Fossil Reveals Secret of Flatfishes

Jun 29, 2012 by Sci-News.com

A new fish fossil from the lower Eocene found by Oxford University researcher Dr Matt Friedman has revealed why flatfishes have one of the most unusual body plans among all backboned animals.

The skeleton of the primitive flatfish Heteronectes (M. Friedman)

How did flatfishes, with both of their eyes on one side of their head, evolve? The new discovery, described in the Journal of Vertebrate Paleontology, finally solves this mystery.

Dr Friedman’s fossil fish, named Heteronectes (meaning ‘different swimmer’), was found in 50 million year old marine rocks from northern Italy.

The study provides the first detailed description of a primitive flatfish, revealing that the migrated eye had not yet crossed to the opposite side of the skull in early members of this group. 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,” Dr Friedman said. “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.”

The skull of the primitive flatfish Heteronectes, with views of the left- and right-hand sides (M. 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.”

Dr Friedman said that “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.”

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Bibliographic information: Friedman M. 2012. Osteology of †Heteronectes chaneti (Acanthomorpha, Pleuronectiformes), an Eocene stem flatfish, with a discussion of flatfish sister-group relationships. Journal of Vertebrate Paleontology (32) 4: 735-756; doi: 10.1080/02724634.2012.661352