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RE: marine reptile McMenu



-------- Original-Nachricht --------
> Datum: Thu, 28 Feb 2008 19:58:13 -0600
> Von: Tim Williams <twilliams_alpha@hotmail.com>

> Molecular analyses have suggested that krill (Euphausiidae) appeared
> some time during the Cretaceous period (Jarman, 2001; Biol. J. Linn.
> Soc. 73: 199-212).

This paper simply assumed a strict molecular clock.

So it used a single calibration point: the split between Syncarida and 
Mysidacea + Euphausidae, fixed at 350 Ma ago. "This calibration point is 
slightly older than the maximum ages of fossil Syncarida and Mysidacea of  ~340 
Mya because several species of each group were present at this time." (p. 204)

While the use of a single calibration point is mostly due to the lack of a 
fossil record for krill (and the lack of good programs for molecular dating in 
2001!), the author apparently considers it a feature rather than a bug, 
incredibly: "Molecular dating methods are generally considered inaccurate [...] 
and especially when they rely on a single DNA region [...]. The use of 
calibration points derived from fossil evidence further compound[s] inaccuracy 
as a result of the incompleteness of the fossil record." (p. 208)

And even so, only two lineages of krill have survived the K-Pg boundary mass 
extinction according to the results (p. 208).

I conclude that there's no evidence for Cretaceous krill, and only negative 
evidence against it.
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