The oldest eugaleaspiform (Galeaspida) from the Silurian Fentou Formation (Telychian, Llandovery) of Wuhan, South China

摘要

A new genus of Eugaleaspidiformes (Galeaspida, Agnatha), Jiangxialepis gen. nov., is described from the Llandovery (Silurian) Fentou Formation from Wuhan, Hubei, South China. The new genus belongs to the family Shuyuidae, as evidenced by the splayed posterior supraorbital canals, absence of 'U'-shaped median dorsal canals, and three lateral transverse canals issuing from the infraorbital canals. The new genus possesses a middle dorsal spine, which is found in eugaleaspidiforms for the first time. Jiangxialepis differs from other genera of Shuyuidae based on the following characteristics: presence of a middle dorsal spine, the anterior end of the median dorsal opening slightly disrupting the rostral margin of the head shield, large oval orbital openings, pineal opening level with the centre of the orbital opening, and seven lateral transverse canals on each side. The new genus is the first discovery of a eugaleaspidiform in the Fentou Formation, which extends the age of eugaleaspiforms to the middle Telychian and illuminates the early morphological conditions of the Eugaleaspidiformes. Cladistic analysis shows that the new genus is the most primitive group of Eugaleaspidiformes (Shuyuidae) and forms a trichotomy with Shuyu and Meishanaspis. We interpret Jiangxialepis as a sub-demersal fish that lived in the brackish waters of an estuarine-deltaic environment.