Biology Reference
In-Depth Information
sorption is infl uenced by temperature and humidity, which vary over time.
Also, the exact thickness of the hydration layer was diffi cult to determine
with the optical microscope because it appears to vary somewhat with the
focal level. An ion beam is now used to measure thickness because the re-
fl ection pattern of the beam changes at the boundary between hydrated and
nonhydrated layers. Old sequences are being recalculated, and new ones are
being established by using obsidian directly associated with the site (rather
than from the general vicinity). These techniques are showing greater con-
sistency within the stratigraphic sequence and with the ages of artifacts
from comparable levels from multiple sites.
Once radiometric dating techniques were developed, it was possible
to date and correlate sequences based on relative ages with those having
absolute ages. The zones of planktonic foraminifera, calcareous nanno-
plankton communities, NALMAs, and magnetic reversals (fi gs. 3.7, 3.8)
could be viewed in real time. Ages are being determined with ever-increas-
ing precision, and the results are being coordinated through projects such
as EARTHTIME (2007). Slowly, geologic, biologic, and climatic histories
are emerging that show consistency across methodologies, with patterns
of environmental change in distant regions, and in coevolutionary pat-
terns between plant and animal groups. One example of the latter is the
recent discovery of a fossil bee in amber from the Cretaceous of Myan-
mar (Poinar and Danforth 2006). One of the defi ning features of the an-
giosperms that give them an advantage over the wind and water-pollinated
ferns, as well as the wind-pollinated gymnosperms they replaced as the
Earth's dominant vegetation, is a more effi cient system of insect pollina-
tion. Bees are prominent among these pollinators, and even though fl ow-
ering plants originated about 135 Ma, and soon after were successfully
radiating and differentiating, the oldest known bees were seemingly late
at 35 to 45 Ma. Crepet and Nixon (1998) anticipated bee pollination for
some Cretaceous Clusiaceae based on fl oral structure, and the 100 Ma Bur-
mese fossil bee signifi cantly closes the gap between the angiosperms and
one of their principal pollinators. Other examples include correspondence
between Cretaceous paleotemperature estimates using
16
O/
18
O ratios and
otoliths, the rings in fossilized calcite spheres from the inner ear of fi sh
(chap. 5), and between glacial events and temperature changes based on
preserved amino acids (allioisoleucine/isoleucin, A/I ratios) in fossilized
gastropod shells from loess (chap. 8). There are numerous examples of
consistencies gradually emerging in lineage and ecosystem histories with
new technologies and that are improving the chronological framework of
Earth history.
