Page 180 - Introduction to Paleobiology and The Fossil Record

MASS EXTINCTIONS AND BIODIVERSITY LOSS 167

Sediment type

Zone
C28R Sample numbers Brazos core,
Texas
Lower Tertiary (Danian) P1a P1b C29N

C29R 1.0 0.5 0 0.5 1.0 1.5m 217 225 242 256 268 279 289 KT boundary
P0

Upper Maastrichtian Pseudotextularia deformis

C30N

Figure 7.4 Patterns of extinction of foraminifera in a classic KT section spanning about 1.5 myr. A
species loss of 53% occurred in two steps close to the KT boundary and iridium anomaly. Dating is
based on magnetostratigraphy, and the KT boundary falls in the C29R (reversed) zone. Planktonic
zones (P0, P1a, P1b) are indicated; sediment types are mudstones (darker grey) and limestones (pale
grey); meter scale bar shows height above and below a particular extinction level, 0. (Based on Keller
et al. 1993.)

answers to questions such as these refer to raphy (see p. 24) and exact radiometric dating
ecological time scales – that is, times of years (see p. 38) are also needed.
or decades at most.

It is just as difficult, if not more so, to
answer questions of the timing of ancient
events from region to region or continent to Selectivity and mass extinctions
continent. How can a paleontologist be sure The second defi ning character of mass extinc-
that the supposed KT boundary in Montana tions (see p. 164) was that they should be
is the same as the supposed KT boundary in ecologically catholic, that there should be
Mongolia? Perhaps the boundary is marked little evidence of selectivity. Ecological selec-
as the next sedimentary rock layer above the tivity implies that some organisms might be
appearance of the last dinosaur fossil. But of better able to survive a mass extinction event
course this definition is perfectly circular: the than others. Mass extinctions do not seem to

KT boundary is marked by the disappearance have been particularly selective, even though
of dinosaurs; dinosaurs disappeared just it might seem that, for example, large reptiles
below the KT boundary. Other fossils, such were specially selected for extinction during
as pollen, may be used to date the boundary, the KT event. The dinosaurs and some other
but additional evidence, from magnetostratig- large reptiles certainly died out then, but a