Geoscience Reference
In-Depth Information
and spread out over wide areas, making them handy for index-
ing rock formations. Their sensitivity to temperature and salinity also
make them useful in interpreting ancient environments. Foramini-
fera occur in the hundreds of thousands, even in a single hand-sized
specimen.
At the time the Alvarez theory appeared, the paleontologists
who had been studying fossil plants believed they had largely sur-
vived the K-T boundary; those who had been studying ammonites
believed they had gone extinct well before it. A return to the field
for more collecting showed that both interpretations were wrong:
The ammonites and the plants each suffered a massive extinction
right at the K-T boundary. In contrast, the intensive study of the
forams in the years following the appearance of the Alvarez theory
led some paleontologists to just the opposite conclusion: What ap-
peared to be a mass extinction was, they claimed, something else.
In 1980 few doubted that the foram extinction had been nearly
complete and had been timed exactly to the K-T boundary; indeed,
so many foram species disappeared that their level of departure
almost defined the boundary. At Snowbird I, Hans Thierstein of
the Scripps Institute of Oceanography showed that over 97 percent
of foram species and 92 percent of the genera became extinct at the
K-T boundary,
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6
and Jan Smit reported that the K-T foram ex-
tinction was so thorough that only one species,
Guembelitria creta-
cea,
survived, with all the subsequent foram species having evolved
from it.
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7
The puzzle for students of earth history is how creatures that
made their living floating in the sea all could be killed at once. A
clue comes from another group of forams—the benthic variety that
live on the seafloor—which suffered a much lower rate of extinction
at the K-T boundary. Some believe that the difference in survival
rate stems from the dependence of the floating plankton on "pri-
mary productivity"—that is, they ate the even smaller plantlike
phytoplankton and therefore would die if those organisms were
not available. The benthic forams, on the other hand, lived down in
the detritus of the seafloor where they could feed on the accumu-
lated organic debris, which would have been abundant after so
many other creatures, including their floating cousins, had died in
the K-T extinction. Thus if the upper layers of the oceans became
sufficiently poisoned to kill the phytoplankton, the floating forams
would die out but the deeper benthic variety would live on.
The simple picture of nearly complete foram extinction right at
the K-T boundary did not go long unchallenged. Gerta Keller, who
emerged as Smit's leading opponent in the interpretation of K-T
