They are considered to be primarily freshwater and terrestrial, but the tubeworm, Spirorbis , almost certainly marine or brackish, points to a mixing of environments. The most impressive fossils are the upright lycopod trees. The two most common types found are Lepidodendron and Sigilaria , which grew to over 30 feet in height. These trees had overlapping scalelike bark with a pithy inner pulp. The fossils themselves have lost their pulp and all that remains is a cylinder of coalified bark filled with sediments often different from the surrounding material.
Horizontal logs are rare, but are usually flattened, crushed by overlying sediment.
The roots or rootlets of the trees, called Stigmaria , are often seen separated from the main trunks. Uniformitarian geology, the mainstream view ever since Lyell, holds that these trees grew in the place where they are now found. It is supposed that surrounding the base of the trees, a layer of forest litter collected, which if thick enough, could become peat. The trees eventually died when sediments buried their roots and lower sections. Finally, the tops broke off, and the insides were hollowed.