July 13. Pennsylvanian scale trees

Scale trees – named for the scale-like symmetrical arrangement of close-spaced leaf scars on the trunks – were common and abundant in the forests of the Pennsylvanian Period. The classic example is lepidodendron – its name means “scale tree” – which grew to over 30 meters or 100 feet tall. Some trunks were a meter in diameter.  

Lepidodendron

Lepidodendrons are related to modern club mosses and quillworts and probably evolved from the earliest vascular plants including the Silurian Cooksonia that we talked about April 25. Together with many other plants large and small, lepidodendrons formed world-wide rain forests during the Pennsylvanian, and ultimately contributed to much of the coal that formed when they died. Lepidodendron fossils are common in coal beds.


They didn’t have bark like modern trees but rather had photosynthetic skin covered with leaves and pores. The skin was tough, like bark, and protected the internal structures of the plants that delivered water and nutrients to the plant’s extremities. The leaves fell off as the tree grew, leaving a tall pole-like plant with branching fronds only at the top. They formed huge stands in some places, with hundreds or even thousands of individuals per acre.

Sigillaria (public domain)

Sigillaria was a related tree-like plant common in the Pennsylvanian coal swamps. Unlike lepidodendrons, sigillaria sometimes grew into branched trees. Otherwise, the main difference between them in fossils is the geometry of the leaf scars. In lepidodendron, the leaves spiraled around the trunk, while sigillaria leaves were arranged in vertical rows up the trunk. Sigillaria fossils look a lot like tire treads.

Both lepidodendron and sigillaria reproduced with spores that were encapsulated, but were not true seeds. The spore capsules were carried in cone-like structures at the ends of the leafy branches at the top of the plants.

Both these plants declined precipitously in late Pennsylvania time when the rainforest ecosystem collapsed, and both were extinct by sometime in the Permian.

—Richard I. Gibson

Lepidodendron photograph taken by Mark A. Wilson (Department of Geology, The College of Wooster) public domain.