The eloquence of the otoliths

May 16, 2013

Fish fossils that are about 23 million years old give unprecedented insight into the evolutionary history of the gobioid order, one of the most species-rich groups among the modern bony fishes.

Researchers led by paleontologist Professor Bettina Reichenbacher from the Division of Paleontology and at LMU's Department of Earth and Environmental Sciences have completed a comprehensive analysis of fish fossils which they assign to the group of bony fishes that includes the gobies. Their results, which have just appeared in the journal PLOS ONE, provide new insights into the of these fish and also have implications for their taxonomy.

The fossil material examined is unusually well preserved. "This has allowed us to describe a gobioid fossil in greater detail than ever before," says Reichenbacher. Indeed, the authors of the new study have been able to show that the concerned does not belong to the true gobies at all, in contrast to what earlier investigators had concluded. It is a member of an enigmatic family now known as the Butidae. Until very recently Butidae had been classified among the sleeper gobies. The family is now recognized as a separate clade, whose members are found in tropical river systems of Africa, Madagascar, Asia and Australia. Furthermore, no fossil specimens that could be attributed to this family have been identified until now. Indeed, datable gobioid fossils are comparatively rare in the . Since fossils of known age provide chronological markers of , this has hampered understanding of the evolutionary history of this highly successful group of fishes.

The signature ear-stones

The new description published by the LMU team, in collaboration with a group of French researchers, is based on material that was discovered in the South of France and made available for study by the Cuvier Museum in Montbéliard. The specimens were excavated from sediments that had been laid down in a shallow lagoon near the coast of the Tethys Sea, the precursor of the modern Mediterranean, towards the end of the Oligocene epoch, around 23 million years ago. Among the many unusual features of the find is the fact that the otoliths (also known as ear-stones), which are small calcified particles that form part of the balance organs in the inner ear of bony fish, are perfectly preserved. Reichenbacher, who specializes in the analysis of fossil otoliths, explains the significance of this: "Otoliths are made up of the mineral aragonite, together with a minor fraction of organic material. What makes them of such interest for us is that they can be read like a genetic code. Otoliths allow us to deduce what sort of fish they belonged to, even if nothing else has survived," she says. This is why the ear-stones play such a crucial role in studies of the paleontology, evolutionary history and biodiversity of the teleosts.

The otoliths revealed to the researchers that the fossils did not actually belong among the true gobies, but should be assigned to either the sleeper gobies or the butids. "Among the skeletal elements of the fossils, we then identified other traits that confirmed this assessment and enabled us to place the species among the butids," says doctoral student Christoph Gierl, who analyzed the structural anatomy of the skull and the dorsal and pelvic fins.

This is the first butid fossil to be found anywhere. Interestingly, no members of the Butidae are found in European waters today. The new findings show that, back in the Oligocene, butids were distributed in estuaries and lagoons around the Tethys and the Paratethys (the remnant sea to the northeast that was cut off from the rest of the Tethys Sea, today's Mediterranean, when the Alps were formed), which were then located in subtropical latitudes. The family vanished from these waters during the Early Miocene, about 22 million years ago. "They were probably displaced by true gobies that were more adaptable," says Reichenbacher.

The researchers expect that their study will lead to a better picture of the evolutionary history of the gobioids as a whole. "Our results also demonstrate that otoliths can play a much greater role in the classification of gobioids than has previously been appreciated," Bettina Reichenbacher concludes. (PLOS ONE 2013)

Explore further: Oldest evidence of split between Old World monkeys and apes discovered

More information: dx.plos.org/10.1371/journal.pone.0064117

Related Stories

Recommended for you

Crowd-sourcing Britain's Bronze Age

Apr 17, 2014

A new joint project by the British Museum and the UCL Institute of Archaeology is seeking online contributions from members of the public to enhance a major British Bronze Age archive and artefact collection.

Roman dig 'transforms understanding' of ancient port

Apr 17, 2014

(Phys.org) —Researchers from the universities of Cambridge and Southampton have discovered a new section of the boundary wall of the ancient Roman port of Ostia, proving the city was much larger than previously ...

User comments : 0

More news stories

Study finds law dramatically curbing need for speed

Almost seven years have passed since Ontario's street-racing legislation hit the books and, according to one Western researcher, it has succeeded in putting the brakes on the number of convictions and, more importantly, injuries ...