Re-evaluating the phylogenetic relationships of zosterophylls with a comprehensively sampled dataset and a...

The known diversity of zosterophylls, the dominant tracheophytes at the beginning of the Devonian, has expanded significantly due to discoveries that intervened since the latest large phylogenetic treatment targeting the group, almost 30 years ago. Past phylogenetic treatments reached incomplete resolution or conflicting results on relationships among zosterophylls and between zosterophylls and lycopsids (which are traditionally thought to have evolved from among zosterophylls). This state of knowledge emphasizes the need for re-evaluation of zosterophyll relationships. Here, we address the phylogenetic relationships of zosterophylls based on the most extensive taxonomic sampling of the group and explore the evolutionary implications of the results.

We assembled and analysed a data matrix of 44 taxa (38 of them zosterophylls) and 42 morphological characters using parsimony and Bayesian approaches. We evaluated the resulting topologies using a new approach aimed at reconciling phylogenetic conflict and identifying shared patterns of relationships, alongside traditional stratigraphic fit tests and ancestral character state reconstruction.

Within the Lycophytina clade, zosterophylls form a grade paraphyletic to the lycopsids. A Sawdoniaceae clade of zosterophylls is sister to the lycopsids; the Sawdoniaceae-lycopsids clade is sister to a clade that includes the barinophytes.

A major radiation before and during the late Silurian gave rise to the main zosterophyll lineages (including lycopsids) and led to diversification within each of these lineages. Cladogenesis continued through the Lochkovian and relented in the Pragian, with virtually no diversification afterwards. Lycophytes (Lycophytina) are ancestrally defined by bivalvate sporangia opening along their distal margin, attached laterally on axes. Lycophytina comprises two clades: one characterized by irregularly distributed sporangia and the other (including the majority of zosterophylls, along with lycopsids and barinophytes) characterized by reniform sporangia arranged in rows, 3-D branching architectures and K-/H-branching in the rhizomatous portions.