Taxon sampling and alternative hypotheses of relationships in the euphyllophyte plexus that gave rise to seed plants: insights from an Early Devonian radiatopsid

Mosaic assembly of regulatory programs for vascular cambial growth: a view from the Early Devonian

Evidence for secondary growth extends into the Early Devonian, 407 million years ago, raising questions about tempo and mode of origination of this key developmental feature. To address such questions, we analyze anatomy in the four oldest fossil plants with well-characterized woody tissues; one of these represents a new genus, described here formally. The new fossil is documented using the cellulose acetate peel technique and associated methods. We use the paradigm of structural fingerprints to identify developmental components of cambial growth based on fossil anatomy. We integrate developmental inferences within a theoretical framework of modular regulation of secondary growth. The fossils possess structural fingerprints consistent with four different combinations of regulatory mechanisms (modules) acting in cambial growth, representing four distinct modes of secondary growth. The different modes of secondary growth demonstrate that cambial growth is an assemblage of regulatory modules whose deployment followed a mosaic pattern across woody plants, which may represent ancestors of younger lineages that exhibit woody growth. The diverse modes of wood development occupy a wide morphospace in the anatomy of wood in the Early Devonian, suggesting that the origins of secondary growth and of its modular components pre-date this interval.

Complex wound response mechanisms and phellogen evolution - insights from Early Devonian euphyllophytes

We analyze the oldest fossil occurrences of wound-response periderm to characterize the development of wound responses in early tracheophytes. The origin of periderm production by a cambium (phellogen), an innovation with key roles in protection of inner plant tissues, is poorly explored; understanding periderm development in early tracheophytes can illuminate key aspects of this process. Anatomy of wound-response tissues is characterized in serial sections in a new Early Devonian (Emsian; c. 400 Ma) euphyllophyte from Quebec (Canada) - Nebuloxyla mikmaqiana sp. nov. - and compared to previously described euphyllophyte periderm from the same fossil locality to reconstruct periderm development. Characterizing development in these oldest periderm occurrences allows us to propose a model for the development of wound-response periderm in early tracheophytes: by phellogen activity that is poorly coordinated laterally but bifacial, producing secondary tissues initially outwardly and subsequently inwardly. The earliest occurrences of wound periderm pre-date the oldest known periderm produced systemically as a regular ontogenetic stage (canonical periderm), suggesting that periderm evolved initially as a wound-response mechanism. We hypothesize that canonical periderm evolved by exaptation of this wound sealing mechanism, whose deployment was triggered by tangential tensional stresses induced in the superficial tissues by vascular cambial growth from within.

A protoxylem pathway to evolution of pith? An hypothesis based on the Early Devonian euphyllophyte Leptocentroxyla

BACKGROUND AND AIMS

The Early Devonian (Emsian, 400-395 Ma) tracheophyte Leptocentroxyla tetrarcha Bickner et Tomescu emend. Tomescu et McQueen combines plesiomorphic Psilophyton-type tracheid thickenings with xylem architecture intermediate between the plesiomorphic basal euphyllophyte haplosteles and the complex actinosteles of Middle Devonian euphyllophytes. We document xylem development in Leptocentroxyla based on anatomy and explore its implications, which may provide a window into the evolution of pith.

METHODS

Leptocentroxyla is preserved by permineralization in the Battery Point Formation (Quebec, Canada). Serial sections obtained using the cellulose acetate peel technique document branching pattern, anatomy of trace divergence to appendages, protoxylem architecture, and variations in tracheid size and wall thickening patterns.

KEY RESULTS

Leptocentroxyla has opposite decussate pseudo-whorled branching and mesarch protoxylem, and represents the earliest instance of central histological differentiation in a euphyllophyte actinostele. Tracheids at the centre of xylem exhibit simplified Psilophyton-type wall thickenings and are similar in size (at the axis centre) or smaller than the surrounding metaxylem tracheids (at the centre of appendage traces).

CONCLUSIONS

The position and developmental attributes of the simplified Psilophyton-type tracheids suggest they may have been generated by the protoxylem developmental pathway. This supports the delayed and shortened protoxylem differentiation hypothesis, which explains the evolution of pith by (1) delay in the onset of differentiation and lengthening of cell growth duration in a central protoxylem strand; and (2) shortening of the interval of differentiation of those tracheids, leading to progressive simplification (and eventual loss) of secondary wall thickenings, and replacement of tracheids with a central parenchymatous area. NAC domain transcription factors and their interactions with abscisic acid may have provided the regulatory substrate for the developmental changes that led to the evolution of pith. These could have been orchestrated by selective pressures associated with the expansion of early vascular plants into water-stresses upland environments.

The role of paleontological data in bryophyte systematics

Systematics reconstructs tempo and mode in biological evolution by resolving the phylogenetic fabric of biodiversity. The staggering duration and complexity of evolution, coupled with loss of information (extinction), render exhaustive reconstruction of the evolutionary history of life unattainable. Instead, we sample its products-phenotypes and genotypes-to generate phylogenetic hypotheses, which we sequentially reassess and update against new data. Current consensus in evolutionary biology emphasizes fossil integration in total-evidence analyses, requiring in-depth understanding of fossils-age, phenotypes, and systematic affinities-and a detailed morphological framework uniting fossil and extant taxa. Bryophytes present a special case: deep evolutionary history but sparse fossil record and phenotypic diversity encompassing small dimensional scales. We review how these peculiarities shape fossil inclusion in bryophyte systematics. Paucity of the bryophyte fossil record, driven primarily by phenotypic (small plant size) and ecological constraints (patchy substrate-hugging populations), and incomplete exploration, results in many morphologically isolated, taxonomically ambiguous fossil taxa. Nevertheless, instances of exquisite preservation and pioneering studies demonstrate the feasibility of including bryophyte fossils in evolutionary inference. Further progress will arise from developing extensive morphological matrices for bryophytes, continued exploration of the fossil record, re-evaluation of previously described fossils, and training specialists in identification and characterization of bryophyte fossils, and in bryophyte morphology.