size, predatory ecology, and evolutionarily derived characters possessed by Eusthenodon likely contributed to its ability to occupy and flourish in the numerous localities across the world mentioned above. Eusthenodon is attributed to being just one of many cosmopolitan genera within the "Old Red Sandstone" fish faunas of the Upper Devonian.[3][8][9] As a result, it has been hypothesized that diversification of Eusthenodon and other morphologically similar tristichopterids were not restricted by biogeographical barriers and were instead limited only by their individual ecologies and mobility.[9]
Most of the Eusthenodon remains found at these globally distributed localities consisted largely of cranial elements and largely not known from complete skeletons.[5][9][8] Consequently, the majority of available literature covering Eusthenodon primarily focus on the intricacies of the bones associated with the skull in order to investigate the genus and while others draw conclusions from the known characters of Tristichopteridae.[9] Johanson & Ahlberg (1997), in their assessment of new sarcopterygian material, present such conclusions proposing Eusthenodon likely possessed the same trifurcate or diamond-shaped caudal fin with an axial lobe turned slightly dorsally known in other tristichopterids (referred to as eusthenopterids by Johanson) along with a triangular-shaped first dorsal fin.[5]
History and discovery
In 1952, Swedish paleontologist Erik Jarvik first described the first species, Eusthenodon wangsjoi of the genus Eusthenodon.[10] The specimen was retrieved in 1936 from the richly fossiliferous sediments of the Upper Devonian sequences of East Greenland, a region that gained tremendous attraction by vertebrate paleontologists after the discovery of the early limbed vertebrate Ichthyostega.[11] The given name of the genus, Eusthenodon, refers to the distinctly large tusks present in the upper and lower jaws.[12]
The
Description
Skull
In his initial diagnosis of the first Eusthenodon remains published in 1952, Jarvik describes the features present in the remains of Eusthenodon wangsjoi including those that are significant characters of tristichopterid fishes (referred to as
The positions and relative sizes of additional fenestrae present in Eusthenodon, including the fenestra exonarina,
Of the three
The external cheek plate is well documented in Eusthenodon being 3.5 times longer than the parietal shield and 3.0 times as long as it is high.[10] The cheek plate and lower jaw in Eusthenodon are significantly longer proportionately than in any other Osteolepiformes. Eusthenodon exhibits a lower jaw that diminishes in height moving from its posterior end to anterior end and is significantly lower in height at its anterior portion. [10]
Dentition
As the name implies, Eusthenodon has large tusks that protrude from the upper and lower jaws of the skull.[17] Specifically, along the midline of the snout, two large and stout teeth emerge on the premaxilla.[5] From the incomplete material collected upon the discovery of Eusthenodon, the largest tusks are estimated to have been at least 50 millimeters in length.[10] These two teeth are anteroposteriorly flattened and have distinctive, sharp cutting edges.[8][4] In a study presented by Gael Clement in 2009, in which a newly discovered tristichopterid assemblage was described, it was found that the enlarged teeth were predominantly in line with the tooth row of the premaxilla and they did not occur in pairs.[9] Consequently, the enlarged premaxillary teeth were described as ‘pseudo fangs’ rather than the true fangs previously thought to be present in Eusthenodon.[9] A horizontal cross-sectionally analysis of the first fang reveals simple and irregularly folded orthodentine.[5] Within the pulp cavity, osteodentine is found.[8][5] The presence of the enlarged pseudo fangs on the premaxilla in Eusthenodon, supported its phylogenetic position within the Tristichopteridae clade as similar dentition patterns are found in other closely related derived tristichopterids.[9] The number of small pointed teeth along the tooth row further supports dentition trends over time as in more derived genera, a greater number of teeth are found relative to more primitive species such as Eusthenopteron.[8]
Despite possessing sets of premaxillary pseudo fangs, Eusthenodon and other large, phylogenetically derived tristichopterids exhibit elaborate anterior dentition and distinctive enlarged dentary fangs.[5][9] The faintly concave denticulated field of the parasphenoid bone is raised in primitive tristichopterids while it is notably recessed in Eusthenodon. Additionally, the presence of a distinctive blade-like vertical lamina present on the anterior coronoid exists in most other tristichopterids but is absent in derived genera such as Eusthenodon.[9] In tristichopterids, the anterior and middle coronoids carry at least a single fang pair while in Eusthenodon, the posterior coronoid possesses two fang pairs.[7][9] Furthermore, marginal coronoid teeth are known to be present in practically all other tristichopterids (except the known absence in a single genus, Cabonnicthys), yet in Eusthenodon and the closely related Mandageria, there is considerable marginal coronoid teeth missing along the anterior portion of the jaw.[8] This reduction of marginal coronoid teeth supports the phylogenetic association of Eusthenodon, Mandageria, and Cabonnichthys and serves as a derived characteristic of late tristichopterids.[8] Eusthenodon possesses a small parasymphysial plate attached to the splenial via the small attachment of the plate onto the anterior portion of the mesial lamina.[9][14] The shape and size of the parasymphysial plate exhibited in Eusthenodon is present in all tristichopterids and is a diagnostic characteristic of the family.[9][10]
Scales
In line with the features described by Berg (1955) to be the significant diagnostic characters of Tristichopteridae, Eusthenodon possesses proportionately large, distinctively round scales without cosmine that exhibit a reticular pattern of ridges with rare appearance of independent tubercles.[18][10][19] Furthermore, each of these round cosmineless scales include a proximal central attachment boss, also diagnostic of Tristichopteridae.[8][10] In contrast to most other tristichopterids, the ornamentation of Eusthenodon scales exhibit ridges forming distinct networks whereas scales from Eusthenopteron tend to have an ornamentation of considerably shorter ridges present in the incompletely fused tubercles.[10][8] The area of overlap between scales in Eusthenodon is also larger than the scales of Eusthenopteron.[10]
Classification
Eusthenodon belongs to the family Tristichopteridae, a subdivision of the order Osteolepiformes within the greater class of Sarcopterygii.[8] Sarcopterygii is the major clade that diverted from the ray-finned Actinopterygii with the evolution of lobed-fins. The phylogeny of Tristichopteridae was described by Gael Clement, Daniel Snitting, and P.E. Ahlberg (2008) after performing a maximum parsimony analysis of the interrelationships of within the clade.[9]
Tetrapodomorpha |
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References
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- ^ a b Blom, Henning; Clack, Jennifer; Ahlberg, Per. (2007). "Devonian vertebrates from East Greenland: A review of faunal composition and distribution". Geodiversitas. 29: 119–132 – via ResearchGate.
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- ^ Daniel Snitting & Henning Blom, 2009, "Correcting Taxon Names Containing Diacritics—Examples from Paleozoic Vertebrates", Journal of Vertebrate Paleontology, 29(1): 269-270
- ^ OCLC 952685457.
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