Sterling J. Nesbitt & Julia A. Clarke (2016)
The anatomy and taxonomy of the exquisitely preserved Green River Formation (early Eocene) lithornithids (Aves) and the relationships of Lithornithidae.
Bulletin of the American Museum of Natural History 406: 1-91
Fossil remains of Paleogene Palaeognathae are poorly documented and are exceedingly rare. One group of palaeognaths, the lithornithids, is well represented in the Paleogene of North America. Nevertheless, few specimens of the same species are known from each of those Paleogene geologic units. Here, we report five new partial skeletons of lithornithids from the Fossil Butte Member of the Green River Formation (early Eocene) of Wyoming. One spectacularly preserved specimen is identified as the holotype of a new species, Calciavis grandei, gen. et sp. nov., and fully described. Preserved soft tissues (e.g., feathers, pes scales) surround the nearly articulated and complete skeleton. A second well-preserved but disarticulated skeleton is referred to this new taxon. We conclude that there are only two lithornithid taxa in the Green River Formation after careful comparisons with the other known taxon from the same geological unit, Pseudocrypturus cercanaxius. Morphological data generated from the new taxon and other Green River Formation lithornithid specimens were integrated into a osteology-only phylogenetic data set containing stem avians as outgroups and extinct and extant members of Palaeognathae (Tinamidae, ratites) and Neognathaes (Anseriformes, Galliformes, Neoaves), unnamed lithornithid specimens, and the following named lithornithid taxa: Lithornis plebius, Lithornis promiscuus, Lithornis celetius, Paracathartes howardae. We find a monophyletic Lithornithidae (containing Calciavis grandei, Pseudocrypturus cercanaxius, Lithornis plebius, Lithornis promiscuus, Lithornis celetius, Paracathartes howardae) as the sister taxon of Tinamidae at the base of Palaeognathae and also recover a monophyletic Ratitae in the morphology-only analysis. A Lithornithidae-Tinamidae relationship, which could imply a broad Northern Hemisphere distribution in the Paleogene for this total group retracted to the present day Neotropical distribution after the Eocene, is weakly supported in our analysis and is also supported by other lines of evidence such as eggshell morphology. Relationships among flightless palaeognaths and assessment of character homology in this group remain problematic. Indeed, when the morphological analyses were constrained to enforce topologies recovered from all recent analyses of molecular sequence data and retroelement insertions, Lithornithidae is no longer recovered with Tinamidae, which is nested within the now paraphyletic ratites, but remains at the base of Palaeognathae. Thus, regardless of the position of Tinamidae, Lithornithidae is recovered at the base of the clade. However, evidence that many, if not all, of these "ratite" lineages independently evolved similar morphologies related to large size and flight loss suggests that the proposed position of the Lithornithidae remains tentative. Significant morphological variation within Lithornithidae should be captured in inclusive future analyses through use of species terminals.