Bird tracks from the Green River Formation (Eocene) of Utah: ichnotaxonomy, diversity, community structure and convergence

ABSTRACT

Abundant well-preserved bird tracks from lacustrine Green River Formation (Eocene) deposits in Utah, are diverse and paleoecologically significant, but remain poorly known. Three of the four previously reported morphotypes (A-D) are named here for the first time with two representing new ichnotaxa. In order of increased footprint length (L) the ichnotaxa are: cf. Avipeda phoenix (L = 2.0 cm); Quadridigitus semimembranus ichnogen. et ichnosp. nov. (L = ~3.5 cm); Jindongornipes falkbuckleyi ichnosp. nov. (L ~ 8.0 cm); and previously named, web-footed Presbyornithiformipes feduccii (L = 9.5 cm). Collectively these avian ichnotaxa, associated invertebrate traces including ubiquitous nematode trails (Cochlichnus), and other rarer, small tetrapod tracks represent the “shorebird” ichnofacies or ichnocoenosis. However, the local track assemblages from the Uinta Basin do not correspond to the avian body fossil record from multiple Green River basins in the larger region. Thus, both records are essential for a cumulative picture. The Green River ichnotaxa are morphologically similar to those from lake basins in the Korean Cretaceous, and modern shorebird assemblages. As modern shorebirds are not known from Cretaceous or Eocene body fossils, the most plausible explanation is community convergence, among taxonomically disparate, but ecologically convergent trackmakers. Community convergence concepts help refine the ichnofacies paradigm.

KEYWORDS:

• Eocene
• bird tracks
• ichnofacies
• community convergence

Acknowledgments

We thank Jacob Van Veldhuizen University of Colorado Museum, Julia McHugh, Museum of Western Colorado, and Kenneth Carpenter, Prehistoric Museum, Price Utah for help with access to specimens and catalogue information used in this study. Unless otherwise stated, the tracks illustrated in this paper mostly originate from USDA Forest Service land at the Soldier Summit site, Utah County, Utah, traditionally identified as the U-3 site (Grande 1984) and catalogued as the UCM 2012121 locality. We thank Bruce Schumacher for his advice on referring to this site in print and acknowledge the jurisdiction of the USDA Forest Service over the fossil resources. The Paleontological Resources Protection Act (PRPA) of 2009 does not permit publication of precise locality information, regardless of previous practice prior to 2009. We thank Ricardo Melchor, Universidad Nacional de La Pampa, Argentina, Gareth Dyke, editor of Historical Biology, and an anonymous reviewer for helpful comments which led to improvements of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.