Archaeopterid root anatomy and architecture: new information from permineralized specimens of Famennian age from Anti-Atlas (Morocco)
Résumé
The evolutionary history of lignophyte roots is poorly known, and direct evidence of root systems in early members of this clade is scant. Whereas Middle Devonian aneurophytalean progymnosperms possessed rhizomatous stems bearing small, shallow roots, the root system of archaeopteridalean trees is expected to have been much more extended and complex. Current evidence, however, has not provided sufficient information for substantiating these expectations. Five root specimens with a Callixylon anatomy were found in a Late Devonian locality of Anti-Atlas. They are 25-90 mm wide and show abundant secondary xylem. Radial pits with a horizontal aperture are documented in some parts of the wood of three specimens, two with trifilievii-type anatomy and the third one with erianum-type anatomy. This character, unknown in archaeopteridalean stems, may have been specific to the roots. The secondary phloem contains fibers. Anatomical studies also indicate two origins for the roots. Dichotomous branching is reported in a single specimen and was probably the least common. All other lateral roots are interpreted to have an endogeneous origin. They are arranged along several rows corresponding in number to the number of lobes in the protostele of the supporting axis. Last-order roots are relatively densely inserted. They have narrow, short vascular traces running perpendicularly to the supporting axis; their growth is determinate.