Organs and tissues of Rhynie chert plants

The role of fossils for reconstructing the evolution of plant development

Many of the developmental innovations that underpin the diversity of plant form alive today, such as those facilitating apical growth, branching, leaves, roots, wood and seeds, all evolved over 360 million years ago. Fossils, as our only direct record of plant form in the past, are thus essential for interpreting the origin and evolution of these innovations. The focus of this Spotlight is to showcase the rich plant fossil record open for developmental interpretation and to cement the role that fossils play at a time when increases in genome sequencing and new model species make tackling major questions in the area of plant evolution and development tractable for the first time.

Hidden functional complexity in the flora of an early land ecosystem

The first land ecosystems were composed of organisms considered simple in nature, yet the morphological diversity of their flora was extraordinary. The biological significance of this diversity remains a mystery largely due to the absence of feasible study approaches. To study the functional biology of Early Devonian flora, we have reconstructed extinct plants from fossilised remains in silico. We explored the morphological diversity of sporangia in relation to their mechanical properties using finite element method. Our approach highlights the impact of sporangia morphology on spore dispersal and adaptation. We discovered previously unidentified innovations among early land plants, discussing how different species might have opted for different spore dispersal strategies. We present examples of convergent evolution for turgor pressure resistance, achieved by homogenisation of stress in spherical sporangia and by torquing force in Tortilicaulis-like specimens. In addition, we show a potential mechanism for stress-assisted sporangium rupture. Our study reveals the deceptive complexity of this seemingly simple group of organisms. We leveraged the quantitative nature of our approach and constructed a fitness landscape to understand the different ecological niches present in the Early Devonian Welsh Borderland flora. By connecting morphology to functional biology, these findings facilitate a deeper understanding of the diversity of early land plants and their place within their ecosystem.

Air spaces bend light in plant stems

Intercellular air spaces are necessary for phototropism in Arabidopsis thaliana.

Hapalosiphonacean cyanobacteria (Nostocales) thrived amid emerging embryophytes in an early Devonian (407-million-year-old) landscape

Cyanobacteria have a long evolutionary history, well documented in marine rocks. They are also abundant and diverse in terrestrial environments; however, although phylogenies suggest that the group colonized land early in its history, paleontological documentation of this remains limited. The Rhynie chert (407 Ma), our best preserved record of early terrestrial ecosystems, provides an opportunity to illuminate aspects of cyanobacterial diversity and ecology as plants began to radiate across the land surface. We used light microscopy and super-resolution confocal laser scanning microscopy to study a new population of Rhynie cyanobacteria; we also reinvestigated previously described specimens that resemble the new fossils. Our study demonstrates that all are part of a single fossil species belonging to the Hapalosiphonaceae (Nostocales). Along with other Rhynie microfossils, these remains show that the accommodation of morphologically complex cyanobacteria to terrestrial ecosystems transformed by embryophytes was well underway more than 400 million years ago.

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.

Leaves and sporangia developed in rare non-Fibonacci spirals in early leafy plants

Lateral plant organs, including leaves and reproductive structures, are arranged on stems in distinct patterns termed phyllotaxis. Most extant plants exhibit phyllotactic patterns that are mathematically described by the Fibonacci series. However, it remains unclear what lateral organ arrangements were present in early leafy plants. To investigate this, we quantified phyllotaxis in fossils of the Early Devonian lycopod Asteroxylon mackiei. We report diverse phyllotaxis in leaves, including whorls and spirals. Spirals were all n:(n+1) non-Fibonacci types. We also show that leaves and reproductive structures occurred in the same phyllotactic series, indicating developmental similarities between the organs. Our findings shed light on the long-standing debate about leaf origins and demonstrate the antiquity of non-Fibonacci spirals in plants.

Evolution of meristem zonation by CLE gene duplication in land plants

In angiosperms, a negative feedback pathway involving CLAVATA3 (CLV3) peptide and WUSCHEL transcription factor maintains the stem-cell population in the shoot apical meristem and is central for continued shoot growth and organogenesis. An intriguing question is how this cell-signalling system was established during the evolution of land plants. On the basis of two recent studies on CLV3/ESR-related (CLE) genes, this paper proposes a model for the evolution of meristem zonation. The model suggests that a stem-cell-limiting CLV3 pathway is derived from stem-cell-promoting CLE pathways conserved in land pants by gene duplication in the angiosperm lineage. The model can be examined in the future by genomic and developmental studies on diverse plant species.

The origin and evolution of stomata

The acquisition of stomata is one of the key innovations that led to the colonisation of the terrestrial environment by the earliest land plants. However, our understanding of the origin, evolution and the ancestral function of stomata is incomplete. Phylogenomic analyses indicate that, firstly, stomata are ancient structures, present in the common ancestor of land plants, prior to the divergence of bryophytes and tracheophytes and, secondly, there has been reductive stomatal evolution, especially in the bryophytes (with complete loss in the liverworts). From a review of the evidence, we conclude that the capacity of stomata to open and close in response to signals such as ABA, CO₂ and light (hydroactive movement) is an ancestral state, is present in all lineages and likely predates the divergence of the bryophytes and tracheophytes. We reject the hypothesis that hydroactive movement was acquired with the emergence of the gymnosperms. We also conclude that the role of stomata in the earliest land plants was to optimise carbon gain per unit water loss. There remain many other unanswered questions concerning the evolution and especially the origin of stomata. To address these questions, it will be necessary to: find more fossils representing the earliest land plants, revisit the existing early land plant fossil record in the light of novel phylogenomic hypotheses and carry out more functional studies that include both tracheophytes and bryophytes.

An evidence-based 3D reconstruction of Asteroxylon mackiei, the most complex plant preserved from the Rhynie chert

The Early Devonian Rhynie chert preserves the earliest terrestrial ecosystem and informs our understanding of early life on land. However, our knowledge of the 3D structure, and development of these plants is still rudimentary. Here we used digital 3D reconstruction techniques to produce the first well-evidenced reconstruction of the structure and development of the rooting system of the lycopsid Asteroxylon mackiei, the most complex plant in the Rhynie chert. The reconstruction reveals the organisation of the three distinct axis types - leafy shoot axes, root-bearing axes, and rooting axes - in the body plan. Combining this reconstruction with developmental data from fossilised meristems, we demonstrate that the A. mackiei rooting axis - a transitional lycophyte organ between the rootless ancestral state and true roots - developed from root-bearing axes by anisotomous dichotomy. Our discovery demonstrates how this unique organ developed and highlights the value of evidence-based reconstructions for understanding the development and evolution of the first complex vascular plants on Earth.

Extending the fossil record for foliicolous Dothideomycetes: Bleximothyrium ostiolatum gen. et sp. nov., a unique fly-speck fungus from the Lower Cretaceous of Virginia, USA

PREMISE

Fossils can reveal long-vanished characters that inform inferences about the timing and patterns of diversification of living fungi. Through analyzing well-preserved fossil scutella, shield-like covers of fungal sporocarps, we describe a new taxon of early Dothideomycetes with a combination of characters unknown among extant taxa.

METHODS

Macerated clays from the Potomac Group, lower Zone 1, from the Lower Cretaceous (Aptian, 125-113 Ma) of Virginia USA yielded one gymnospermous leaf cuticle colonized by 21 sporocarps of a single fungal morphotype. We inferred a tree from nuclear ribosomal DNA of extant species, and coded morphological characters to evaluate alternative, equally parsimonious placements of the fossil in a molecular constraint tree of extant species.

RESULTS

Bleximothyrium ostiolatum gen. et sp. nov. has an ostiolate scutellum of radiate, dichotomizing hyphae. Unlike otherwise similar extant and fossil taxa, B. ostiolatum has tangled hyphae at its scutellum margin. Scutella of B. ostiolatum are connected to superficial mycelium, to intercalary and lateral appressoria, and to extensive subcuticular "mycélium en palmettes". The gymnospermous host has characters consistent with identity as a non-papillate ginkgophyte or cycad.

CONCLUSIONS

Bleximothyrium ostiolatum is the oldest known fossil fly-speck fungus that occurs on plant cuticles and has the radiate, ostiolate scutellum known only from Dothideomycetes. Its combination of characters, its scutellum margin, and mycélium en palmettes are unknown in other extant and fossil species, and Bleximothyrium ostiolatum likely represents a new group of fly-speck fungi that may now be extinct.

The Rhynie chert

In 1912, William Mackie, a medical practitioner surveying the regional geology west of Aberdeen, Scotland, happened on some unusual rocks (Figure 1) near the village of Rhynie. Dark gray to nearly black and shot through with cylindrical structures a few millimeters in diameter, these rocks differed markedly from the shales and volcanic rocks of local hills. Mackie had discovered the Rhynie chert - paleobotany's most iconic deposit - with its exceptionally preserved fossils that provide a uniquely clear view of early terrestrial ecosystems in statu nascendi. Early research by Robert Kidston and William Lang showed that the cylindrical structures in Rhynie rocks were the axes of early plants, preserved in remarkable cellular detail. A century of subsequent research confirmed that Rhynie provides not only an unparalleled record of early tracheophyte (vascular plant) evolution, but also offers additional paleontological treasures, including animals (mostly arthropods) and microorganisms ranging from fungi, algae, and oomycetes to testate amoebozoans, and even cyanobacteria. A captivating snapshot of life on land more than 400 million years ago, the Rhynie chert provides our earliest and best view of how terrestrial ecosystems came to be.

Rhynie chert fossils demonstrate the independent origin and gradual evolution of lycophyte roots

Mapping fossil traits onto the land plant phylogenetic framework indicates that there were at least two independent origins of roots among extant vascular plants - once in lycophytes and independently in euphyllophytes. At least two rooting structural types are found among extinct species preserved in the Rhynie chert. First, species that lacked roots and developed horizontal axes that developed rhizoids. Second, the rooting axes of Asteroxylon mackiei resembled the roots of extant lycopsids but lacked root hairs and root caps. These two rooting structures preceded the evolution of the roots of extant lycophytes comprising axes on which root hairs and root caps developed. These data demonstrate the defining root characters evolved gradually in the lycophyte lineage.

Testate Amoebae in the 407-Million-Year-Old Rhynie Chert

The Lower Devonian Rhynie chert is justly famous for the clear glimpse it offers of early terrestrial ecosystems [1]. Seven species of stem- and crown-group vascular plants have been described from Rhynie, many preserved in growth position [2], as well as 14 species of invertebrate animals, all arthropods [3] save for a single nematode population [4]. While these shed welcome light on early tracheophytes and land animals, modern terrestrial ecosystems additionally contain a diversity of microscopic organisms that are key to ecosystem function, including fungi, protists, and bacteria. Fungi ranging from mycorrhizae to saprophytes are well preserved in Rhynie rocks ([5] and references therein), and oomycetes are also present [5]. Both green algae (charophytes) and cyanobacteria have also been documented locally [6, 7, 8]. To date, however, phagotrophic protists have not been observed in Rhynie cherts, even though such organisms contribute importantly to carbon, nitrogen, and silica cycling in modern terrestrial communities [9]. Here, we report a population of organic tests described as Palaeoleptochlamys hassii gen. nov., sp. nov. from a pond along the Rhynie alluvial plain, which we interpret as arcellinid amoebozoans. These fossils expand the ecological dimensions of the Rhynie biota and support the hypothesis that arcellinids transitioned from marine through freshwater environments to colonize soil ecosystems in synchrony with early vascular plants.

Deciphering Silicification Pathways of Fossil Forests: Case Studies from the Late Paleozoic of Central Europe

The occurrence and formation of silicified wood from five late Paleozoic basins in Central Europe was investigated. Fossil wood from diverse geological settings was studied using field observations, taphonomic determinations as well as mineralogical analyses (polarizing microscopy, cathodoluminescence (CL) microscopy and spectroscopy). The results indicate that silicification is either a monophase or multiphase process under varying physico-chemical conditions. In particular, CL studies revealed complex processes of silica accumulation and crystallization. The CL characteristics of quartz phases in silicified wood can mostly be related to blue (390 and 440 nm), yellow (580 nm), and red (650 nm) emission bands, which may appear in different combinations and varying intensity ratios. Yellow CL is typical for initial silicification, reflecting quick precipitation under oxygen-deficient conditions caused by initial decay of the organic material. Blue CL is predominantly of secondary origin, resulting from replacement of precursor phases by a secondary hydrothermal quartz generation or subsequent silicification of wood. The red CL can be related to a lattice defect (non-bridging oxygen hole center—NBOHC).

Stepwise and independent origins of roots among land plants

Roots are one of the three fundamental organ systems of vascular plants¹, and have roles in anchorage, symbiosis, and nutrient and water uptake^2-4. However, the fragmentary nature of the fossil record obscures the origins of roots and makes it difficult to identify when the sole defining characteristic of extant roots-the presence of self-renewing structures called root meristems that are covered by a root cap at their apex^1-9-evolved. Here we report the discovery of what are-to our knowledge-the oldest meristems of rooting axes, found in the earliest-preserved terrestrial ecosystem¹⁰ (the 407-million-year-old Rhynie chert). These meristems, which belonged to the lycopsid Asteroxylon mackiei^11-14, lacked root caps and instead developed a continuous epidermis over the surface of the meristem. The rooting axes and meristems of A. mackiei are unique among vascular plants. These data support the hypothesis that roots, as defined in extant vascular plants by the presence of a root cap⁷, were a late innovation in the vascular lineage. Roots therefore acquired traits in a stepwise fashion. The relatively late origin in lycophytes of roots with caps is consistent with the hypothesis that roots evolved multiple times² rather than having a single origin¹, and the extensive similarities between lycophyte and euphyllophyte roots^15-18 therefore represent examples of convergent evolution. The key phylogenetic position of A. mackiei-with its transitional rooting organ-between early diverging land plants that lacked roots and derived plants that developed roots demonstrates how roots were 'assembled' during the course of plant evolution.

History and contemporary significance of the Rhynie cherts-our earliest preserved terrestrial ecosystem

The Rhynie cherts Unit is a 407 million-year old geological site in Scotland that preserves the most ancient known land plant ecosystem, including associated animals, fungi, algae and bacteria. The quality of preservation is astonishing, and the initial description of several plants 100 years ago had a huge impact on botany. Subsequent discoveries provided unparalleled insights into early life on land. These include the earliest records of plant life cycles and fungal symbioses, the nature of soil microorganisms and the diversity of arthropods. Today the Rhynie chert (here including the Rhynie and Windyfield cherts) takes on new relevance, especially in relation to advances in the fields of developmental genetics and Earth systems science. New methods and analytical techniques also contribute to a better understanding of the environment and its organisms. Key discoveries are reviewed, focusing on the geology of the site, the organisms and the palaeoenvironments. The plants and their symbionts are of particular relevance to understanding the early evolution of the plant life cycle and the origins of fundamental organs and tissue systems. The Rhynie chert provides remarkable insights into the structure and interactions of early terrestrial communities, and it has a significant role to play in developing our understanding of their broader impact on Earth systems.This article is part of a discussion meeting issue 'The Rhynie cherts: our earliest terrestrial ecosystem revisited'.