Heterosteidae
| Heterosteidae Temporal range: Emsian to Givetian |
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| Comparison of genera | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Subphylum: | Vertebrata |
| Infraphylum: | Gnathostomata |
| Class: | Placodermi |
| Order: | Arthrodira |
| Suborder: | Brachythoraci |
| Family: |
Heterosteidae Jaekel, 1903 |
| Genera | |
Heterosteidae is an extinct family of moderately large to giant, flattened, benthic arthrodire placoderms with distinctive, flattened, triangular skulls that are extremely broad posteriorly, but become very narrow anteriorly.
This genus is known from freshwater deposits of the early Eifelian-aged Verdalen member of the Stjørdalen formation in the Wood Bay series, Spitzbergen, Norway. In addition to being smaller than Heterosteus, Herasmius differs by having a comparatively broader, shorter skull.
This genus includes the largest species in the family, and are among the largest arthrodires, as well, with the type species, H. asmussi, having an estimated body length of up to 6 metres (19 ft 8 in). The genus differs from Herasmius by having the orbits on slightly longer eyes talk-like projections. The various species are found in Givetian-aged deposits in Europe and Greenland. With the except of the German H. rhenanus, all species are known from freshwater deposits: H. rhenanus is based on fragments found in a marine deposit.
This genus is known from Emsian-aged specimens from the Wuding region of Yunnan, China. In overall anatomy, it is extremely similar to the European genera, though it differs from them in its slightly smaller size, the shape of its nuchal plate, and in various body proportions.
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