Fossils reveal intriguing behaviour of a prehistoric water bird
Trace fossils from the Green River Formation in Utah reveal the intricate behaviour of a prehistoric water bird, previously thought to filter feed like the modern duck.
Researchers reveal the feeding behaviours of a proposed species of wetland-dwelling prehistoric bird proposed to have lived sometime between the middle-late Wasatchian (55.4 to 50.3 million years ago, Ma) and the Bridgerian (50.3 to 46.2 Ma). The research team described the discovery of the ichnotaxon footprints, Presbyornithiformipes feduccii, which took place in the 1900s, and the morphology of the fossilised traces.
The discovery
Except for one specimen identified by Dr. Erickson in 1967, the Gunther family gathered and donated the majority of the fossil specimens. The original trace fossils were discovered in the Green River Formation (Utah) by a student during a school trip. Lloyd Gunther found and gathered additional fossils, which were subsequently presented to Brigham Young University between 1970 and 2023. After examining the fossils, scientists were able to determine the feeding marks and footprints that the prehistoric birds had left behind.
Inferring behaviour
The team arrived at four new ichnospecies (distinct fossilised traces left by an extinct species of animal). Based on their commonalities, the researchers categorised the ichnospecies (including Aptosichnus diatarachi, Ravdosichnus guntheri, Erevnoichnus blochi, and Erevnoichnus strimmena) placing them into three larger groups (ichnogenera). Consider it similar to identifying many footprint forms and classifying them into broad groups, like "reptile tracks" or "bird tracks," but with unique traits for each. These discoveries aid scientists in understanding past behaviours and surroundings since trace fossils are categorised based on their shape and pattern and not the specific species that produced them.
The investigators of the present study contend that referring to the traces as "dabble marks" is inaccurate because dabbling is a particular feeding habit that is often observed during swimming so could not possibly leave any kind of fossilised trace. To feed on silt, some birds flip upside down, tipping forward to leave their bums and tails in the air—which might result in pit clusters but no imprints. These traces may easily be misread if one is not familiar with the behaviour of water birds. The researchers opted to completely omit the term "dabble marks" to maintain scientific accuracy.
The researchers believe that the four fossilised bird foraging behaviors: head poking/sweeping (E. blochi), bill-stirring (A. diatarachi), forward probing (E. strimmena) and bill-dragging (R. guntheri), was achieved by a relative of the extant duck, Presbyornis pervetus (P. pervetus).
Initially, the scientific community believed that P. pervetus filter fed like ducks, which was deduced from observations of bill structure. However, the latest research suggests that P. pervetus might have been a tactile feeder, using its bill to locate micro- and macro-scopic through sediments rather than filter feeding, as was once supposed.
News reference
Waterbird foraging traces from the early Eocene Green River Formation, Utah. February 2025. Journal of Paleontology. Zonneveld, J.; Naone, S., and Britt, B.