Mesozoic (and Pleistocene) biomass issues

["Thomas R. Holtz, Jr." <tholtz@geol.umd.edu>]

Greetings,

Man, go away from the office for a day, and suddenly everyone gets very 
chatty... :-)

Okay, in any case, biomass issues are really significant for dinosaurian (and 
any organisms!) biology. The two main factors involved
in per area biomass would be a) productivity of that environment and b) energy 
flow up the trophic levels.

In the past there was a tendancy to assume productivity levels were the same in 
the past as now. That is clearly incorrect. We have
various lines of geological information on changing atmospheric composition, 
temperatures, water vs. land exposure, etc. at
different points in time (some better resolved than others). Recent work by 
graduate student Sara Decherd (NCSU) and colleagues show
that plants which retain largely Mesozoic morphologies (such as gingko) have 
elevated productivity rates (2-3x Holocene) in modelled
mid-Cretaceous atmospheres. So it might well be that plants were putting out 
more energy per unit area per unit time than at
present. I won't even go into additional discussion about black shales here, 
but could...

How about energy flow through the system? Well, let's just say that 
bradymetabolism vs. tachymetabolism in dinosaurs remains a
matter for professional debate.

As for the Pleistocene: productivity rates in the polar regions may well have 
been higher in the presence of mammoths. Dale Guthrie
and others have proposed a biome called the "mammoth steppe", based on the very 
different pollen ratios and memberships in
Pleistocene Arctic regions to today. The model of this biome goes something 
like this:
        * Mammoths rip up the vegetation in the north, clearing up spots for 
colonization of new plants.
        * Mammoth droppings allow for the quick recycling of nutrients in the 
North.
        * The combiination of these two above factors means that the nutrients 
are not isolated into a solid tunda and permafrost, but
available for a higher diversity of plants, and thus a higher diversity of 
animals. The latter themselves provide more dung and
continued high rates of nutrient recycling.

Then bad times happen to the mammoths and some other megafauna:
        * Other animals are less well adapted to ripping up the vegetation mats.
        * Less and less space is available for non-tundra type plants to 
colonize.
        * Nutrients are less available (less dung), and animals starve or leave.
        * Less biotic activity in the soil allows for establishment of 
nutrient-poor dense tundra layers and of permafrost.
        * Things suck for the North.

These issues are by no means agreed upon by all Quaternary workers, but I think 
that they have a lot of merit.

Take care,

                Thomas R. Holtz, Jr.
        Senior Lecturer, Vertebrate Paleontology
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
        Mailing Address:
                Building 237, Room 1117
                College Park, MD  20742

http://www.geol.umd.edu/~tholtz/
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