A new phylogeny of the cladoxylopsid plexus: contribution of an early cladoxylopsid from the Lower Devonian (Emsian) of Quebec.

PREMISE: Cladoxylopsids formed Earth's earliest forests and gave rise to the ancestors of sphenopsids and ferns. Lower Devonian (Emsian) strata of the Battery Point Formation (Quebec, Canada) contain new anatomically preserved cladoxylopsids, one of which is described in this article. To assess the phylogenetic position of this fossil and address questions of cladoxylopsid phylogeny, we conducted a comprehensive phylogenetic study. METHODS: Permineralized axes were studied in serial sections using the cellulose acetate peel technique. We evaluated phylogenetic relationships among cladoxylopsids using a data set of 36 new morphological characters and 31 species, in parsimony-constrained analyses. RESULTS: We describe Adelocladoxis praecox gen. et sp. nov., a cladoxylopsid with small actinostelic axes bearing dichotomously branched, helically arranged ultimate appendages and fusiform sporangia. Adelocladoxis provides the oldest evidence of cladoxylopsid anatomy, including ultimate appendages and sporangia. In agreement with non-phylogenetic classification schemes, our phylogenetic analysis resolves a basal grade of iridopterids and a clade of cladoxylopsids s.s., which includes a pseudosporochnalean cladoxylopsid clade, a cladoxylalean cladoxylopsid clade, and Adelocladoxis. CONCLUSIONS: Our phylogenetic analysis illuminates aspects of tempo and mode of evolution in the cladoxylopsid plexus. Originating prior to the Emsian, cladoxylopsids reached global distribution by the Frasnian. Iridopterids and cladoxylopsids s.s. radiated in the Emsian-Eifelian. The sequence of character change recovered by our phylogeny supports a transition from actinostelic protosteles to dissected steles, associated with an increase in xylem rib number and medullation generating a central parenchymatous area.

Unique growth strategy in the Earth's first trees revealed in silicified fossil trunks from China.

Cladoxylopsida included the earliest large trees that formed critical components of globally transformative pioneering forest ecosystems in the Mid- and early Late Devonian (ca. 393-372 Ma). Well-known cladoxylopsid fossils include the up to ∼1-m-diameter sandstone casts known as Eospermatopteris from Middle Devonian strata of New York State. Cladoxylopsid trunk structure comprised a more-or-less distinct cylinder of numerous separate cauline xylem strands connected internally with a network of medullary xylem strands and, near the base, externally with downward-growing roots, all embedded within parenchyma. However, the means by which this complex vascular system was able to grow to a large diameter is unknown. We demonstrate-based on exceptional, up to ∼70-cm-diameter silicified fossil trunks with extensive preservation of cellular anatomy from the early Late Devonian (Frasnian, ca. 374 Ma) of Xinjiang, China-that trunk expansion is associated with a cylindrical zone of diffuse secondary growth within ground and cortical parenchyma and with production of a large amount of wood containing both rays and growth increments concentrically around individual xylem strands by normal cambia. The xylem system accommodates expansion by tearing of individual strand interconnections during secondary development. This mode of growth seems indeterminate, capable of producing trees of large size and, despite some unique features, invites comparison with secondary development in some living monocots. Understanding the structure and growth of cladoxylopsids informs analysis of canopy competition within early forests with the potential to drive global processes.