New papers part II ;-)

[David Marjanovic <david.marjanovic@gmx.at>]

Part I was... last winter on the description of the mammal *Schowalteria* 
which is Cretaceous but looks Paleocene. :-]

Marcel van Tuinen & S. Blair Hedges: The effect of external and internal 
fossil calibrations on the avian evolutionary timescale, Journal of 
Paleontology 78(1), 45 -- 50 (January 2004)

Abstract: "Molecular clocks can provide insights into the evolutionary 
timescale of groups with unusually biased or fragmentary fossil records, 
such as birds. In those cases, it is advantageous to establish internal 
anchor points -- molecular time estimates -- using the group under study. 
This method also avoids the inherent problems in drawing conclusions about 
the evolution of a group based on data tied to the poor fossil record of 
that same group [...except it still relies on the fossil record for... 
calibration!]. The galliform-anseriform divergence ( ~ 90 million years ago 
[hopefully]) is an example of such an ideal [hear, hear] anchor point for 
molecular clock analyses in birds."

More quotes:

> >
The majority of the fossils used for calibration of avian molecular clocks 
have uncertain phylogenetic relationships. Incorrect phylogenetic placement 
may lead to older divergence times [...]. Furthermore, the assumption of 
well-constrained fossil calibrations is not always clearly met. For example, 
a study on parrot biogeography (Miyaki et al., 1998) chose a calibration age 
based on a midpoint between the molecular estimates from Hedges et al. 
(1996) and [the hopelessly misrooted] Cooper and Penny (1997). While it 
represents a compromise, using an average of multiple ages may increase 
calibration error and compounding [sic] all other errors. Another example 
involves ratites, where Haddrath and Baker (2001) used a calibration age of 
35 Myr for the emu-cassowary divergence to time several other ratite 
divergences. This age is based on *Emuarius gidju* (Boles, 1992) at 25 Myr, 
a fossil clearly on the emu line. To account for uncertainty in calibration 
point, the authors [arbitrarily] added 10 million years to the minimum age 
to approximate the divergence of emus and cassowaries. Similarly, Waddell et 
al. (1999) used the gamefowl-waterfowl divergence (or stem anseriform) at 68 
Myr to set their mitochondrial protein clock. They reasoned that the 
currently oldest waterfowl fossils were 55 Myr old, applied this calibration 
and estimated the time of stem Anseriformes to be around 78 Myr. Their 
subsequent 68 Myr estimate is based on an average (plus three Myr) of the 
minimum crown age (55 Myr) and this molecular stem estimate (78 Myr). Yet, 
no clear evidence exists from either genetics or fossils that 68 Myr closely 
approximates the age of stem Anseriformes. To the contrary, recent fossil 
evidence exists that pushes even the crown Anseriformes into the late 
Cretaceous [...] while nuclear estimates of stem Anseriformes points to a 90 
Myr age [...].

CALIBRATION WITH CRETACEOUS LOONS

An example involving fossil "loons" demonstrates that different phylogenetic 
interpretations can have major effects on time estimation. *Neogaeornis* and 
*Polarornis* are two supposed modern loons that have been described from the 
late Cretaceous ( ~ 70 Myr) of South America and Antarctica (Lambrecht, 
1929; Chatterjee, 1989). *Neogaeornis* is based on tarsometatarsus material 
that shows the presence of a hypotarsus as seen in modern foot-propelled 
diving birds (loons and grebes). Originally considered to be part of a 
[fictitious] larger hesperornithid-loon-grebe grouping, hesperornithids were 
later removed from this group. In 1992, Olson redescribed *Neogaeornis* as a 
modern foot-propelled diving bird because it lacked the defining [sic] 
characters of hesperornithids. Although he thought it conceivable that some 
other group could have given rise to *Neogaeornis* during the Mesozoic, he 
did not consider it the most parsimonious hypothesis. A close affinities 
with modern loons was based on the shape of the trochlea (Olson, 1992; Hope, 
2002) and the placement of the distal foramina similar to that seen in 
Miocene loons (*Colymboides*). *Polarornis* is based on more extensive 
material (including cranial) apparently showing several defining 
characteristics of loons and a neognathous palate (Olson, 1992). However, 
the material has not been formally described (Chatterjee, 1989, 1997). 
[AFAIK it has been -- Chatterjee, 1998, or suchlike. Is in the archives of 
this list, IIRC.] Despite limited descriptions and the lack of thorough 
phylogenetic analyses on both fossil "loons", these taxa are almost 
universally interpreted as modern loons (e.g., Chiappe, 1996; Dingus and 
Rowe, 1998; Padian and Chiappe, 1998; Hope, 2002) and have even served as 
molecular clock calibrations (Cooper and Penny, 1997).
   However, three interpretations can be ascribed to the phylogenetic 
position of these fossils, assuming a sister group relationship between 
*Polarornis* and *Neogaeornis* [just to simplify things] (Fig. 1). First, a 
minimum age of 70 Myr can be applied to the stem of modern loons. This 
interpretation was used to calibrate portions of a nuclear and a 
mitochondrial gene resulting in deep Cretaceous estimates for the origins of 
several modern bird orders (Cooper and Penny, 1997). Even deeper divergence 
times (up to 200 Myr) would be obtained if such a calibration were applied 
to DNA hybridization or immunological data. If, instead, the 70 Myr "loons" 
represent the crown loon family Gaviidae (sensu Chatterjee, 1997), 
reanalyses of those molecular data would indicate a Precambrian origin for 
modern orders of birds. [!!! !!! !!!] Alternatively, some diagnostic 
characters of Gaviiformes may have evolved repeatedly in other neognathous 
birds. This is the most likely explanation for two reasons: 1) convergent 
evolution is a frequent phenomenon within the crown of modern aquatic groups 
[...] and 2) more consistent time estimates (e.g., 80 Myr for the 
Galliformes. Anseriformes divergence) are obtained when assuming that these 
"loons" were neognathous foot-propelled diving birds near the base of stem 
Ciconiiformes [sensu Sibley & Ahlquist] (the modern clade that includes 
nearly all aquatic radiations within modern birds). If true, this hypothesis 
would again imply that crown Gaviiformes is restricted to the Northern 
Hemisphere. Thus, *Polarornis* and *Neogaeornis* could be viewed as Southern 
Hemisphere analogs of modern day loons.
<<

Here is a feeble attempt to explain what fig. 1 tries to say.

--+--Hesperornithiformes
 `--+--Palaeognathae
    `--A--Remaining ciconiiforms
       `--B--Procellariidae (tubenoses)
          |--Spheniscidae (penguins)
          `--C--Gavi-
             `--idae

*Polarornis* and *Neogaeornis* were inserted infinitely shortly after 
divergence A respectively B or C and used to calibrate them.
       If they are put at C ( = as crown gaviids), divergence B is about 
Devonian in age, and A is at least as old as Ediacara. Neornithes as a whole 
is a billion years old, and so on. Makes one think about the etymology of 
"looney".
       If they are put at B ( = as stem gaviiforms), A is nearly as old as 
*Archaeopteryx*, and (as mentioned) Neornithes approaches a Triassic age.
       If they are put at A ( = as neornithine waterbirds, but far away 
from loons), then things make pretty much sense. C becomes late Miocene, as 
probably expected from the fossils. A becomes Eocene, which ironically fits 
the first known fossil appearances of all three groups, making fun of the 
southern hemisphere's bad Eocene and older fossil record! Neornithes as a 
whole is then about as old as, or slightly older than, then Late Cretaceous.

Another paper in the same issue calls this the reductio ad absurdum 
approach. As demonstrated above, it works.