Abstract

The stem lineage relationships and early phenotypic evolution of Charadriiformes
(shorebirds) and Gruiformes (rails, cranes, and allies) remain unresolved. It is still
debated whether these clades are sister-taxa. New phylogenetic analyses incorporating
Paleogene fossils have the potential to reveal the evolutionary connections of these
two speciose and evolutionarily critical neoavian subclades. Although Gruiformes have
a rich Paleogene fossil record, most of these fossils have not been robustly placed.
The Paleogene fossil record of Charadriiformes is scarce and largely consists of
fragmentary single elements. Only one proposed Eocene charadriiform-like taxon,
Scandiavis mikkelseni of Denmark, is represented by a partial skeleton. Here, we
describe a new species from the early Eocene Green River Formation of North America
comprising a partial skeleton and feather remains. Because the skeleton lacks the
pectoral girdle and forelimbs as in S. mikkelseni, only features of the skull, axial skeleton,
and hind limb are available to resolve the phylogenetic placement of this taxon. These
anatomical subregions initially showed features seen in Charadriiformes and Gruiformes.
To assess placement of this taxon, we use a matrix consisting of 693 morphological
characters and 60 taxa, including S. mikkelseni and the oldest known charadriiform
taxa represented by single elements. These more fragmentary records comprise two
distal humeri from the earliest Eocene Naranbulag Formation of Mongolia and the early
Eocene Nanjemoy Formation of Virginia. Our phylogenetic analyses recover the new
taxon and S. mikkelseni alternatively as a charadriiform or as a stem-gruiform; placement
is contingent upon enforced relationships for major neoavian subclades recovered
by recent molecular-based phylogenies. Specifically, when constraint trees based on
results that do not recover Charadriiformes and Gruiformes as sister-taxa are used, the
new taxon and S. mikkelseni are recovered within stem Gruiformes. Both Paleogene
fossil humeri are consistently recovered within crown Charadriiformes. If placement of
these humeri or the new taxon as charadriiforms are correct, this may indicate that
recent divergence time analyses have underestimated the crown age of another major
crown avian subclade; however, more complete sampling of these taxa is necessary,
especially of more complete specimens with pectoral elements.