Mesozoic plants?

[Jonathon Woolf <jwoolf@erinet.com>]

Random musings . . .

A question that I wonder about from time to time: how much do we know
about Mesozoic flora, and specifically about the plants that dinosaurs
ate?  

Over and over, I read reports about the Mesozoic including large numbers
of very massive herbivorous dinosaurs: eight or ten tons apiece for the
average hadrosaur, anywhere between eight and eighty tons for a
sauropod, ten tons or so for a ceratopsid, etc.  Not only that, but we
also find evidence of _herds_ of these beasts.  Ten thousand maiasaurs. 
Thousands of centrosaurs.  Vast bone beds of styracosaurs and
gryposaurs.  Sauropods moving in bands of ten or twenty.  It would
appear from the evidence that western North America during the Late
Jurassic or Late Cretaceous supported at least ten times the herbivore
biomass supported by the Serengeti/Masai Mara of today or the American
Great Plains of a few centuries ago.  

What were all those dinosaurs eating?  What plants were around then that
could have fed so many _big_ herbivores, and given them all enough
nutrition to keep going?  No matter what kind of metabolism it had, it
would still take a _lot_ of munchies to power a fifty-ton brachiosaur. 
What was it eating?  Doesn't seem that any modern plant life can deliver
that much nutrition.  Modern antelope rarely mass more than a few
hundred kg, and they seem to spend an awful lot of time eating.  Was
Mesozoic flora more nutritional than modern flora?  Or were dinosaurs
vastly more efficient than mammals at using what they ate?  Or is there
some other answer I've missed?

Comments, thoughts, ideas, anyone?

-- JSW