The origin of trees by the mid-Devonian epoch (398-385 million years ago) signals a major change in terrestrial ecosystems with potential long-term consequences including increased weathering, drop in atmospheric CO(2), modified climate, changes in sedimentation patterns and mass extinction. However, little is known about the ecology of early forests or how changes in early terrestrial ecosystems influenced global processes. One of the most famous palaeontological records for this time is the 'oldest fossil forest' at Riverside Quarry, Gilboa, New York, USA, discovered in the 1920s. Hundreds of large Eospermatopteris sandstone casts, now thought to represent the bases of standing cladoxylopsid trees, were recovered from a horizon that was originally interpreted as a muddy swamp. After quarry operations ceased, relatively minor outcrops of similar fossils at nearby localities have provided limited opportunities to evaluate this pervasive view using modern methods. In 2010, removal of the quarry backfill enabled reappraisal of the palaeoecology of this important site. Here we describe a 1,200 m(2) map showing numerous Eospermatopteris root systems in life position within a mixed-age stand of trees. Unexpectedly, large woody rhizomes with adventitious roots and aerial branch systems identified as aneurophytalean progymnosperms run between, and probably climb into, Eospermatopteris trees. We describe the overall habit for these surprisingly large aneurophytaleans, the earliest fossil group having wood produced by a bifacial vascular cambium. The site also provides evidence for arborescence within lycopsids, extending the North American range for trees in this ecologically critical group. The rooting horizon is a dark grey sandy mudstone showing limited root penetration. Although clearly belonging to a wetland coastal plain environment, the forest was probably limited in duration and subject to periodic disturbance. These observations provide fundamental clarification of the palaeoecology of this mixed-group early forest, with important implications for interpreting coeval assemblage data worldwide.

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The evolution of trees of modern size growing together in forests fundamentally changed terrestrial ecosystems. The oldest trees are often thought to be of latest Devonian age (about 380-360 Myr old) as indicated by the widespread occurrence of Archaeopteris (Progymnospermopsida). Late Middle Devonian fossil tree stumps, rooted and still in life position, discovered in the 1870s from Gilboa, New York, and later named Eospermatopteris, are widely cited as evidence of the Earth's 'oldest forest'. However, their affinities and significance have proved to be elusive because the aerial portion of the plant has been unknown until now. Here we report spectacular specimens from Schoharie County, New York, showing an intact crown belonging to the cladoxylopsid Wattieza (Pseudosporochnales) and its attachment to Eospermatopteris trunk and base. This evidence allows the reconstruction of a tall (at least 8 m), tree-fern-like plant with a trunk bearing large branches in longitudinal ranks. The branches were probably abscised as frond-like modules. Lower portions of the trunk show longitudinal carbonaceous strands typical of Eospermatopteris, and a flat bottom with many small anchoring roots. These specimens provide new insight into Earth's earliest trees and forest ecosystems. The tree-fern-like morphology described here is the oldest example so far of an evolutionarily recurrent arborescent body plan within vascular plants. Given their modular construction, these plants probably produced abundant litter, indicating the potential for significant terrestrial carbon accumulation and a detritus-based arthropod fauna by the Middle Devonian period.

known phylls, have the longest history of any group of vascular land plants. The early evolution of the group is reviewed concentrating on the Late Silurian and Devonian record of Lycopsida and Zosterophyllopsida. Distinct root-like and shoot-like axes in zosterophyllopsids and lycopsids are first recorded in the Early Devonian and can be compared to the later development of pseudobipolar growth of tree-shaped lycopsids with upward-growing trunks and branch systems and downwardgrowing rooting systems. The development of stigmarian rootlets postdates the evolution of pseudobipolar growth, first recorded in the Middle Devonian, according to current evidence. Other important events in early lycophyte evolution during the Devonian include changes in leaf morphology, modification of sporophylls, and the eventual appearance of the lycopsid strobili.

The Middle to early Late Devonian transition from diminutive plants to the first forests is a key episode in terrestrialization. The two major plant groups currently recognized in such "transitional forests" are pseudosporochnaleans (small to medium trees showing some morphological similarity to living tree ferns and palms) and archaeopteridaleans (trees with woody trunks and leafy branches probably related to living conifers). Here we report a new type of "transitional" in-situ Devonian forest based on lycopsid fossils from the Plantekløfta Formation, Munindalen, Svalbard. Previously regarded as very latest Devonian (latest Famennian, 360 Ma), their age, based on palynology, is early Frasnian (ca. 380 Ma). In-situ trees are represented by internal casts of arborescent lycopsids with cormose bases and small ribbon-like roots occurring in dense stands spaced ~15-20 cm apart, here identified as Protolepidodendropsis pulchra Høeg. This plant also occurs as compression fossils throughout most of the late Givetian-early Frasnian Mimerdalen Subgroup. The lycopsids grew in wet soils in a localized, rapidly subsiding, short-lived basin. Importantly, this new type of Middle to early Late Devonian forest is paleoequatorial and hence tropical. This high-tree-density tropical vegetation may have promoted rapid weathering of soils, and hence enhanced carbon dioxide drawdown, when compared with other contemporary and more high-latitude forests.

We present the rationale for a cross-disciplinary investigation addressing the 'Devonian plant hypothesis' which proposes that the evolutionary appearance of trees with deep, complex rooting systems represents one of the major biotic feedbacks on geochemical carbon cycling during the Phanerozoic. According to this hypothesis, trees have dramatically enhanced mineral weathering driving an increased flux of Ca 2+ to the oceans and, ultimately, a 90% decline in atmospheric CO 2 levels through the Palaeozoic. Furthermore, experimental studies indicate a key role for arbuscular mycorrhizal fungi in soil-plant processes and especially in unlocking the limiting nutrient phosphorus in soil via Ca-phosphate dissolution mineral weathering. This suggests co-evolution of roots and symbiotic fungi since the Early Devonian could well have triggered positive feedbacks on weathering rates whereby root-fungal P release supports higher biomass forested ecosystems. Long-standing areas of uncertainty in this paradigm include the following: (1) limited fossil record documenting the origin and timeline of the evolution of tree-sized plants through the Devonian; and (2) the effects of the evolutionary advance of trees and their in situ rooting structures on palaeosol geochemistry. We are addressing these issues by integrating palaeobotanical studies with geochemical and mineralogical analyses of palaeosol sequences at selected sites across eastern North America with a particular focus on drill cores from Middle Devonian forests in Greene County, New York State.

This is a repository copy of Mid-Devonian Archaeopteris roots signal revolutionary change in earliest fossil forests.

Lycophyte trees, up to 50 m in height, were the tallest in the Carboniferous coal swamp forests. The similarity in their shoot and root morphology led to the hypothesis that their rooting (stigmarian) systems were modified leafy shoot systems, distinct from the roots of all other plants. Each consists of a branching main axis covered on all sides by lateral structures in a phyllotactic arrangement; unbranched microphylls developed from shoot axes, and largely unbranched stigmarian rootlets developed from rhizomorphs axes. Here, we reexamined the morphology of extinct stigmarian systems preserved as compression fossils and in coal balls from the Carboniferous period. Contrary to the long-standing view of stigmarian systems, where shoot-like rhizomorph axes developed largely unbranched, root-hairless rootlets, here we report that stigmarian rootlets were highly branched, developed at a density of ∼25,600 terminal rootlets per meter of rhizomorph, and were covered in root hairs. Furthermore, we show that this architecture is conserved among their only extant relatives, herbaceous plants in the Isoetes genus. Therefore, despite the difference in stature and the time that has elapsed, we conclude that both extant and extinct rhizomorphic lycopsids have the same rootlet system architecture.evolution | paleobotany | Carboniferous forests | stigmarian root systems | Isoetes T he spread of the first wetland forests with tall trees during the Carboniferous period (359-300 million years ago) had a dramatic impact on the carbon cycle by burying large amounts of organic carbon in the form of peat in coal swamps (1, 2). Lycophyte trees up to 50 m in height (3, 4) were dominant components of coal swamp forests (5, 6). They were key components of coal-forming environments throughout the Carboniferous period but dominated in the lower-middle Pennsylvanian (Namurian-Wetsphalian) where they typically contribute between 60% and 95% of the biomass in buried peat (7-13). The preserved remains of lycophyte trees form some of the most extensive fossil plant deposits of any geological period. This is in part because of their size and ecological dominance but also the result of the high probability of preservation in the waterlogged conditions in which these trees grew (4). Detailed descriptions of the morphology of these plants on a range of scales-from entire in situ tree lycophyte forests (14, 15) to cellular descriptions of developing spores (16)-have made these trees some of the best understood fossil plants of the Carboniferous coal swamps.The rooting system of the arborescent lycopsids-stigmarian systems-consist of large shoot-like axes (rhizomorphs) that develop lateral organs called rootlets (4,(17)(18)(19)(20). Rootlets, which have been described as largely unbranched and root hairless (4,5,(17)(18)(19)(20)(21)(22)(23)(24), are arranged in a characteristic pattern or rhizotaxy on the rhizomorph (25). It is the arrangement of these largely unbranched leaf-like rootlets on a shoot-like axis that first led to the theory that stigm...