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Eusthenodon (Greek for “strong-tooth” – eusthenes- meaning “strong”, - odon meaning “tooth”) is an extinct genus of marine tristichopterid from the Devonian period, ranging between 365 and 359 million years ago (Late Famennian). They are well known for being a cosmopolitan genus with remains being recovered from East Greenland, Australia, Central Russia, USA (Pennsylvania), and Belgium. Compared to the other closely related genera of the Tristichopteridae clade, Eusthenodon was one of the largest Sarcopterygii (approximately 2.5 meters in length) and among the most derived tristichopterids alongside its close relatives Cabonnichthys and Mandageria.
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. 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.
Most of the Eusthenodon remains found at these globally distributed localities consisted largely of cranial elements and largely not known from complete skeletons. 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. 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.
The specific name honours Gustav Wängsjö as discoverer. Jarvik originally spelled it as wängsjöi. However, the ICZN forbids the use of diacritics in binomials. In 1962 Vorobyeva emended the specific name to waengsjoei. In 2009 Daniel Snitting and Henning Blom pointed out that this erroneously assumed German umlauts had been present. According to article 32.5.2.1 ICZN the correct spelling is wangsjoi.Daniel Snitting & Henning Blom, 2009, "Correcting Taxon Names Containing Diacritics—Examples from Paleozoic Vertebrates", Journal of Vertebrate Paleontology, 29(1): 269-270
The alleged species " E. gavini" Johanson & Ritchie, 2000 from the Hunter Siltstone of Australia is only known from very fragmentary material that cannot be confidently assigned to this genus, and likely requires revision.
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:
The positions and relative sizes of additional fenestrae present in Eusthenodon, including the fenestra exonarina, pineal foramen, and pineal fenestra are further diagnostic characters of the genus. The triangular pineal fenestra is well known in Eusthenodon for its large size and the distinctive posterior tail of the fenestra coming near or in-contact to the posterior Frontal bone margin. On the contrary, the pineal foramen is much smaller in size and is positioned distinctively posterior both to the center of radiation of the frontal and the postorbital bone of the frontoethmoidal shield. When viewing the Eusthenodon skull in Dorsally, the fenestra exonarina can be seen positioned high and laterally in the snout.
Of the three Temporal bone that make up the parietal shield present in Osteolepiformes (intertemporal, supratemporal, and extratemporal), the presence of the extratemporal bone in a ‘postspiracular’ position, defined as the shift of the bone from a position lateral to the supratemporal to a more posterior-lateral position, is a significant and diagnostic character of the Tristichopteridae clade. The extratemporal bone present in Eusthenodon is notable for its complete postspiracular position resulting in no contact between the supratemporal and extratemporal bones, a condition known only to exist in Eusthenodon. One theory to explain the trend observed in the posterior shift of the extratemporal bone in more derived fishes suggests that the change in head proportions contributed to a more streamlined body shape and enhanced its maneuverability and speed in its aquatic environment.
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. 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.
Despite possessing sets of premaxillary pseudo fangs, Eusthenodon and other large, phylogenetically derived tristichopterids exhibit elaborate anterior dentition and distinctive enlarged Mandible fangs. The faintly concave denticulated field of the Parasphenoid is raised in primitive tristichopterids while it is notably recessed in Eusthenodon. Additionally, the presence of a distinctive blade-like present on the anterior coronoid exists in most other tristichopterids but is absent in derived genera such as Eusthenodon. In tristichopterids, the anterior and middle coronoids carry at least a single fang pair while in Eusthenodon, the posterior coronoid possesses two fang pairs. 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. This reduction of marginal coronoid teeth supports the phylogenetic association of Eusthenodon, Mandageria, and Cabonnichthys and serves as a derived characteristic of late tristichopterids. 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. The shape and size of the parasymphysial plate exhibited in Eusthenodon is present in all tristichopterids and is a diagnostic characteristic of the family.
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