What it is
A fern that grows as a tree: one upright trunk with a crown of fronds at the top. It made spores, not seeds. The best known coal-age tree fern, Psaronius, grew 30 to 50 feet tall, shorter than the giant scale trees around it.
A trunk made of roots
It had no real wood. Its thin stem was wrapped in a thick coat of roots that grew down its sides and spread wider toward the ground, sometimes more than a meter across. That coat held the tree up, light but tough, and other plants grew in it.
Why it matters
About 305 million years ago, the tropics were drying, and nobody is sure what tipped them. Across what is now Europe and North America, the giant scale trees died back, and tree ferns took their place. In China, the old swamp forest held on for millions of years more.
Read the sources
- Psaronius, an extinct tree fern · Carl R. Woese Institute for Genomic Biology, University of Illinois
- The late Palaeozoic tree fern Psaronius: an ecosystem unto itself · Review of Palaeobotany and Palynology (2000)
- Rainforest collapse triggered Carboniferous tetrapod diversification · Geology (2010)