Gruiformes

• 1. Introduction

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  Gruiformes is an order of birds whose very name means "crane form" - and yet the cranes themselves are only one small corner of a group that has spent more than a century resisting easy classification. Fifteen species of large cranes share this order with roughly 145 species of smaller crakes and rails, plus a scatter of families so unusual that scientists could not figure out where else to put them. That problem of placement turns out to be the central story of the Gruiformes: a tidy-looking taxonomic group that dissolved, over decades of study, into something far stranger. Who actually belongs here? What does it mean when a group of birds turns out not to be a group at all? And what became of the outsiders who were quietly shunted in and then quietly shown the door? The answers reach back to the Late Cretaceous, run through a German anatomist's influential 1888 classification, and are still being worked out today.

• 2. A Taxonomy of Convenience

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  Max Fürbringer, a German avian comparative anatomist, established the traditional order Gruiformes in 1888. His approach was logical given the tools of the time: bird families that did not seem to fit anywhere else were gathered together on anatomical grounds. The group that resulted included not only cranes and rails but also the limpkin, the trumpeters of the family Psophiidae, the finfoots and sungrebe of the Heliornithidae, and a range of other families each containing only one to three species. For decades, ornithologists added to this miscellany rather than questioning it. It was not until the review work of Olson in 1985 and Sibley and Ahlquist in 1990 that the accumulated doubts were formally surveyed. Researchers had noticed, for instance, that the sunbittern might actually be related to herons, and that the seriemas might be closer to cuckoos. The suspicion was growing that the Gruiformes held together not because its members were genuinely related, but because nobody could think of a better place to put them.

• 3. Dismantling the Order

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  Olson and Steadman in 1981 were the first to correctly remove any family from the traditional Gruiformes. They showed, using skeletal characters, that the Australian plains-wanderer of the family Pedionomidae was actually a member of the shorebirds, the order Charadriiformes. DNA-DNA hybridization work by Sibley and Ahlquist in 1990 confirmed this finding and also removed the button-quails of the family Turnicidae from the Gruiformes on the basis of large genetic distances to other members of the group. Later work by Paton and colleagues in 2003 and 2004, and by Fain and Houde in 2004 and 2006, established the correct placement of buttonquails within Charadriiformes through phylogenetic analysis of multiple genetic loci. A broader picture emerged in 1997 when Houde and colleagues used 12S ribosomal DNA sequences to present the first molecular genetic evidence that the Gruiformes as traditionally defined were polyphyletic - meaning the group's members do not all share a common ancestor unique to them. By the time Fain and Houde published again in 2004, followed by Ericson and colleagues in 2006 and Hackett and colleagues in 2008, the evidence was decisive: the traditional Gruiformes consist of between five and seven unrelated clades.

• 4. Core and Periphery

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  What remained after the dismantling is a smaller, genuine group called the core Gruiformes, organized under the suborder Grues. Rails of the family Rallidae, flufftails of the Sarothruridae, finfoots and sungrebe of the Heliornithidae, the adzebills of the Aptornithidae, trumpeters of the Psophiidae, the limpkin of the Aramidae, and cranes of the Gruidae all belong here as genuine relatives. Sitting separately in the suborder Eurypygae are the kagu of the Rhynochetidae and the sunbittern of the Eurypygidae - birds that, as the source text puts it, are not even remotely related to Grues. Fain and Houde in 2004 proposed that all Neoaves - the great radiation of modern birds - can be divided into two large clades called Metaves and Coronaves. The sunbittern, kagu, and mesites all fall within Metaves, while the remaining lineages once called Gruiformes group either among waterbirds or among landbirds within Coronaves. This Metaves-Coronaves division was supported by combined analyses of as many as 30 independent genetic loci by Ericson and colleagues in 2006 and Hackett and colleagues in 2008, though it depends on the inclusion of specific loci and is contradicted in part by mitochondrial DNA data.

• 5. The Kagu and Sunbittern

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  Among the most striking reclassifications concerns the kagu and the sunbittern, which are now understood to be each other's closest relatives. Joel Cracraft proposed in 2001 that these two birds, together with the recently extinct adzebills of New Zealand, form a distinct Gondwanan lineage - a remnant of the ancient southern supercontinent. That idea proved only partly right. Subsequent work by Houde and colleagues established that the adzebills are actually members of the Grues, firmly within core Gruiformes, not a Gondwanan outlier. More importantly, the sunbittern and kagu are believed to have diverged from one another long after the break-up of Gondwanaland itself, which undercuts the Gondwanan hypothesis for their shared origin. Together they now form the basis for a proposed new order, Eurypygiformes. Their case illustrates how a plausible biogeographic story - two strange birds on opposite sides of the world, descended from Gondwana - can dissolve under molecular scrutiny.

• 6. Fossil Families and Deep Time

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  Many families known only from fossils were historically assigned to the Gruiformes, among them the Ergilornithidae, Phorusrhacidae, Messelornithidae, Eogruidae, Idiornithidae, and Bathornithidae. Several of these are now recognized as belonging to a distinct order, Cariamiformes. The terror birds of the family Phorusrhacidae, for instance, fall within Cariamiformes rather than core Gruiformes. No completely extinct family can be confidently assigned to core Gruiformes, despite some being superficially crane-like. The divergence of "gruiform" lineages is proposed to predate the Cretaceous-Paleogene extinction event of roughly 66 million years ago, making it among the earliest splits within Neoaves - though no unequivocal basal gruiforms are known from the fossil record. Specific genera remain tantalizing: Propelargus from the Late Eocene or Early Oligocene of Quercy in France may be a cariamid or idiornithid; Badistornis from the Middle Oligocene of Shannon County, Missouri, may be an aramid; and Aramornis from the Middle Miocene of Snake Creek Quarries in the United States may be a gruid or aramid. The question marks attached to each of these names reflect how difficult it is to place fragmentary fossil material within a group whose living members have themselves only recently been properly sorted out.