Secondary sexual characteristics are present in all major vertebrate groups. They are features that appear at sexual maturity and separate male and female individuals but are not directly related to the reproductive system, like the extravagant set of feathers a male peacock has. While it might be obvious in living animals, it can be difficult to see in fossils due to poor preservation or fragmentation. These features are important in understanding the reproduction of animals as well as aspects related to reproduction such as behavior and evolution.
Researchers Xu Guang-hui and Li-Jun Zhao described a new sexually dimorphic fish, Venusichthys comptus, by analyzing 30 well-preserved fossils. The fossils were found in the Luoping Lagerstätte of eastern Yunnan, China and are thought to be from the Middle Triassic.
Each of the 30 fossils are 25-38 mm long. Some of them show tiny bumps called tubercles on the first rays of their fins, as well as a hook-like organ at the front of their anal fin. These individuals were presumed to be male. The hooklets may have played a role in sperm transfer from male to female fish.