Club Mosses

There's no need to make big branching leaves.  There's so much carbon available in the atmosphere that all you need is a clump of leaves at the top.


Club mosses grow into huge, tall trees, dropping their simple leaves as they are shadowed by new leaves growing out from the top.  They look somewhat like modern palm trees, for the same reason: get a bunch of leaves up as high as you can and let the rest die back rather than repair any damage.
Giant club mosses form the majority of what are now called 'coal forests'.

This period is the carboniferous period.  It is the time when all the coal reserves on our planet were laid down.  Massive forests spread across the globe, far bigger than our current rain forests.  They lay down huge, deep peat bogs, which eventually form into coal as they are crushed under the weight of more and more earth.  The forests are alive with simple insects, fish and reptiles, all of which grow bigger and bigger as the air becomes enriched with oxygen, left over from the carbon-fixing carried out by the forests.

As the oxygen level rises, possibly as high as 35%, many of the insects actually grow much bigger than we currently see.  Hornets the size of your hand fly over the landscape.  But, eventually, the atmosphere runs out of carbon dioxide, at any rate in the levels needed for most of the giant club mosses.  Without a megaphyll leaf, the giant club moss species die out.


All of your giant varieties become extinct, now visible only as fossils in coal.  Only a thousand or so much smaller varieties remain extant today.

That's as far as you can evolve here!  
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