Thursday, July 28, 2011

Taxonomic Revisions To Iconic Species: Wood Warblers and Fossil Birds

Yesterday the AOU published its 52nd Supplement to the AOU Checklist (pdf). Each year, the supplement contains additions, splits, lumps, and other revisions made by the AOU's checklist committee at its spring meeting. There are a number of significant changes in this year's supplement. One of them is a split in the moorhens: the Eurasian form retains the name Common Moorhen (Gallinula chloropus), while the American birds become Common Gallinule (Gallinula galeata).

Formerly Dendroica coronata, now Setophaga coronata / Photo by Frank Miles (USFWS)
The change likely to cause the most controversy is a full-scale revision of the wood warbler family (Parulidae). The move has been rumored for almost a year since the publication of a proposed reclassification of the family based on genetic analysis. I have avoided talking about the proposed changes since I do not have access to the original research informing the changes, but there are several summaries on the web. A good discussion of the reasoning behind the changes is at Biological Ramblings. There are some additional diagrams of the changes on David Sibley's blog. The short version is that genetic relationships do not match the traditional division of the wood warbler family into genera. The result is a reordering of the genera, with some being folded into others.

Here is the ordering of the wood warbler genera as they now stand:
  • Seiurus: This genus has one species, Ovenbird, which is unlike most of the other warblers in its appearance and habits.
  • Helmitheros: Another genus with one species, Worm-eating Warbler.
  • Parkesia: The two waterthrush species, which were split from Seiurus last year.
  • Vermivora: This genus lost several species to a split last year and now contains only the extinct Bachman's Warbler and the closely-related Blue-winged and Golden-winged Warblers.
  • Mniotilta: One species, Black-and-white Warbler.
  • Protonotaria: One species, Prothonotary Warbler.
  • Limnothylpis: Yet another genus with only one species, this time Swainson's Warbler.
  • Oreothlypis: This genus was created last year with most of the former Vermivora warblers and two members of the Parula genus.
  • Leucopeza: One species, the possibly-extinct Semper's Warbler.
  • Oporornis: Most of the former members of this genus were moved into the genus below; it now contains only the Connecticut Warbler, which has the unusual habit (for a warbler) of walking on the ground.
  • Geothlypis: Traditionally this genus only the yellowthroats. Now it also contains the species exiled from Oporornis (like Mourning and Kentucky Warblers).
  • Setophaga: While originally this was a small genus, containing only the American Redstart, it now contains all of genus Dendroica and two species of genus Parula, as well as the Hooded Warbler. Even though this is basically the old Dendroica genus, it is called Setophaga because that name has priority (i.e., it was published earlier in the historical record). The move results is an old situation where the family Parulidae no longer has a matching genus Parula, and the name Dendroica disappears from the checklist.
  • Myiothlypis: One species in North America, Buff-rumped Warbler. There are more species in this genus, but most of them are in South America.
  • Basileuterus: This is another Neotropical genus. This genus and Myiothlyis were traditionally considered a single genus.
  • Cardellina: This genus contains the Red-faced Warbler, as well as two members from Wilsonia and two from Ergaticus
  • Myioborus: These are the Neotropical redstarts and whitestarts. 
  • Zeledonia: Wrenthrush, a Neotropical species.
  • Icteria: One species, Yellow-breasted Chat, an oddball among warblers.
  • Xenoligea: One species, White-winged Warbler.
  • Microligea: One Neotropical species, Green-tailed Warbler.
  • Teretistris: Two Neotropical species.
The last few genera, including the Yellow-breasted Chat, may not belong in Parulidae at all, but the AOU has held off reclassifying them, perhaps until there is better evidence about where they truly belong.

Over the past few months, several people have contacted me out of concern about the changes, especially the removal of the name Dendroica from the official taxonomy. I have been using the name "dendroica" as an internet handle since 2005, on this blog, on Twitter, and elsewhere, so I am sad to see that it will not longer match a warbler genus. At the same time, taxonomy is constantly subject to revision as new relationships are discovered, and the best available science today may be overturned tomorrow.

On the same day that the AOU published its checklist supplement revising the wood warbler family, another iconic species had its traditional placement questioned. A newly-discovered fossil species, named Xiaotingia, suggests that Archaeopteryx may not belong to the bird lineage, but to a parallel group, the deinonychosaurs. Ed Yong explains the consequences of such a change:
Meanwhile, others have noted that Archaeopteryx has many features that are only found among the deinonychosaurs. For example, they share a distinctive hip bone, and they both have a large hole above their noses (the “premaxillary fenestra”) that other birds and dinosaurs lack. Any many of the features that supposedly characterise Archaeopteryx and other birds, such as feathers, a wishbone and long powerful forearms, are also found in deinonychosaurs.

This is more than just a matter of shuffling cards. Archaeopteryx’s position has been so sacrosanct that its body had guided many of our ideas about the origins of birds. It grounds our understanding of this group. For example, it was previously thought that most primitive birds and their closest dinosaur relatives had lightly built skulls, which might have been useful for flight. “Instead, our study suggests that primitive birds had robust and rigid skulls,” says Xu.

“Our study also suggests that most primitive birds were herbivorous animals,” he says. “Previous studies which suggested that flight evolved in the [meat-eating] context (such as wings evolving for catching prey) need reconsideration.” Witmer writes, “Clearly, without the safety net of good old Archaeopteryx at the base of the birds, we’ve got some fresh work to do.”

But Gerald Mayr, who studies fossil birds at Germany’s Sneckenburg Museum, is unimpressed with the new discovery. “I fear that it is a bit hyped and that the conclusions are not as novel as the authors claim,” he says. Mayr is one of several palaeontologists who think that the deinonychosaurs are actually birds themselves. According to him, they’re flightless members of a group that includes Archaeopteryx and modern birds, like smaller extinct versions of today’s ostriches and emus.
See his blog for pictures of the new species, Xiaotingia, and diagrams of the new proposed taxonomy.

Whether or not the authors are correct in their placement of Archaeopteryx among the deinonychosaurs, it is clear that there were a lot of proto-birds and bird-like creatures and deciding which deserve to be called "birds" and which "dinosaurs" is not necessarily easy. There are relatively few fossils at that point in the lineage, so there is potential for one or two discoveries to disrupt the current alignment. Wherever Archaeopteryx belongs on the new taxonomic tree, I think it is still safe to see it as symbolic of the evolutionary relationship between birds and dinosaurs, even if it does not belong in the direct lineage of modern birds.