Page 122 - Introduction to Paleobiology and The Fossil Record

PALEOECOLOGY AND PALEOCLIMATES 109

Box 4.8 Climate change and fossil size

There is strong evidence that climate and environmental changes have controlled extinctions and

speciations, but do they have a direct influence on the size of organisms? Daniela Schmidt and her
colleagues (2004) have investigated size changes in planktic foraminiferans during the last 70 myr
from well-dated cores furnished by various ocean drilling programs. There was a sharp decrease in
size at the Cretaceous–Paleogene boundary with the disappearance of many large taxa, and after
this extinction event high-latitude taxa remained consistently small. Fluctuations in size, however,
occurred in low-latitude assemblages (Fig. 4.25). A fi rst phase (65–42 Ma) is characterized by dwarfs,

a second (42–12 Ma) contains moderate size fluctuations, whereas the third (12 Ma to present) has
the relatively large-sized taxa that typify Modern assemblages. Size increases are correlated
with intervals of global cooling (Eocene and Neogene), when there were marked latitudinal and
temperature gradients and high diversity. More minor size changes in the Paleocene and Oligocene
may have been associated with changes in productivity. Cenozoic planktic foraminiferans thus
provide strong support for a stationary model of evolutionary change, with size changes being

strongly correlated with extrinsic factors such as fluctuations in latitudinal and surface-water tem-
perature gradients.

[μm]
Age Size assemblages
[Ma] 300 400 500 600
0 Plt.
Plio.
10
Miocene size increase
strong
in low-latitude
20 fauna
Oligocene small size in
30 low and high
latitudes
40
Eocene first separation
of high- and
low-latitude
50
fauna
14 12 10 8 6 4 2 0
Paleocene Mg/Ca
60 bottom water temperature [°C]

70
Cenozoic average 0 1 2 3 4 5
18
δ O[‰]
12 8 4 0
Bottom-water temperature [°C]
Ice-free ocean

Figure 4.25 Size changes in planktic foraminiferans from high and low latitudes during the last

70 Ma, compared to temperature profiles generated from oxygen isotope data and Mg : Ca ratios.
Three phases are recognized, a first (65–42 Ma) with dwarf taxa, a second (42–12 Ma) with

moderate-sized taxa, and a third (12 Ma to present) with large-sized taxa. Size increases are
correlated with intervals of global cooling. (Courtesy of Daniela Schmidt.)