Editing Polystrate fossil (section)

===Formation by regeneration===
Geologists have also found that some of the larger upright fossil trees found within Carboniferous coal-bearing strata show evidence of regeneration after being partially buried by sediments. In these cases, the trees were clearly alive when they were partially buried by sediments. The accumulated sediment was insufficient to kill the trees immediately because of their size. As a result, some of them developed a new set of roots from their trunks just below the new ground surface. Until they either died or were overwhelmed by the accumulating sediments, these trees would likely continue to regenerate by adding height and new roots with each increment of sediment, eventually leaving several meters of former "trunk" buried underground as sediments accumulated.<ref name=Gastaldo2004a/><ref name= Gastaldo1997a>Gastaldo, R.A., 1992, ''Regenerative growth in fossil horsetails (Calamites) following burial by alluvium''. Historical Biology, 6(3):203-220.</ref>