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Re: New Antarctic findings of Upper Cretaceous and lower Eocene loons

Not having seen the paper yet I cannot say much about this. However, Chatterjee 
seems convinced that at least some Polarornis material = Vegavis, which would 
make it anseriform instead of gaviiform.



David Černý <david.cerny1@gmail.com> schrieb am So, 8.11.2015:

 Betreff: Re: New Antarctic findings of Upper Cretaceous and lower Eocene loons
 An: dinosaur@usc.edu
 Datum: Sonntag, 8. November, 2015 10:14 Uhr
 If confirmed, the
 conclusions of this paper would have pretty
 far-reaching implications for the early
 evolution of modern birds. For
 phylogenomic dating analyses suggest that loons diverged
 from their extant sister group in the Paleocene
 (Jarvis et al. 2014;
 Prum et al. 2015), so
 this would be one of the few cases in the bird
 tree of life where molecular data underestimate
 (rather than
 overestimate) the age of a node
 relative to the fossil record.
 It would also mean that the earliest phase of
 diversification, which corresponds
 to a "bush" of extremely short
 branches in molecular trees and which even
 whole-genome analyses might
 be unable to
 resolve into a neat sequence of bifurcations, happened
 prior to the K--Pg boundary. Recently, some
 authors on both sides of
 "rocks-vs.-clocks" debate (e.g., Ksepka &
 Phillips 2015; Prum et
 al. 2015; Suh et al.
 2015) apparently came to agree that this region
 of bird phylogeny represents an explosive
 radiation into ecological
 niches emptied by
 the K--Pg extinction. If Acosta Hospitaleche and
 Gelfo are right in assigning the Maastrichtian
 fossils to Gaviiformes,
 this explanation
 obviously doesn't work, which makes me wonder if they
 could have misidentified the material. They
 seem to have good enough
 evidence that the
 fossils belong to neornithines, although they do
 mention that the morphology of the hypotarsus
 is unclear in what they
 interpret as an
 indeterminate Cretaceous loon. However, their
 phylogenetic analysis is based on Livezey and
 Zusi's (2007) data
 matrix that includes
 very few fossils and routinely fails to recover
 well-supported clades, as should be obvious
 from the fact that they
 get 100% bootstrap
 support for a loon-grebe group. Could the fossils
 perhaps belong to some other, earlier-diverged
 neornithine lineage?
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 David Černý