Asteroxylon is an extinct genus of terrestrial vascular plants that thrived during the Early Devonian period. Specimens of this ancient plant are recognized from anatomically preserved fossils found in the Rhynie Chert and Windyfield Chert in Scotland, which are renowned fossil sites. Discovered and described by Kidston and Lang in 1920, with further details on its reproductive structures resolved by Lyon in 1964, Asteroxylon holds a significant position in the fossil record, offering a glimpse into early land plant evolution.
Characteristics of Asteroxylon
Asteroxylon consisted of aerial stems that branched, reaching up to 40 cm long and 12 mm in diameter. These stems likely emerged from a leafless rhizome, a horizontal underground stem, which bore smaller, root-like branches. These branches could penetrate the substrate up to 20 cm deep. The rhizomes represent an independent origin of roots.
The aerial axes were covered with scale-like enations, unbranched, strap-shaped protrusions up to 5 mm long. These enations are not considered true leaves because their vascular tissue extended only to their base, not throughout their entire length. However, these enations and the main stems possessed stomata, indicating their capacity for photosynthesis.
Internally, the vascular tissue, or xylem, in the center of the aerial stems had a distinctive star-shaped appearance in cross-section, forming an early actinostele or “Asteroxylon-type” protostele. The xylem cells, called tracheids, were of a primitive annular or helical type. Reproductive structures, known as sporangia, were kidney-shaped and attached to the main axis by a short stalk, interspersed among the non-vascularized enations.
The Ancient Environment of Asteroxylon
Fossils of Asteroxylon are primarily found in the Rhynie Chert, a Scottish sedimentary deposit that preserves an ancient hot spring ecosystem. This site offers a unique window into the earliest terrestrial environments, where plants, animals, fungi, and microbes were preserved in their original positions.
During the Devonian, land colonization by plants accelerated, with a diverse flora becoming established. Early terrestrial vegetation, including zosterophylls and trimerophytes, had begun to spread. Many of these plants lacked true roots or leaves and were relatively small. The Devonian seas were rich with brachiopods, corals, crinoids, and ammonites, while the first land animals, such as arthropods, also became well-established.
Asteroxylon’s Evolutionary Importance
Asteroxylon is considered a significant fossil for understanding plant evolution due to its pioneering features and its transitional form. It represents an early member of the Lycopodiophyta, a group that includes modern clubmosses. The development of its vascular system, with specialized xylem and phloem, was a major step for plants to colonize land, allowing for efficient transport of water and nutrients. Asteroxylon’s vascular tissue, while conductive, likely lacked safety features to prevent damage from excessive water loss, suggesting that it relied on stomatal regulation to prevent desiccation.
The unique leaf-like structures, or enations, of Asteroxylon are noteworthy. While not fully vascularized like true microphylls in later lycophytes, the presence of a vascular trace extending to the base of each enation suggests an intermediate stage in leaf evolution. This feature supports the “enation hypothesis” for leaf origins, where simple outgrowths gradually developed vascular connections. Asteroxylon’s complex body plan, featuring distinct rooting axes, root-bearing axes, and leafy shoots, provides insights into the development of the first complex vascular plants on Earth.