fossils

The 244-million-year-old male fish endowed with a hook

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Jackson
Chambers

Writer
A study done in January 2017 involving 30 fish fossils from Middle Triassic China revealed anatomical differences that helped scientists set apart male from female fish.

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.


Despite these differences between the presumed sexes of these specimens, Venusichthys males lacked the specialized anal fin of closely related fishes which suggests that it may have used a different reproductive strategy.


Several living groups of ray-finned fishes also have breeding tubercles and organs that come into contact with the females during breeding. Comparing the behavior of living fishes, these traits could have three functions; maintaining body contact between sexes during mating, stimulation of the females during breeding, or the defense of territories or nests.


Venusichthys belongs to the sub-group Neopterygii within the ray-finned fishes (Actinopterygii) that appeared sometime in the Late Permian. Neopterygians evolved lighter skeletons and modified skull elements than their ancestors. They also had increased swimming capabilities brought on by better control of their dorsal and anal fins. Their modified skulls allowed them to eat a wider variety of prey and therefore become successful in many ecological niches.


Sources:
Phys.org, Neopterygian fish with secondary sexual characteristics found from the Middle Triassic of China
Chinese Academy of Sciences, Neopterygian Fish with Secondary Sexual Characteristics Found from Middle Triassic of China

Image Credit: Lucas Lima

D0ffa1e0b9aae1b8eb82bb07c0e01c81

Jackson
Chambers

Writer


http://sulc.us/g5a96
http://www.eartharchives.org/articles/neopterygian-fish-with-secondary-sexual-characteristics-found-from-the-middle-triassic-of-china/