Gansuselache

Gansuselache is a genus of extinct elasmobranch in the order Hybodontiformes, comprising one species, Gansuselache tungshengi (monotypy) from Gansu Province (Beishan Hills), China. It is known from Fangshankou Formation, which previously interpreted as Permian in age, but reinterpreted as the Early Triassic in later study. A tooth remain tentatively assigned to this genus is also known from Late Permian strata of Poland.

Description
G. tungshengi is known from single specimen which is nearly complete and has traces of soft tissue preserved. With a total length reaching 49 cm, it had large spines on two dorsal fins, a feature seen in other hybodontiform sharks, as well as two pairs of cephalic spines with retrorse barbs. The dorsal spines were found to be covered in placoid scales, as well.

Although its classification is still uncertain, it is tentatively placed in the family Lonchidiidae on the basis of tooth its morphology. The teeth themselves measure up to 3 mm long, though the tooth base is shorter and deeper than the crown. The crown/base junction on each tooth is at it highest on the labial (lip) side of the tooth, but the overall profile of the teeth are low with a short, slightly pyramid shaped central cusp. Up to two pairs of lateral cusplets flank this central cusp, and the first pair is well-separated from the main cusp. All cusps on the teeth of this genus are ornamented by vertical ridges across the crown that do not bifurcate. These ridges begin at crown apecies and descent down to the crown's shoulder which is weakly developed compared to the teeth of other hybodontiform genera. There is also a thin horizonal ridge with runs along the crown in the direction of the tongue.

Paleoecology
Gansuselache is known from Fangshankou Formation, which was originally considered as Permian. However, study in 2020 reinterpreted the geologic formation as Early Triassic in age. The Hongyanjing Formation sits at the same stratigraphic horizon, and it is even possible that fossils from both formations were recovered from the locality which produced Gansuselache. Both formations represent lacustrine environment, Although Fangshankou Formation has so far produced only one other fish taxon (an acipenseriform genus, Eochondrosteus), the Hongyanjing Formation produced multiple taxa such as several thousand specimens of actinopterygians (including Beishanichthys, Plesiofuro, Boreosomus), other hybodontiform sharks which are evidenced by preserved narial and oral barbels, a cynodont (Beishanodon), lizard-like diapsids, temnospondyls and possible late-surviving lepospondyl.