Geoscience Reference
In-Depth Information
As Johnson has continued to sample the fossil plant record in
Montana and North Dakota, he has found more and more new plant
species in the uppermost Cretaceous, but next to no new Tertiary
ones. This means that more Cretaceous species died out than he had
measured earlier. Johnson now estimates the percentage extinction
at close to 90 percent.
2
5
Thus the fossil plant evidence thoroughly
corroborates predictions 1 and 2: As many as 90 percent of Creta-
ceous plant species disappeared suddenly, right at the K-T bound-
ary; none of them are found above the iridium level.
FORAMINIFERA
Planktonic foraminifera (Figure 21], nicknamed forams, are one-
celled, amoebae-like protozoa that float at various depths in the
oceans, eating the still smaller photosynthetic algae and secreting cal-
careous shells that survive the foram's demise. They evolve rapidly
FIGURE
2
I
Foraminifera.
[Photo courtesy of Brian
Huber and National
Museum of Natural History. ]
