Phylogeny of "core Gruiformes" (Aves: Grues) and resolution of the Limpkin-Sungrebe problem

Opinions on the systematic relationships of birds in the avian order Gruiformes have been as diverse as the families included within it. Despite ongoing debate over monophyly of the order and relationships among its various members, recent opinion has converged on the monophyly of a "core" group of five families classified as the suborder Grues: the rails (Rallidae), the cranes (Gruidae), the Limpkin (Aramidae), the trumpeters (Psophiidae), and the finfoots (Heliornithidae). We present DNA sequence data from four mitochondrial (cytochrome b, 12S rRNA, Valine tRNA, and 16S rRNA) and three nuclear loci (intron 7 of beta-fibrinogen, intron 5 of alcohol dehydrogenase-I, and introns 3 through 5 of glyceraldehyde-3-phosphate dehydrogenase) to test previous hypotheses of interfamilial relationships within Grues, with particular attention to the enigmatic family Heliornithidae. Separate and combined analyses of these gene sequences confirm the monophyly of Grues as a whole, and of the five families individually, including all three species of Heliornithidae. The preferred topology unambiguously supports relationships among four of the five families, with only the position of Psophiidae remaining equivocal. Bayesian "relaxed-clock" dating methods suggest that the divergences of the three heliornithid species occurred in the mid-Tertiary, suggesting that their present disjunct pantropical distribution is a result of early- to mid-Tertiary dispersal.