The emergence of asymmetrical flatfishes, such as halibut, flounder, and sole, has long been an evolutionary puzzle. Their truly strange asymmetrical heads are one of the most unusual body features among vertebrates, the evolution of which has been hard to explain. How did an animal evolve to feature both their eyes on one side of their head? What were the intermediate steps? The puzzle of their anatomy was one of the main examples used in early arguments against the theory of natural selection. How could such unusual features have slowly evolved while remaining advantageous to survival?
The discovery of a previously unknown fish species answers some of those questions. Oxford University researcher Dr Matt Friedman found the fossil fish, named Heteronectes (meaning ‘different swimmer’), in 50 million year old marine rocks from northern Italy.
This discovery has given researchers the first detailed description of a primitive flatfish, one in which the migrated eye had not yet crossed all the way over to the opposite side of the skull.
“Heteronectes, with its flattened form, shows the perfect intermediate stage between most fish with eyes on each side of the head and specialized flatfishes where both eyes are on the same side.”
“This fossil comes from Bolca in northern Italy, a site that has literally been mined for hundreds of years for its fossil fishes. This remarkable site provides a snapshot of an early coral reef assemblage. Reefs are well known as biodiversity hotspots, so it is perhaps not surprising that Bolca provides us with the first evidence of many modern fish groups,” said Friedman. “Our understanding of the relationships of some of these groups is in a state of change with the increasing influx of molecular genetic studies. Fossils have not contributed very much to this debate, but specimens like that of Heteronectes reveal the superb level of detail that can be extracted from extinct species.”
“The specimen itself was discovered — with no identification — in a museum collection in Vienna. It just goes to show that even well-known fossil sites can yield important surprises, and that not all new discoveries take place in the field,” Friedman said.
“This is a profound discovery which clearly shows that intermediate fossil forms, which according to certain creationist theories shouldn’t exist, are regularly turning up as scientists keep looking for them,” said Dr. John Long of the Natural History Museum of LA County, who is an expert in fossil fishes and was not involved in the study.
The discovery is featured in the latest issue of the Journal of Vertebrate Paleontology.
Image Credits: M. Friedman