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

Abstract

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.