[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]
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."
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.
*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
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.