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New papers part II ;-)

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 [...].


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.

    `--A--Remaining ciconiiforms
       `--B--Procellariidae (tubenoses)
          |--Spheniscidae (penguins)

*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.