A Middle Triassic stem-neopterygian fish from China shows remarkable secondary sexual characteristics

【摘要】：Secondary sexual characteristics are features that appear at sexual maturity and distinguish the two sexes of a species. They are readily observed and studied in living animals, but the phenomenon is rather more difficult to identify in fossil taxa. Here we report a new sexually dimorphic stem-neopterygian fish, Venusichthys comptus gen. et sp. nov., based on 30 exceptionally well-preserved specimens from the Middle Triassic(Pelsonian, Anisian)Luoping Lagersta¨tte of eastern Yunnan, China. The discovery represents the oldest known secondary sexual characteristics in Neopterygii. These characteristics,including pointed tubercles on cranial bones, scales and fins, and hook-like contact organ anterior to the anal fin,have three inferred primary functions: maintenance of body contact between the sexes during prespawning behavior or spawning; stimulation of the females during breeding; and defense of nests and territories. Lacking a specialized anal fin in the presumed males, Venusichthys would likely have a different reproductive strategy from peltopleurids and other potentially viviparous stem-neopterygians. Moreover,Venusichthys shows a unique character combination distinguished from any other stem-neopterygian families and consequently represents a new family of this clade. Assuch, the new finding provides an important addition for understanding the behavior, reproduction, and early diversification of Neopterygii.