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



If confirmed, the conclusions of this paper would have pretty
far-reaching implications for the early evolution of modern birds. For
starters, 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 neoavian
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
the "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?


*Refs:*

Jarvis ED, Mirarab S, Aberer AJ, Li B, Houde P, Li C, Ho SYW,
Faircloth BC, Nabholz B, Howard JT, Suh A, Weber CC, Fonseca RRd, Li
J, Zhang F, Li H, Zhou L, Narula N, Liu L, Ganapathy G, Boussau B,
Bayzid MdS, Zavidovych V, Subramanian S, Gabaldón T, Capella-Gutiérrez
S, Huerta-Cepas J, Rekepalli B, Munch K, Schierup M, Lindow B, Warren
WC, Ray D, Green RE, Bruford M, Zhan X, Dixon A, Li S, Li N, Huang Y,
Derryberry EP, Bertelsen MF, Sheldon FH, Brumfield RT, Mello CV,
Lovell PV, Wirthlin M, Schneider MPC, Prosdocimi F, Samaniego JA,
Velazquez AMV, Alfaro-Núñez A, Campos PF, Petersen B, Sicheritz-Ponten
T, Pas A, Bailey T, Scofield P, Bunce M, Lambert DM, Zhou Q, Perelman
R, Driskell AC, Shapiro B, Xiong Z, Zeng Y, Liu S, Li Z, Liu B, Wu K,
Xiao J, Yinqi X, Zheng Q, Zhang Y, Yang H, Wang J, Smeds L, Rheindt
FE, Braun M, Fjeldså J, Orlando L, Barker K, Jønsson KA, Johnson W,
Koepfli KP, O'Brien S, Haussler D, Ryder OA, Rahbek C, Willerslev E,
Graves GR, Glenn TC, McCormack J, Burt D, Ellegren H, Alström P,
Edwards SW, Stamatakis A, Mindell DP, Cracraft J, Braun EL, Warnow T,
Jun W, Gilbert MTP, Zhang G 2014 Whole genome analyses resolve the
early branches in the tree of life of modern birds. Science 346(6215):
1320–31

Ksepka DT, Phillips MJ 2015 Avian diversification patterns across the
K-Pg boundary: Influence of calibrations, datasets, and model
misspecification. Ann Mo Botan Gard 100(4): 300–28

Livezey BC, Zusi RL 2007 Higher-order phylogeny of modern birds
(Theropoda, Aves: Neornithes) based on comparative anatomy: II. –
Analysis and discussion. Zool J Linn Soc 149(1): 1–95

Prum RO, Berv JS, Dornburg A, Field DJ, Townsend JP, Moriarty Lemmon
E, Lemmon AR 2015 A comprehensive phylogeny of birds (Aves) using
targeted next-generation DNA sequencing. Nature 526(7574): 569–73

Suh A, Smeds L, Ellegren H 2015 The dynamics of incomplete lineage
sorting across the ancient adaptive radiation of neoavian birds. PLoS
Biol 13(8): e1002224

-- 
David Černý