Recently, paleontologists Dr. John-Paul Zonneveld, Dr. Sarah Naone, and Dr. Brooks Britt described the discovery and classification of four new ichnotaxa (fossilized trace taxa) from the Eocene mudflat successions of Utah.
The study, published in the Journal of Paleontology, describes the morphologies and behavioral implications of these traces found in close association with the footprint taxon Presbyornithiformipes feduccii. These trace fossils are interpreted as recording foraging strategies employed by lake-margin waterbirds searching for prey.
The fossils were recovered from the Green River Formation, which spans parts of Utah, Wyoming, and Colorado, and were formed around the major mountain-building event called the Laramide Orogeny. Despite not being directly dated, the fossils are estimated to have been made between the middle-late Wasatchian (55.4 to 50.3 Ma) and the Bridgerian (50.3 to 46.2 Ma).
The fossils were all collected and donated by the Gunther family, with the exception of one fossil collected and later published by researcher Dr. Erickson in 1967.
“The spectacular, original foraging traces were found by a student during a class field trip looking at outcrops of the Green River Formation, but the class did not find the horizon. They showed Lloyd Gunther where the specimen was found. He found the upslope track horizon and collected a number of tracks, most of which were then donated to BYU beginning in 1970. Most were donated circa 1972 and 1973. The last batch was donated by Val Gunther in 2023,” says Britt.
After their donation, the fossil trackways were collectively analyzed, and footprints were assigned the ichnotaxon Presbyornithiformipes feduccii, while the trace marks were collectively referred to as “dabble marks.”
However, according to Zonneveld, “dabble marks” was a rather inaccurate description that did not properly encompass the feeding strategies employed by this waterbird.
“We chose not to call them ‘dabble marks’ out of interest in accuracy. Although some researchers use the term for any waterbird or shorebird feeding trace, that is technically inaccurate. Dabbling refers to a very specific behavior practiced by a sub-group of the Family Anatidae (which, not surprisingly, are referred to as ‘dabbling ducks’),” explained Zonneveld.
“Most actual dabbling is conducted while the bird is swimming and does not involve interaction between the animal and the substrate surface and thus would leave no trace fossils.
“The word actually has a couple of meanings, depending on if it is used by North American or British ornithologists. Here, it is most commonly used for birds grasping food in the water with their bills and sieving a mouthful of water with structures in their bills called lamellae.
“The term is also used for birds which ‘upend’ (which is what you see when you see ducks floating with their butts and legs in the air and their heads underwater). During upending, the birds may grasp mouthfuls of sediment from the substrate surface and, again, sieve the sediment for food particles.
“Traces emplaced during this feeding would look like clusters of ovoid pits on the sediment surface and would not be associated with bird footprints. They would likely be misinterpreted in the rock record unless someone was well-associated with waterbird feeding behavior.”
Thorough investigation of the trace marks led to the identification of four new ichnospecies (trace fossil taxa): Erevnoichnus blochi, E. strimmena, Ravdosichnus guntheri, and Aptosichnus diatarachi.
The four foraging behaviors encompassed the bird poking its head into the mud, then sweeping its bill side to side, gaping its mouth to expose buried (R. guntheri), “bill-stirring” which encompassed agitating the sediment with random, vigorous sweeping motions (Aptosichnus diatarachi), and probing via a behavior called touch-base probing which involved moving the head vertically (up and down) as well as oblique-lateral movements (E. strimmena).
Based on the bill morphology and associated footprints, the researchers hypothesized that the now-extinct Presbyornis pervetus (a relative of the modern duck) was likely responsible for these trace fossils.
However, early studies of this fossil species suggested these birds were filter feeders, similar to modern ducks. This interpretation was based on their bill anatomy as well as the presence of lamellae on their bills. However, more recent studies have suggested that the quadrate bone (a skull bone critical for jaw movement) was unlike modern filter-feeders and would have hindered their filter-feeding capabilities.
The study by Zonneveld and his colleagues may provide evidence supporting the hypothesis that these birds were instead tactile feeders, foraging along shorelines instead of feeding on microorganisms in the water like modern filter-feeders such as flamingos and some ducks.
More information: John-Paul Zonneveld et al, Waterbird foraging traces from the early Eocene Green River Formation, Utah, Journal of Paleontology (2025). DOI: 10.1017/jpa.2023.49
Journal information: Journal of Paleontology
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