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Re: Age division?
Jordan Mallon (email@example.com) wrote:
<I've heard things like "_T. rex_ was from the Late Maastrichtian," but
does this really mean anything quantitatively? Are geological ages
separated according to time like that? (i.e., would "Late Maastrichtian"
mean something like 68-65 Ma?) I'm just looking to put some numbers to
these distinctions. I get the impression that anything past the Age
category is fairly subjective.>
Based on faunal correllation for the Mesozoic, at least, taxa can be
placed within a formation and levels in the formation correllated to more
precisely dated marine formations. One can demonstrate that a taxon is
upper Albian, mid-Turnonian, lower Hauterivian. Standardization of age
boundaries permits placement of volcanically-derived isotope degradation
ages, and therefore the fauna in them. Meaning, the Leatoli footprints of
the Ngorogoro crater can be precisely dated, though volcanic dates have a
greater variance the further back in time they derive. We can be pretty
certain that the bulk of geologic dating is accurate around 10my, volcanic
dates vary around 2my, and so we can determine where in the Berriasian,
for instance, the Yixian may date to, marine correlation of the Judith
River Group, and faunal comparisons to those. The Hell Creek overlies the
Bearpaw, which accurately dates it to the upper half of the Maastrichtian.
Otherwise, nearness of correlated boundaries permits some ease in
placement within an age or formation. For instance, the vertebrate fossils
of the Anacleto Member of the Río Colorado Formation occur throughout, but
*Aucasaurus* is from the upper third, and titanosaur and egg fossils of
the Auca Mahuevo section are restricted to the middle and lower thirds;
there is so far no correllation between *Aucasaurus* and the Auca Mahuevo
sauropods or nests, though it is possible there is.
My two cents,
Jaime A. Headden
Little steps are often the hardest to take. We are too used to making leaps
in the face of adversity, that a simple skip is so hard to do. We should all
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