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Gojira VS Nyuupeipaasu
In today's Nature:
Wible, J. R., G. W. Rougier, M. J. Novacek & R. J. Asher. 2007.
Cretaceous eutherians and Laurasian origin for placental mammals near
the K/T boundary. Nature 447: 1003-1006.
'Estimates of the time of origin for placental mammals from DNA
studies span nearly the duration of the Cretaceous period (145 to 65
million years ago), with a maximum of 129 million years ago and a
minimum of 78 million years ago. Palaeontologists too are divided on
the timing. Some support a deep Cretaceous origin by allying certain
middle Cretaceous fossils (97?90 million years old) from Uzbekistan
with modern placental lineages, whereas others support the origin of
crown group Placentalia near the close of the Cretaceous. This
controversy has yet to be addressed by a comprehensive phylogenetic
analysis that includes all well-known Cretaceous fossils and a wide
sample of morphology among Tertiary and recent placentals. Here we
report the discovery of a new well-preserved mammal from the Late
Cretaceous of Mongolia and a broad-scale phylogenetic analysis. Our
results exclude Cretaceous fossils from Placentalia, place the origin
of Placentalia near the Cretaceous/Tertiary (K/T) boundary in
Laurasia rather than much earlier within the Cretaceous in the
Southern Hemisphere, and place afrotherians and xenarthrans in a
nested rather than a basal position within Placentalia.'
The new critter is called _Maelestes gobiensis_ (no connection to Mae
West). For once, the analysis includes more than just hedgehogs as
representatives of modern placentals. As the abstract says,
everything that has even looked vaguely Cretaceous-ish is outside the
placental crown - even _Purgatorius_ forms a clade with
_Protungulatum_ and _Oxyprimus_ that is the sister group to the crown
(WARNING - more traditional positions for these taxa were apparently
not significantly rejected). Zalambdalestids - out. Asioryctitheres -
out. Zhelestids - don't make me laugh.
I'm reminded of something that I've been wondering about for a while
- not only do all Cretaceous eutherians fall outside the crown group,
but a significant number of early Cenozoic taxa appear to as well.
The Nature paper shows at least cimolestids and leptictids (assuming
a Palaeocene age for the Bug Creek taxa, it would be equivocal
whether they split from the crown group before or after the K-T
transition). That's leaving out other Palaeocene groups that have
been allied to one or the other of those families, such as
taeniodonts and pantolestids (and taeniodonts also appear to have
crossed the K-T). So how come so many mammal groups seem to have
crossed the K-T unscathed, only to dribble out afterwards?