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The Evolution of Birds in the Early Cenozoic

Photograph of Septencoracias Morsensis, a Roller-Like Bird from the Danish Mo-Clay. Credit: Estelle Bourdon.

The fossil record of modern birds prior to the Cretaceous-Paleogene boundary is scarce and fragmentary. Early Cenozoic bird fossils are more abundant, but typically disarticulated and/or flattened. 

Photo 1: Photograph of the Mo-Clay in the Island of Fur. Mo-Clay sediments crop out in Northern Jutland, Denmark, and consist of 60 m thickness of diatomite interbedded with over 180 volcanic ash layers. Credit: Estelle Bourdon

This project has focused on 3D-preserved specimens from the Early Eocene Fur Formation (Mo-Clay) of Denmark, which are among the most exquisite Early Cenozoic fossil birds. Researchers report the oldest roller (Coracii), Septencoracias, based on a new fossil from the Fur Formation (about 54 million years ago). This specimen is one of the best preserved Eocene fossil birds. 

Living rollers are colourful birds found in warm climates in the Old World. The Mo-Clay biota indicates a tropical to subtropical environment in the Early Eocene. This discovery adds to the evidence that rollers had a widespread northern hemisphere distribution in the Eocene, with subsequent restriction of the group to the Old World tropics and subtropics. Septencoracias is the oldest significant record of a large avian group that includes rollers, kingfishers, hoopoes and woodpeckers. It provides a reliable calibration points for molecular clock studies. 

The study of avian evolution across geological time is part of a global project aimed at assembling the Tree of Life. Paleoenvironmental inferences based on studies of Mo-Clay faunas (including birds) may have significance for elucidating long-term effects of the current climate change.

Introduction

Photo 2: Photograph of Lithornis vulturinus (Lithornithidae) from the Danish Mo-Clay. Lithornithidae were flying palaeognathous birds that formed part of the Northern Hemisphere bird faunas in the Paleocene and Eocene. They resemble living tinamous (Tinamidae) from South America. Credit: Estelle Bourdon.

Although some aspects of the evolutionary radiation of modern birds (Neornithes) are now well understood, phylogenetic relationships among living birds and their extinct relatives are still hotly debated. Recent molecular works indicate that most major avian lineages originated in the Cretaceous, but well-preserved fossils of this age that substantiate this hypothesis are exceedingly rare. In contrast, the fossil record of birds is much more abundant after the Cretaceous/Paleogene boundary. Several key deposits of Paleocene and Eocene age have been documented across Europe, North America and North Africa, but the avian fossils that they contain are typically disarticulated and/or flattened.

The Fur Formation (Mo-Clay) of Denmark is a unique marine deposit of Early Eocene age (55.8 to 54 million years old), which is famous for its spectacular preservation of numerous fossils, including insects, fishes, turtles and birds. Fossil birds from the Early Eocene Fur Formation represent the earliest post-Cretaceous bird fauna with early representatives of over 10 major groups of modern birds. The three-dimensionally preserved specimens from the Mo-Clay are among the best preserved and most complete early Cenozoic fossil birds.

Photo 3: Septencoracias morsensis, a roller-like bird from the Danish Mo-Clay: (a) Photograph; (b) 3D white light scanning; (c) Interpretative drawing. Fish remains located in the abdominal region of the fossil are interpreted as preserved stomach content. Credit: Estelle Bourdon.


The present project mainly focuses on the fossil birds from the Early Eocene Mo-Clay. The project has been housed at the Section of Biosystematics, Natural History Museum of Denmark (SNM) at the University of Copenhagen. The material is deposited in the collections of the SNM, Fur Museum and Moler Museum. The study has also incorporated some fossil birds from other Early Cenozoic fossiliferous localities, such as the La Borie clay deposits (France). Phylogenetic assessments of these well-dated fossils constitute powerful tools for elucidating early branches in the tree of modern birds.

A Roller-Like Bird from the Early Eocene of Denmark

The main result of this project documents one of the best preserved bird fossils of the Eocene Epoch. The new Mo-Clay fossil is approximately 54 millions years old (based on radiometric dating), and lived around 12 million years after the Cretaceous/Paleogene extinction event.

“The new fossil bird is a nearly complete, three-dimensionally preserved and articulated skeleton, which is exceptional. The fossil even preserves soft tissue remains and stomach content” says Dr. Estelle Bourdon, who has received a postdoctoral fellowship from the Carlsberg Foundation to carry out the research project THE EVOLUTION OF BIRDS IN THE EARLY CENOZOIC.

“The fossil was found in 1986 during a geology field course in an abandoned diatomite quarry on the Island of Mors. I pointed to a large block that had fallen from the vertical wall, so that students could identify the ash layers in this laminated limestone boulder. The students saw the preserved bird skull exposed on the surface of the boulder along with a few neck vertebrae.” said Dr. Niels Bonde, the main collaborator of this study.

Photo 4: Life reconstruction of Septencoracias morsensis from the Danish Mo-Clay. The fossil is quite small compared to most living species of rollers. However, its skull is larger than in other rollers, and is more similar in proportion to that of kingfishers. Credit: Estelle Bourdon.

The publication finds that this new species, Septencoracias morsensis, is the oldest representative of the rollers (Coracii). Living species of rollers are medium-sized birds with brightly coloured feathers, large heads and stout beaks. They are restricted to the Old World, and all species occur in tropical to subtropical regions, except the European roller and the Dollarbird, which also occur in the temperate zone.

The study also reveals that Septencoracias is the oldest substantial record of a large avian group that includes rollers, kingfishers, bee-eaters, motmots, hoopoes, hornbills and woodpeckers (Picocoraciae). As such, it provides a reliable calibration points for molecular clock studies.

The Danish Mo-Clay was deposited just after the Paleocene-Eocene Thermal Maximum (PETM). The Early Eocene climate was tropical to subtropical in the Northern Hemisphere, as indicated by the Mo-Clay fauna and flora. The new species from the Mo-Clay provides evidence that the rollers (Coracii) had a widespread northern hemisphere distribution in the Eocene, with subsequent restriction of the group to the Old World tropics and subtropics. This pattern occurs recurrently in modern birds, in which groups now restricted to low latitude tropical environments were present in higher latitudes in the Eocene, when temperatures were several degrees warmer than they are today.

Photo 5: Photograph of the upper beak of Gastornis parisiensis from the Early Eocene of La Borie (France). Gastornis was a giant flightless bird related to ducks and geese. This two-metre-tall herbivorous bird was widespread in the Northern Hemisphere in the Early Eocene. Credit: Estelle Bourdon.

Scientific Social Responsability

Elucidating some aspects of the evolutionary tree of birds is part of a more global project aimed at assembling the ‘Tree of Life’. Having a reliable Tree of Life is a worthwhile attempt, because it makes it possible for us to: 1) Understand the huge numerical, morphological, genetic and ecological diversity of the Living World; 2) Trace how every living species evolved over time from a common stem; 3) Appreciate the origin of ourselves as a species; 4) Communicate unambiguously about any group of living organisms; 5) Set priorities in the conservation and management of living resources.

Mo-Clay Fauna and Climate Change

The Stolleklint Clay, which is located just below the Mo-Clay, represents a unique global warming event that defines the PETM. Therefore, Mo-clay fauna and flora record the immediate aftermath of this global warming event. Many modern groups have their oldest known representatives in the Fur Formation, as exemplified above. Further studies of Mo-Clay fauna (including birds) and flora can be used for reconstructing Early Eocene environments, in conjunction with sedimentological studies. These paleoenvironmental inferences may have significance for elucidating long-term effects of the current climate change.

Conclusion

“This project has been an excellent opportunity to develop collaborations with Danish researchers. Research in a foreign institution is critical for building an international network and a strong research profile. The financial support I have received from the Carlsberg Foundation allows me to improve the knowledge of avian evolution across geological time. In turn these results can provide key elements for better understanding how today’s avian biodiversity may evolve in response to environmental change. Our project also contributes to Danish excellence, because it is based on Danish natural history objects of outstanding scientific value. Our work is being presented this year at the UNESCO World Heritage Symposium in Glyngøre (Denmark), which aims to highlight the potential Outstanding Universal Value of the Fur Formation Lagerstätten”, Estelle Bourdon concludes.