Cladoxylopsida

The cladoxylopsids are a group of plants known only as fossils that are thought to be ancestors of ferns and horsetails. They had a central trunk, from the top of which several lateral branches were attached. Fossils of these plants originate in the Middle Devonian to Early Carboniferous periods (around 390 to 320 million years ago), mostly just as stems. Cladoxylopsida contains two orders. The order Hyeniales is now included in Pseudosporochnales. Intact fossils of the Middle Devonian cladoxylopsid Wattieza show it to have been a tree, the earliest identified in the fossil record as of 2007. A recent (2017) discovery in Xinjiang in China of early Late Devonian (Frasnian, ca. 374 Ma) silicified fossil cladoxylopsid tree trunks (diameter up to c.70 cm) with preserved cellular anatomy showed an internal arrangement with many xylem bundles in the outer part and none in the interior; each bundle was surrounded by its own cambium layer, by which the tree's trunk widened.