Modern roots evolved at least twice, and developed their characteristic features gradually, reports a study in this week’s Nature. This conclusion comes from the discovery of a transitional root fossil from the earliest-known land ecosystem.
The defining feature of modern-day plant roots is the meristem - a self-renewing structure that is covered by a cap at its apex. Root meristems are hard to spot in the fragmentary fossil record, however, making it challenging to unearth the evolutionary origin of roots.
Sandy Hetherington and Liam Dolan studied the Rhynie chert, a 407 million-year-old sedimentary deposit that contains the exceptionally well-preserved remains of the oldest known terrestrial ecosystem. Located in Aberdeenshire, Scotland, the chert yields various fossils including plants, lichen and various arthropods. Examining chert samples under the microscope, the authors find root meristems belonging to the lycopsid plant Asteroxylon mackiei. Lycopsids, represented today by club mosses, are vascular plants (those with tissues that internally move resources) whose lineage branched off early, before the other higher plants (the euphyllophytes).
The authors find that the meristems of A. mackiei lack both root hairs and caps - they are covered instead by a continuous layer of surface tissue. This structure makes these roots unique among the vascular plants. The authors conclude that A mackiei’s roots are a transitional step towards modern-style, rooted vascular plants - and support the idea that, as this cap-less transitional structure appears in a plant that is already a lycopsid, roots with caps evolved separately in lycopsids and euphyllophytes from their common, root-less ancestors.