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Mike Keesey  

Why Just Avians?

November 1st, 2005 by Mike Keesey :: Comment »

We’ve all heard that the dinosaurs went extinct at the end of the Cretaceous, but that’s not entirely true: modern birds (a.k.a. crown clade avians, or neornitheans) did survive (and, at about 9,000 extant species, are doing quite well today).

It’s been debated for a while whether the crown clade (i.e., the last common ancestor of the living forms, plus all descendants thereof) originated before or after the K/T boundary. There hasn’t been a great deal of evidence either way. If it originated afterward, then there was only one avian progenitor species that made it past the Cretaceous, into the Paleogene. But if it originated earlier, there would have to be several species surviving across the boundary.

Some Cretaceous fossils have been identified as members of crown clade lineages, which would push the origin of crown clade Aves back into the Cretaceous, but these are typically fragmentary (e.g., the supposed Cretaceous loriid psittaciform, known from one mandible), or have turned out to belong elsewhere (e.g., hesperornitheans are not loon [gaviiform] relatives, but more distant relatives of crown clade avians).

But earlier this year, a new Antarctican species was named: Vegavis iaai. Cladistic analysis places it within Anseriformes, the avian group including ducks, geese, and screamers. According to the most widely accepted phylogenetic trees, this means that, at the very least, five species made it across the boundary: the ancestors of Paleognathae (ratites and tinamous), Galliformes (chickens, grouse, etc.), Anhimae (screamers), Anseres (ducks and geese), and Neoaves (other birds).

But while these five or more species made it across, all other dinosaurs died out, including many types of bird: the aquatic hesperornitheans, the gull-like ichthyornithids, and the highly diverse enantiornitheans. Why did crown clade avians make it when all these other forms did not?

We don’t yet know.

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