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Age of the Amur region, and the Horseshoe Canyon and Javelina formations (was: Re: Age of formations)



KELL00BELL@aol.com wrote:

"Palynomorphs are said to correlate the Tsagayan with the Wodehousia spinata
palynozone in NA.  Is a Lancian age necessarily implied, or certain?"

    Not sure where Tsayagan is... The idea, put forth by Godefroit et al.
(2000), is that the Wodehousia spinata Zone in North America corresponds to
"Lancian" time, so the presence of that species in Asian rocks indicates
those rocks are Lancian. This is a misinterpretation of Markevitch's (1994)
zonation, in which Wodehousia spinata occurs as part of a zone representing
Early Maastrichtian time in Russia (palynomorphs are nasty about showing up
at different times in different places). Markevitch et al. (2000) later
interpreted the Amur faunas as representing "middle" Maastrichtian. How this
relates to the Eurpoean stages, or North American stratigraphy, is not
clear, so I am inclined to default to Markevitch's earlier conclusion. So,
long story short, don't try to use pollen on one continent to correlate to
another... be very skeptical of palynologically correlated "Lancian" faunas
in Asia.

-------------------------------------------------

      "-Does a Late Maastrichtian age for the Nemegt seem likely now, [...]"
    [Other stuff about the Age of the Horsehoe Canyon excerpted]

    OK, you may be making a mountain out of a molehill:

    Vertical succession of faunas does not imply discontinuity. Dinosaurian
faunas show some evidence of altitudinal segregation, with putative "upland"
faunas looking rather different from "lowland" faunas. In a progradational
sequence, such as the Horseshoe Canyon Formation, inland facies overly
shoreline facies... thus, going up vertically in section you are crossing
"space" as well as "time." Therefore, it is not entirely out of wack to
interpret the "distinct" Campanian and Edmontonian faunas of the Horseshoe
Canyon Formation as contemporaries, even if we only have sequential records
of them (see my next point).

    The Campanian/ Maastrichtian boundary is not a magical barrier to faunal
continuity, it is an arbitrary level in a series of strata somewhere in
Europe. Recent revision has messed up our traditional, happy view of
"Judithian" faunas as Late Campanian and "Edmontonian" faunas as Early
Maastrichtian... it turns out the C/M stage boundary of the European system
is probably somewhere within "Edmontonian" time. The "Edmontonian" of our
youths is now a bastard child of two stages... this does not oblige us to
treat its faunas as any more different than we did before. The faunal
heterogeneity of the Horseshoe Canyon Formation has been known for decades,
and the usual explanation (that I have read, anyway), is facies-based.

    That said, Anchiceratops *has* been found in "Judithian" rocks. Poop!

    Now, as for the Nemegt, I have always suspected it to be of
"Edmontonian" age. As I recall, Radiometric dates in the Russian Dinosaurs
book seem to confirm this, although I don't have the book handy to double
check.

----------------------------------------------------

      "-How good is the evidence for a late Maastrichtian age for the
Javelina?"

    Good. Not perfect, but good. It is based partially on the uninterrupted
sedimentary sequence at the top of the formation which *definitely* spans
the boundary... If there is an hiatus anywhere, it is between the Aguja and
Javelina (not very likely, though). There is a biostratigraphic correlation
with the better constrained Naashoibito Member of the Kirtland Shale
(bracketted radiometrically and correlated with mammals) and the North Horn
Formation (The dates for which have been constrained, I can't recall how)
based on Alamosaurus. Not that I am very comfortable with that. The presence
of specimens referable to Torosaurus is a little more damning, in my
opinion.

    "Does Javelina material eg TMM41436-1, suggest a smaller, hence earlier
tyrannosaur? "

    I had the opportunity to discuss this specimen with Dr. Molnar at SVP.
He stands by his assertion that it is not T. rex. I feel we should regard it
as Theropoda incerta sedis until such time as a proper study can be
conducted. In other words: ignore it.

    Hope this helps! :)

    Wagner


REFERENCES:

GODEFROIT, P. ZAN S. AND JIN L. 2000. Charonosaurus jiayinensis n.g., n.sp.,
a lambeosaurine dinosaur from the Late Maastrichtian of northeastern China.
Compte Rendu de l'Academie du Sciences Naturelles, Paris, series 2, Sciences
de la Terre et des planétes, 330:875-882.

MARKEVITCH, V. S. 1994. Palynological zonation of the continental Cretaceous
and lower [sic] Tertiary of eastern Russia. Cretaceous Research, 15:165-177.

MARKEVITCH, V. S., E. V. BUGADEVA AND Y. L. BOLOTSKY. 2000. Playnological
evidence of vegetational change and dinosaur exitinction in the Amur Region.
Paleontological Journal, 34 (Supp. 1): S50-S53.