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

Abstract

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 Lagerstätte 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. As such, the new finding provides an important addition for understanding the behavior, reproduction, and early diversification of Neopterygii.

摘要

第二性征是动物在性成熟所表现的、可以用来分辨物种性别的特征。第二性征比较容易在现生物种中观察到,但是它们很难在化石中识别。根据采集于云南罗平中三叠世安尼期海相地层的三十块鱼化石,本文报道了基干新鳍鱼的一个新属种:多饰维纳斯鱼(Venusichthys comptus)。维纳斯鱼具有明显的性双型:成年雄鱼具有第二性征,包括雄鱼头部、鳞片和鳍条上的尖突,以及臀鳍前面钩状的接触器。这些第二性征可能用于增进雌、雄鱼体产卵前或产卵中的身体接触,刺激雌鱼排卵,以及保护巢穴和领地。维纳斯鱼不具有肋鳞鱼类或其它卵胎生鱼类那样特化的臀鳍,它的生殖方式可能不是卵胎生的。维纳斯鱼代表了新鳍鱼类第二性征的最早记录,它的发现为了解新鳍鱼类的行为、繁殖方式和早期分异提供了重要信息。