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Edwards, Dianne [1], Morris, Jennifer L [2], Duckett, Jeffrey G [3], Pressel, Silvia [4], Kenrick, Paul [5].

Piecing together a new group of early land plants.

The earliest fossil evidence for plant life on land comes from spores dispersed in sediments, and the most distinctive and enigmatic of these are called cryptospores. They dominated assemblages for some 60 million years, first appearing about 470 million years ago during the Ordovician Period and diversifying rapidly. They declined abruptly during the Early Devonian, about 410 million years ago, becoming extinct shortly thereafter. The first and only direct evidence of cryptospore-producing plants comes from minute fossils preserved in charcoal in Upper Silurian and Lower Devonian rocks located near the border between England and Wales (UK). Among the most informative elements are tiny fragments of sporophytes measuring only a few millimetres in length. Previous work has shown that these have simple forking axes with distinctive longitudinal ridges. The axes terminate in sporangia in which cryptospore dyads and tetrads, like those dispersed in sediments, were documented in situ. Sporangia are mostly valvate with rare stomata. The form of the plants is therefore known only in part; further information is needed on overall habit, life cycle, and on internal anatomy, especially the character of the vascular system. Here we present evidence of a distinctive cell wall structure in tissues interpreted as vascular in fossils bearing sporangia that contain cryptospores. Cells are elongate, thick-walled, perforated by numerous plasmodesmata-derived pores, and sheets, strands, or globules of material that line or project into the cell lumen. Comparison with modern plants shows a striking similarity between the cell wall features in the fossils and the cell walls of the desiccation tolerant food- conducting cells of living mosses (e.g., leptoids and conducting parenchyma). When considered in the context of the minute size of the plants and the rarity of stomata, we conclude that the vascular system was composed only of living cells principally involved in food transport. There were no strictly water conducting cells. This suggests a poikilohydric physiology in which hydration was largely serviced through external surface water. It is likely that these early plants equilibrated rapidly with the water potential in their surroundings being either fully hydrated and metabolically active or desiccated and quiescent. Taken together, these new data reveal a suite of characteristics that distinguishes the cryptospore-producing plants from both bryophytes and vascular plants, hinting at the existence of a novel and hitherto unknown major group of early land plants.

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1 - Cardiff University, School of Earth and Ocean Sciences, Main Building, Park Place, Cardiff, CF10 3AT, UK
2 - Natural Environment Research Council, Polaris House, North Star Avenue, Swindon, SN2 1EU, UK
3 - The Natural History Museum, Life Sciences, Cromwell Road, London, SW7 5BD, United Kingdom
4 - The Natural History Museum, Department of Life Sciences, Cromwell Road, London, SW7 5BD, United Kingdom
5 - The Natural History Museum, Earth Sciences, Cromwell Road, London, SW7 5BD, UK

conducting cell.

Presentation Type: Oral Paper
Session: PL4, Paleobotany: Honoring Fran Hueber - Session II
Location: /
Date: Tuesday, July 20th, 2021
Time: 1:30 PM(EDT)
Number: PL4005
Abstract ID:289
Candidate for Awards:None

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