No Cretaceous rainforests?

[bh480@scn.org]

No Cretaceous rainforests?
From: Ben Creisler bh480@scn.org

Kirk Johnson's article in the new issue of Science about 
an early Paleocene rainforest in the Denver region 
contains a statement that surprised me: "no known 
Cretaceous floras exhibit rainforest physiognomy" [i.e., 
features such as large leaf size, smooth leaf margins, and 
elongate drip-tips on leaves to shed high amounts of 
rainfall]. Does this mean there were NO rain forests 
during the Cretaceous?

The following article appeared back in September 2001 but 
I don't recall seeing it cited here-- it's worth seeking 
out for anyone interested in the K/T extinction event:
Sweet, A.R. 2001. Plants, a yardstick for measuring the 
environmental consequences of the Cretaceous-Tertiary 
Boundary Event. Geoscience Canada 28(3): 127-137. 
The article presents evidence based on pollen from Montana 
and Canada that the effect of the K/T impact on plants was 
not the total wipe-out envisioned in some scenarios, and 
instead was much more selective. Although there was 
extensive destruction of the forest canopy across the 
continent, the understory vegetation may have survived the 
event and recovered fairly quickly. However, insect-
pollinated angiosperms suffered much greater extinction 
than wind-pollinated angiosperms. The scenario proposed 
seems to fit with some details of the article by Kevin 
Pope in the February 2000 issue of Geology:
Pope, K. O. 2002. Impact dust not the cause of the 
Cretaceous-Tertiary mass extinction. Geology 30(2): 99-102.
Pope argued that the amount of dust created by the impact 
has been greatly overestimated and would not have created 
the "nuclear winter" type scenario proposed by some, in 
which all sunlight was blocked for a months or years, 
preventing photosynthesis. Instead, vaporized rock spread 
as "fireball condensate" that could have ignited the type 
of continent-wide forest canopy fires indicated in Sweet. 
Soot from the fires and sulfate aerosols created by the 
impact may have had some effect on photosynthesis.

Sweet also discusses the effect of large dinosaurs on the 
landscape:
"If Triceratops and Tyrannosaurus had survived in the 
Tertiary, they would have found conditions alien and in 
strong contrast to the well-drained landscapes of the late 
Maastrichtian in western Canada. However, the very absence 
of large herbivores may have contributed to the 
prevasiveness of marsh/swamp conditions. These prodigious 
Mesozoic Era consumers, and by their movements, degraders 
of organic material, were removed from the ecosystem. No 
large trampling animals were left to break tributaries 
free of dams formed by the grasp of plants and their 
debris, thus decreasing the capacity of the landscape to 
shed itself of Paleocene rain. Until the land was uplifted 
some 2 million years later, swamps and sluggish drainage 
systems dominated the early Tertiary landscape." (p. 136)

So contrary to the old idea that dinosaurs lived in 
swamps, dinosaurs probably PREVENTED swamps!