One of the largest, and sometimes overlooked, fossil record belongs to fishes, spanning hundreds of millions of years since their origin in the mid-Paleozoic. Such an immense fossil record has given ichthyologists an advantage in developing comprehensive hypotheses of evolutionary relationships of fishes both living and extinct. But even with such a expansive fossil record, gaps remain where fossils have not been preserved or discovered, and so many questions remain about the history of fishes. So it is pretty exciting to the field of ichthyology when new discoveries are made, allowing us to examine and reevaluate the history of fishes within a specific region and how it can impact our current understanding of the phylogenetic relationships of fishes globally.
Case in point: this month in PLOS ONE, a paper was published outlining an assemblage of fishes from the Middle Eocene of Libya. The study, led by Olga Otero from the Université de Poitiers in France, along with authors Aurélie Pinton, Henri Cappetta, Sylvain Adnet, Xavier Valentin, Mustapha Salem, and Jean-Jacques Jaeger, helps fill some gaps in our knowledge of the history of African fish genera. The fossil localities, in an area known as Dur At-Talah in southeastern Libya, were first explored over 50 years ago, but recent efforts by the team behind this paper in PLOS ONE have significantly improved upon the known fossil record of African fishes, as well as provide a clearer understanding of the stratigraphy and geology of these sites in Northern Africa.
“The team [who collected this material] is used to working in this kind of context [continental deposits in Libya], notably to find rodent and small monkey teeth. The material collected gives information on the fish taxa preserved, and are diagnosed by small elements, notably the teeth.”
The sites at Dur At-Talah in Libya overly middle Eocene marine deposits, and the stratigraphy clearly shows a complex transition from marine shoreline deposits to tide-dominated stream and delta deposits to stream-dominated deposits in the youngest layers. Most of the fishes described in the paper are freshwater fishes recovered from the middle tide and deltaic deposits, although the layers below and above produce some marine vertebrates, which indicates that the system here is extremely complex in its composition and ecology, causing the authors to proceed with caution to determine a proper interpretation of the geology and sedimentology of the site as it pertains to the fish fauna.
“It was important to discuss [the site] with sedimentologists to be able to be more confident on further ecological inferences,” Otero said. “I suspect that in this continental coastal environment, some marine fish may enter the freshwater system or there maybe be some members who adapt to brackish or deltaic waters.” However, Otero clarifies that of the fishes preserved, some layers show freshwater taxa, and others marine, showing how complicated this system can be. “We use our knowledge of modern relatives to infer the ecology of the fossils, and it also has to be coherent at the scale of the fossil assemblage and with what is assumed from the sedimentology and context.”
In the paper in PLOS ONE, the team identifies and describes remains from several genera found in the Eocene deposits at Dur At-Talah, including lungfish (Protopterus), bichirs (Polypterus), African aba knifefish (Gymnarchus), extinct elopiform fish Egertonia, a claroteid catfish Chrysichthys, mochokid catfishes, characiform tiger fish Hydrocynus, perciform Parachanna, cichlid Tylochromis, and at least six indeterminate actinopterygian taxa. Most of the indeterminate material are isolated teeth and fragments, described in detail in the paper.
When considering the diversity outlined in this study, a lot can be ascertained about the history of fishes in Africa. For example, in a previous post on the PLOS Paleo Community blog, I discussed the long evolutionary history of lungfishes, and a lot of that diversity can be found in the fossil record in Africa. So finding lungfish toothplates in this fossil assemblage was not surprising to the authors of the study. The toothplates belong to the African lungfish Protopterus, based on the number of crests on the tooth, but differences in the shape and size exclude it from belonging to any other known species of Protopterus, and likely belong to a new species.
Other fishes provided surprises for the authors. The morphology of the teeth of Gymnarchus sp. and Hydrocinus sp. are different from any other extinct or extant fishes, and are the earliest record of this tooth morphology in Africa. Likewise, the catfish fossils described in this study present the earliest record of these modern genera. Indeed, most of the fishes outlined in this study, with the exception of the bichir and lungfish, are some of the oldest records for these groups of fishes.
This assemblage fills a gap in the record of fossil fishes in Africa that spans 10 million years (and a lot can happen in 10 million years).
“This study has to be interpreted in association with other studies performed in subcontemporaneous levels in Egypt; together they will give new information on the late Middle/Late Eocene in northern Africa,” Otero said. “Paleogene outcrops with freshwater are scare in Africa, and each time a new assemblage is discovered it provides new information on the early diversification of modern taxa.”
And what does this fossil site at Dur At-Talah tell us about the biogeography of norther Africa at the time? “For the first time, we have an idea about the African freshwater fauna in the Late Eocene, at least in a region covering Libya and Egypt. They belong to a connected hydrographical stream system,” Otero told me.
“This information will be even more valuable when new outcrops will be available elsewhere in Africa to address drainage and basin limits during the Eocene.”
In what other ways is this study useful to ichthyologists? Otero said, “What is very important to me is to give [these fishes] a systematical attribution that is supported by the anatomical information available and that fits in extant members’ phylogenies. That means that this attribution can also be used to calibrate molecular clocks.”
Otero concludes, “I would say that a lot can be learned from a small assemblage of minute remains obtained by screening. They teach us about different taxa and also complement/increase information yielded by articulated fossil fishes that are traditionally solely collected (when available).”
“I wanted to join one of the field missions to complete the material and figure out the outcrop and sedimentology…but then the war [in Libya] started,” Otero told me, “I suspect that it will be a long time before [we can] go back safely. More work is needed over there. Considering the interest and diversity of the fossils yielded by screening, I suspect that further work will be valuable.”
Read the paper here, in PLOS ONE:
Otero O, Pinton A, Cappetta H, Adnet S, Valentin X, Salem M, et al. (2015) A Fish Assemblage from the Middle Eocene from Libya (Dur At-Talah) and the Earliest Record of Modern African Fish Genera. PLoS ONE 10(12): e0144358. doi:10.1371/ journal.pone.0144358