The land plant cover in the Devonian: a reassessment of the evolution of the tree habit

Abstract

Abstract This paper reviews information on the Devonian trees that evolved in the euphyllophyte clade with special focus on the Middle Devonian Pseudosporochnales. The morphology of pseudosporochnalean trees shows analogies with that of extant tree ferns, including the possession of an adventitious root system of limited extent at the base of the trunk. Direct evidence on how these trees were constructed is scarce. We propose a growth model integrating information from younger representatives of the same class known to reach large diameters. According to this model, trunk width in its aerial part results from the large size of its primary body where living tissues are abundant, a condition reached early during growth. Secondary xylem contributes little to trunk diameter. This model sharply diverges from that of the Late Devonian archaeopteridalean trees characterized by an extended root system and where trunk diameter and mechanical support are achieved by the substantial development of secondary vascular tissues. These differences suggest that pseudosporochnalean trees may have had a lesser impact on Devonian environments than the Archaeopteridales. The important investment in living tissues in the Pseudosporochnales probably made them vulnerable to drought and cold.