(Phillips, Tom L. and William A. DiMichele. 1998. A transect through a
clastic-swamp to peat-swamp ecotone in the Springfield Coal, Middle
Pennsylvanian age of Indiana, USA. Palaios 13(2):113-128.)
They looked at plant fossils in coal balls from a transect including both
coal and a clastic paleochannel. The channel has some poorly-developed
paleosols, with in situ roots. The ratio of above-ground plant parts to
roots was about 3 to 1, which they say is unusually high and reflects an
exceptional abundance of pteridosperm leaves. Two different habitats were
represented, based on differing types and numbers of types of plants. The
coal balls formed by reworking of organic-rich mud during a marine
transgression.
The roots, the channel, and the habitat differentiation all suggest that
this is essentially autochthonous.