Fossils Inform Understanding Of Permian Period’s Ray-Finned Fishes

Salvelinus alpinus (Arctic Char); a species of ray-finned fish (Credit: Martin Cathrae, via Flickr CC BY-SA 2.0)

Modern-day fish science and research have come a long way in recent years. In particular, advancements in fish surveys, tracking and other methods of obtaining information have allowed researchers to better understand the behaviors and physical compositions of fish. More recent and accurate record-keeping of species near extinction has helped future researchers find species in texts and references, despite the original being having gone extinct. Such data and information has helped identify when species thought to be extinct have made a spontaneous return.

Observing Characteristics of Ray-Finned Fish

Unfortunately, ancient and long-gone fish are much harder to gather data on. In particular, sexual characteristics of extinct species can be challenging to identify. For example, the Venusichthys comptus, a primitive type of ray-finned fish, went extinct during the Permian Period (almost 300 million years ago). Many other species went extinct during the same time period due to various environmental factors. The Permian extinction event eliminated more than 95% of the marine species and 70% of the terrestrial species.

Despite going extinct millions of years ago, ancestors of these lost species can still be found today. Unfortunately, these descendants aren’t perfect replicas, and so many findings surrounding the sexual characteristics of an extinct species are speculative. As a result of this phenomenon, researchers with the Chinese Academy of Sciences have been interested in figuring out the sexual characteristics of the long-extinct Venusichthys comptus for some time. Fortunately, a recent fossil find may allow the researchers to make these observations.

About 30 well-preserved specimens of the fish were uncovered in the eastern Yunnan Province in China in 2016. The scientists were able to evaluate those to find that about a third of them had hook-like contact organs near their anal fins, and were likely males. Some secondary sexual characteristics emerged as well, in the form of pointed tubercles on the skulls and dorsal fins. All of which are associated with males of species, having also been found in the fossils of male Peltopleurus nuptialis, which lived in the Ladinian Period, just before the Permian Period.

Top image: Secondary sexual characteristics on cranial bones and dorsal fins. (Credit: XU Guanghui)

Conclusion

The fossils collected in 2016 represent the oldest known secondary sexual characteristics in the Neopterygii (ray-finned fish). Just like their ancestors, many modern-day sexually dimorphic ray-finned fish share similar anatomical formations:

• Breeding tubercles,
• Contact organs on their heads,
• Scales and fins (on males and females) that are contacted during prespawning/spawning and/or during fights over territory with other males.

In short, these fossils show that not only are some secondary sexual characteristics similar to anatomical structure and distribution, they may have served similar functions. Considering that the fish cannot be observed in action anymore, the other similarities between the extinct Venusichthys comptus and its modern-day relatives allow for further estimations of the species’ behavior.