Exploring geography and evolutionary history as drivers of variation in floral scent chemistry in western sessile-flowered Trillium using parsimony-constrained phylogenetics.

BACKGROUND AND AIMS: The sessile-flowered Trillium species from western North America have been challenging to distinguish morphologically due to overlapping characters and intraspecific variation. Molecular phylogenetic analyses, currently inconclusive for this group, have not sampled multiple populations of the different species to account for this. Here, we query the diversity of floral volatile composition to understand its bearings on the taxonomy, distribution and evolution of this group. METHODS: We explored taxonomic and geographic patterns in average floral volatile composition (105 different compounds) among 42 wild populations of four sessile-flowered Trillium species and the outgroup, Pseudotrillium, in California, Oregon and Washington by means of parsimony-constrained phylogenetic analyses. To assess the influence of character construction, we coded compound abundance in three different ways for the phylogenetic analyses and compared the results with those of statistical analyses using the same dataset and previously published statistical analyses. KEY RESULTS: Different codings of floral volatile composition generated different phylogenetic topologies with different levels of resolution. The different phylogenies provide similar answers to taxonomic questions but support different evolutionary histories. Monophyly of most populations of each taxon suggests that floral scent composition bears phylogenetic signal in the western sessile-flowered Trillium. Lack of correlation between the distribution of populations and their position in scent-based phylogenies does not support a geographic signal in floral scent composition. CONCLUSIONS: Floral scent composition is a valuable data source for generating phylogenetic hypotheses. The way scent composition is coded into characters is important. The phylogenetic patterns supported by floral volatile compounds are incongruent with previously reported phylogenies of the western sessile-flowered Trillium obtained using molecular or morphological data. Combining floral scent data with gene sequence data and detailed morphological data from multiple populations of each species in future studies is needed for understanding the evolutionary history of western sessile-flowered Trillium.

An early cladoxylopsid with complex vascular architecture: Paracladoxylon kespekianum gen. et sp. nov. from the Lower Devonian (Emsian) of Quebec, Canada.

PREMISE: Cladoxylopsids, one of the first lineages with complex organization to rise from the plexus of structurally simple plants that comprised the earliest euphyllophyte floras, are moniliformopsid euphyllophytes. They formed Earth's earliest forests by the Middle Devonian and are thought to have given rise to the equisetopsids and probably some fern lineages. The Lower Devonian (Emsian) Battery Point Formation (Quebec, Canada) contains previously unrecognized cladoxylopsids preserved anatomically. One of these provides new data on structural evolution among euphyllophytes and is described here. METHODS: The anatomy and morphology of permineralized axes of the new plant were studied with light and electron microscopy on sections produced using the cellulose acetate peel technique. Morphological comparisons and phylogenetic analysis were used for taxonomic placement of the plant. RESULTS: The plant represents a new species, Paracladoxylon kespekianum Chu et Tomescu, gen. et sp. nov., that has tracheids with modern-looking bordered pits and the complex cauline vascular architecture characteristic of the genus Cladoxylon. Its dissected ultimate appendages have complex regular taxis and a pattern of vascularization that suggests bilateral symmetry. CONCLUSIONS: Paracladoxylon kespekianum is one of the largest Early Devonian euphyllophytes, among the oldest representatives of the cladoxylopsid group, and older than any species of the closely related Cladoxylon by at least 35 million years. It is also one of the oldest anatomically preserved representatives of the cladoxylopsid group. Its anatomical organization pushes the rise of complex vascular architecture among moniliformopsid euphyllophytes deeper in time than previously recognized.

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.

Dating the evolution of the complex thalloid liverworts (Marchantiopsida): total-evidence dating analysis supports a Late Silurian-Early Devonian origin and post-Mesozoic morphological stasis.

Divergence times based on molecular clock analyses often differ from those derived from total-evidence dating (TED) approaches. For bryophytes, fossils have been excluded from previous assessments of divergence times, and thus, their utility in dating analyses remains unexplored. Here, we conduct the first TED analyses of the complex thalloid liverworts (Marchantiopsida) that include fossils and evaluate macroevolutionary trends in morphological 'diversity' (disparity) and rates. Phylogenetic analyses were performed on a combined dataset of 130 discrete characters and 11 molecular markers (sampled from nuclear, plastid and mitochondrial genomes). Taxon sampling spanned 56 extant species - representing all the orders within Marchantiophyta and extant genera within Marchantiales - and eight fossil taxa. Total-evidence dating analyses support the radiation of Marchantiopsida during Late Silurian-Early Devonian (or Middle Ordovician when the outgroup is excluded) and that of Ricciaceae in the Middle Jurassic. Morphological change rate was high early in the history of the group, but it barely increased after Late Cretaceous. Disparity-through-time analyses support a fast increase in diversity until the Middle Triassic (c. 250 Ma), after which phenotypic evolution slows down considerably. Incorporating fossils in analyses challenges previous assumptions on the affinities of extinct taxa and indicates that complex thalloid liverworts radiated c. 125 Ma earlier than previously inferred.

Deep origin and gradual evolution of transporting tissues: Perspectives from across the land plants.

The evolution of transporting tissues was an important innovation in terrestrial plants that allowed them to adapt to almost all nonaquatic environments. These tissues consist of water-conducting cells and food-conducting cells and bridge plant-soil and plant-air interfaces over long distances. The largest group of land plants, representing about 95% of all known plant species, is associated with morphologically complex transporting tissue in plants with a range of additional traits. Therefore, this entire clade was named tracheophytes, or vascular plants. However, some nonvascular plants possess conductive tissues that closely resemble vascular tissue in their organization, structure, and function. Recent molecular studies also point to a highly conserved toolbox of molecular regulators for transporting tissues. Here, we reflect on the distinguishing features of conductive and vascular tissues and their evolutionary history. Rather than sudden emergence of complex, vascular tissues, plant transporting tissues likely evolved gradually, building on pre-existing developmental mechanisms and genetic components. Improved knowledge of the intimate structure and developmental regulation of transporting tissues across the entire taxonomic breadth of extant plant lineages, combined with more comprehensive documentation of the fossil record of transporting tissues, is required for a full understanding of the evolutionary trajectory of transporting tissues.

An early snapshot of plant-herbivore interactions: Psilophyton diakanthon sp. nov. from the Early Devonian of Gaspé (Quebec, Canada).

PREMISE: Trimerophytes are a plexus of early tracheophytes that form the base of the euphyllophyte clade and, thus, represent the link between the earliest land plants and modern-day ferns, sphenophytes, and seed plants. As the best-characterized trimerophyte, the genus Psilophyton occupies a key position in the euphyllophyte fossil record. We describe a new Psilophyton species that has implications for the evolution of plant-animal interactions. METHODS: The fossil material is preserved by permineralization in the Lower Devonian (Emsian) Battery Point Formation (Québec, Canada) and was studied in serial sections using the cellulose acetate peel technique. RESULTS: Psilophyton diakanthon sp. nov. differs from other Psilophyton species in possessing fibers that form a discontinuous layer in the inner cortex and two distinct types of spinescent emergences whose anatomy and morphology are consistent with roles in anti-herbivore defense. CONCLUSIONS: Psilophyton diakanthon adds another species to an already diverse genus. Its two morphologically distinct types of spinescence suggest that herbivory was rampant in plant-animal interactions and demonstrate that anti-herbivory defenses had reached a previously unrecognized level of sophistication by 400 million years ago, in the Early Devonian.

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.

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.

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

An abrupt transition in the fossil record separates Early Devonian euphyllophytes with a simple structure from a broad diversity of structurally complex Middle-Late Devonian plants. Morphological evolution and phylogeny across this transition are poorly understood due to incomplete sampling of the fossil record. We document a new Early Devonian radiatopsid and integrate it in analyses addressing euphyllophyte relationships. Anatomically preserved Emsian fossils (402-394 Ma) from the Battery Point Formation (Gaspé, Quebec, Canada) are studied in serial sections. The phylogenetic analysis is based on a matrix of 31 taxa and 50 characters emphasising vegetative morphology (41 discrete, nine continuous). The new plant, Kenrickia bivena gen. et sp. nov., is one of very few structurally complex euphyllophytes documented in the Early Devonian. Inclusion of Kenrickia overturns previously established phylogenetic relationships among Radiatopses, reiterating the need for increased density of Early Devonian taxon sampling. Kenrickia is recovered as the sister lineage to all other radiatopsids, a clade in which paraphyletic Stenokoleales led to a lignophyte clade where archaeopterids and seed plants fall into sister clades. Our results shed light on early euphyllophyte relationships and evolution, indicating early exploration of structural complexity by multiple lineages and reiterating the potential of a single origin of secondary growth in euphyllophytes.

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.

Mosaic modularity: an updated perspective and research agenda for the evolution of vascular cambial growth.

Secondary growth from a vascular cambium, present today only in seed plants and isoetalean lycophytes, has a 400-million-yr evolutionary history that involves considerably broader taxonomic diversity, most of it hidden in the fossil record. Approaching vascular cambial growth as a complex developmental process, we review data from living plants and fossils that reveal diverse modes of secondary growth. These are consistent with a modular nature of secondary growth, when considered as a tracheophyte-wide structural feature. This modular perspective identifies putative constituent developmental modules of cambial growth, for which we review developmental anatomy and regulation. Based on these data, we propose a hypothesis that explains the sources of diversity of secondary growth, considered across the entire tracheophyte clade, and opens up new avenues for exploring the origin of secondary growth. In this hypothesis, various modes of secondary growth reflect a mosaic pattern of expression of different developmental-regulatory modules among different lineages. We outline an approach that queries three information systems (living seed plants, living seed-free plants, and fossils) and integrates data on developmental regulation, anatomy, gene evolution and phylogeny to test the mosaic modularity hypothesis and its implications, and to inform efforts aimed at understanding the evolution of secondary growth.

Grimmiaceae in the Early Cretaceous: Tricarinella crassiphylla gen. et sp. nov. and the value of anatomically preserved bryophytes.

Background and Aims: Widespread and diverse in modern ecosystems, mosses are rare in the fossil record, especially in pre-Cenozoic rocks. Furthermore, most pre-Cenozoic mosses are known from compression fossils, which lack detailed anatomical information. When preserved, anatomy significantly improves resolution in the systematic placement of fossils. Lower Cretaceous (Valanginian) deposits on Vancouver Island (British Columbia, Canada) contain a diverse anatomically preserved flora including numerous bryophytes, many of which have yet to be characterized. Among them is the grimmiaceous moss described here. Methods: One fossil moss gametophyte preserved in a carbonate concretion was studied in serial sections prepared using the cellulose acetate peel technique. Key Results: Tricarinella crassiphylla gen. et sp. nov. is a moss with tristichous phyllotaxis and strongly keeled leaves. The combination of an acrocarpous condition (inferred based on a series of morphological features), a central conducting strand, a homogeneous leaf costa and a lamina with bistratose portions and sinuous cells, and multicellular gemmae, supports placement of Tricarinella in family Grimmiaceae. Tricarinella is similar to Grimmia, a genus that exhibits broad morphological variability. However, tristichous phyllotaxis and especially the lamina, bistratose at the base but not in distal portions of the leaf, set Tricarinella apart as a distinct genus. Conclusions: Tricarinella crassiphylla marks the oldest record for both family Grimmiaceae and sub-class Dicranidae, providing a hard minimum age (136 million years) for these groups. The fact that this fossil could be placed in an extant family, despite a diminutive size, emphasizes the considerable resolving power of anatomically preserved bryophyte fossils, even when recovered from allochthonous assemblages of marine sediments, such as the Apple Bay flora. Discovery of Tricarinella re-emphasizes the importance of paleobotanical studies as the only approach allowing access to a significant segment of biodiversity, the extinct biodiversity, which is unattainable by other means of investigation.

Spore wall ultrastructure and development in a basal euphyllophyte: Psilophyton dawsonii from the Lower Devonian of Quebec (Canada).

PREMISE OF THE STUDY: Euphyllophytes, a clade including living ferns, horsetails, and seed plants, have a rich fossil record going back to the Early Devonian. The euphyllophyte spore wall has a complex structure, the evolutionary origins of which are incompletely understood. Psilophyton is the best-characterized basal euphyllophyte genus; thus, data on this genus can inform current hypotheses on spore wall structure and development, which propose a bilayered spore wall organization of combined spore and sporangial origin for the ancestral euphyllophyte. METHODS: We employed cellulose acetate peel sectioning of permineralized Lower Devonian (Emsian) Psilophyton dawsonii sporangia, combined with electron microscopy, to document spore wall structure and development. KEY RESULTS: The Psilophyton dawsonii spore wall is bilayered. The inner spore wall is homogeneous, probably of lamellar construction. The outer spore wall, loosely attached to the inner wall, covers distal and equatorial spore areas, and has a foveolate base layer upon which stacks of sporopollenin lumps accrete centrifugally, forming the scaffolding for the final apiculate ornamentation. CONCLUSIONS: This is the most complete account on spore wall structure, allowing developmental interpretations, in a basal euphyllophyte. The bipartite organization of the Psilophyton dawsonii spore wall reflects development as a result of two processes: an inner layer laid down by the spore cell and an outer layer of tapetal origin. Providing direct evidence on the spore wall of a basal euphyllophyte, these data confirm previous hypotheses and mark an empirically supported starting point for discussions of the evolution of spore wall development in euphyllophytes.

Wanted dead or alive (probably dead): Stem group Polytrichaceae.

PREMISE OF THE STUDY: The Polytrichaceae are a widespread and morphologically isolated moss lineage. Early attempts to characterize phylogenetic relationships within the family suggested that morphology is not phylogenetically informative. Two well-characterized fossils similar to basal and derived Polytrichaceae (Meantoinea alophosioides and Eopolytrichum antiquum, respectively), are known from Cretaceous rocks. To assess the phylogenetic positions of these fossils and compare hypotheses of relationships recovered using molecular vs. morphological methods, we conducted a comprehensive morphology-based phylogenetic study of Polytrichaceae. METHODS: We evaluated the phylogenetic relationships of Polytrichaceae using a data set of 100 morphological characters (including 11 continuously varying traits codified as continuous characters) scored for 44 species of acrocarpous mosses and parsimony as the optimality criterion. KEY RESULTS: Continuous characters significantly increased the resolving power of the analyses. The overall ingroup topology was sensitive to rooting as determined by outgroup selection, with some analyses yielding results that were incongruent with those of molecular studies. Both fossils had stable phylogenetic relationships, irrespective of outgroup sampling. CONCLUSIONS: Our results suggest that morphology is useful in resolving phylogenetic relationships in the Polytrichaceae, if both discrete and continuous characters are used. However, our rooting experiments demonstrate that there is no superior way to root analyses and indicate that relationships within the family are best evaluated using unrooted networks without outgroup taxa. These rooting problems suggest that additional information is needed to understand the phylogenetic relationships of Polytrichaceae. Such additional information could come from fossils of stem group polytrichaceous mosses, which await discovery.

Buried deep beyond the veil of extinction: Euphyllophyte relationships at the base of the spermatophyte clade.

PREMISE OF THE STUDY: The deep origin and early evolution of seed plants (spermatophytes) are poorly understood. Starting in the Early Devonian, euphyllophytes diversified rapidly into several groups. Two of these groups, progymnospems and Stenokoleales, along with satellite taxa, have been involved in discussions of seed plant origins. Because these early lineages are extinct, the key to the origin and early evolution of seed plants lies in the fossil record. Decades-long paleobotanical work has produced data on the diversity and anatomy of fossil species, which now provide a foundation for exploring seed plant origins in a phylogenetic context. METHODS: We address phylogenetic relationships between early seed plants, aneurophytalean progymnosperms, Stenokoleales, and several Devonian species of uncertain affinities using parsimony analyses that include 28 anatomically preserved species (the most comprehensive taxon sampling to date) and 49 morphoanatomical characters (including nine continuous characters). KEY RESULTS: Our analyses recover monophyletic seed plants, Stenokoleales, and aneurophytes, with the latter placed as sister to a clade (termed the bilateral clade) that includes the former two. When added in the analysis, continuous characters based on anatomy improve phylogenetic resolution. CONCLUSIONS: Our results support the groups defined by traditional taxonomy, resolve Stenokoleales nested among the lignophytes, and indicate that seed plants may share a closer ancestor with Stenokoleales than with aneurophytes. Additionally, our trees suggest a Givetian minimum age for the seed plant ancestor, a late Emsian minimum age for the Stenokoleales, and early Emsian minimum ages for lignophytes, the bilateral clade, and the aneurophyte ancestor.

Origin of Equisetum: Evolution of horsetails (Equisetales) within the major euphyllophyte clade Sphenopsida.

PREMISE OF THE STUDY: Equisetum is the sole living representative of Sphenopsida, a clade with impressive species richness, a long fossil history dating back to the Devonian, and obscure relationships with other living pteridophytes. Based on molecular data, the crown group age of Equisetum is mid-Paleogene, although fossils with possible crown synapomorphies appear in the Triassic. The most widely circulated hypothesis states that the lineage of Equisetum derives from calamitaceans, but no comprehensive phylogenetic studies support the claim. Using a combined approach, we provide a comprehensive phylogenetic analysis of Equisetales, with special emphasis on the origin of genus Equisetum. METHODS: We performed parsimony phylogenetic analyses to address relationships of 43 equisetalean species (15 extant, 28 extinct) using a combination of morphological and molecular characters. KEY RESULTS: We recovered Equisetaceae + Neocalamites as sister to Calamitaceae + a clade of Angaran and Gondwanan horsetails, with the four groups forming a clade that is sister to Archaeocalamitaceae. The estimated age for the Equisetum crown group is mid-Mesozoic. CONCLUSIONS: Modern horsetails are not nested within calamitaceans; instead, both groups have explored independent evolutionary trajectories since the Carboniferous. Diverse fossil taxon sampling helps to shed light on the position and relationships of equisetalean lineages, of which only a tiny remnant is present within the extant flora. Understanding these relationships and early character configurations of ancient plant clades as Equisetales provide useful tests of hypotheses about overall phylogenetic relationships of euphyllophytes and foundations for future tests of molecular dates with paleontological data.

Vascularization of the Selaginella rhizophore: anatomical fingerprints of polar auxin transport with implications for the deep fossil record.

The Selaginella rhizophore is a unique and enigmatic organ whose homology with roots, shoots, or neither of the two remains unresolved. Nevertheless, rhizophore-like organs have been documented in several fossil lycophytes. Here we test the homology of these organs through comparisons with the architecture of rhizophore vascularization in Selaginella. We document rhizophore vascularization in nine Selaginella species using cleared whole-mounts and histological sectioning combined with three-dimensional reconstruction. Three patterns of rhizophore vascularization are present in Selaginella and each is comparable to those observed in rhizophore-like organs of fossil lycophytes. More compellingly, we found that all Selaginella species sampled exhibit tracheids that arc backward from the stem and side branch into the rhizophore base. This tracheid curvature is consistent with acropetal auxin transport previously documented in the rhizophore and is indicative of the redirection of basipetal auxin from the shoot into the rhizophore during development. The tracheid curvature observed in Selaginella rhizophores provides an anatomical fingerprint for the patterns of auxin flow that underpin rhizophore development. Similar tracheid geometry may be present and should be searched for in fossils to address rhizophore homology and the conservation of auxin-related developmental mechanisms from early stages of lycophyte evolution.