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 mikkelseniof 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 inS. 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, includingS. mikkelseniand 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 andS. mikkelsenialternatively 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 andS. mikkelseniare 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.

Highlights

  • There is a growing consensus around the majority of relationships among avian subclades, positions of several subclades have been persistently debated

  • Unconstrained parsimony analysis resulted in 69 most parsimonious trees (MPTs) of 5,147 steps. This analysis recovered a paraphyletic Gruiformes with respect to Charadriiformes, with Nahmavis grandei and Scandiavis mikkelseni being placed as the sister-taxa of all included Charadriiformes (CI = 0.166, RI = 0.475, RC = 0.079, HI = 0.834)

  • Messelornithidae was within a clade sister to extant Ralloidea across all analyses. This is consistent with the three derived characters described by Mayr (2004) as evidence of a monophyletic (Messelornithidae + (Rallidae + Heliornithidae)) clade and with the phylogenetic analyses of Musser et al (2019) which recovered a Messelornithidae + Ralloidea clade

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Summary

Introduction

There is a growing consensus around the majority of relationships among avian subclades, positions of several subclades have been persistently debated. A Charadriiformes + Gruiformes clade was recovered using a variety of datatypes and analytical approaches including recent large genome-based datasets (e.g., Cracraft, 1988; van Tuinen and Hedges, 2001; Livezey and Zusi, 2007; Bertelli et al, 2011; Jarvis et al, 2014; Musser and Cracraft, 2019, primary analysis of Kimball et al, 2019). It is not recovered in other large molecular datasets (Hackett et al, 2008; Prum et al, 2015; Reddy et al, 2017, additional analyses of Kimball et al, 2019)

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