The first Vascular Plants appeared and underwent a dramatic radiation similar to that seem in Animals in the Cambrian, during the Late Silurian and Early Devonian. During this time both vegetative and sexual reproduction appeared, with some of the earliest Plants able to cover large areas through clonal growth, while at the same time producing spores which enabled them to distribute to new areas. During the Early Devonian structures such as leaves, roots, megaspores and secondary xylem also appeared. The Zosterophyllopsids formed a characteristic part of many Early Devonian floras, with species reported from China, North America, Europe and Australia. These Plants lacked leaves, and had a characteristic 'K'; or 'H' branching pattern, combined with lateral spike sporangia.
In a paper published in the journal Proceedings of the Royal Society B on 15 January 2025, Pu Huang of the Nanjing Institute of Geology and Palaeontology, Jia-Shu Wang of the Geological Museum of China and the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University, Yi-Ling Wang, also of the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University, Lu Liu of the National Natural History Museum of China, Jing-Yu Zhao of the School of Resources and Civil Engineering at Suzhou University, and Jin-Zhuang Xue also of the Key Laboratory of Orogenic Belts and Crustal Evolution at Peking University, describe a new species of Zosterophyllopsid Plant from the Early Devonian of Guizhou Province, China.
The new species is described upon the basis of two compression fossils from the Plant-bearing beds from the lower part of the Early Devonian Mangshan Group at Baoyang in Duyun City in Guizhou Province. These beds have been determined to be Pragian in age (410.8-407.6 million years old) on the basis of fossils which are also found in the Posongchong Formation of Yunnan Province, which in turn has been dated on the basis of spore assemblages, Plants and stratigraphic correlation It is placed in the genus Zosterophyllum, and given the specific name baoyangense, meaning 'from Baoyang'.
Zosterophyllum baoyangense comprises a rhizome system with K-shaped branching and upright sporangia-bearing spikes 5.8-10.8 mm high. Each of these spikes has 5-10 sporangia, arranged in a spiral pattern. The sporangia are oval to semicircular, 1.6−2.0 mm high and 0.9−1.4 mm wide, departing from axis at an acute angle by a short stalk.
The Zosterophyllopsids are a distinctive group of Plants found worldwide from the Late Silurian to the Late Devonian. There are currently 37 described genera in the group, although it is not completely certain they form a monophyletic group. The earliest members of the group appear in the Ludlow (427.4-423.0 million years ago), with the group reaching its maximum diversity during the Pragian, then declining during the Emsian (410.62-393.47 million years ago), and eventually disappearing during the Frasnian (382.7-372.2 million years ago.
Zosterophyllum baoyangense is notably smaller that other members of same genus, with a complete specimen measuring 45.4 mm in length and 0,5-1.3 mm in width, with a spike 10.8 mm high. For comparison, the contemporary Zosterophyllum confertum from western Germany can reach 5.1 mm in width with a preserved length of 440 mm. Silurian members of the genus were typically smaller, though they grew over time, while Emsian species, while rarer, show a wider range of sizes.
All species of Zosterophyllum lack any form of spikes, leaves, or leafy structures. However, they are presumed to have been able to photosynthesize due to the presence of stomata. With such a simple bodyplan, the only way that these plants would have been able to increase their photosynthetic surface would have been to get larger, a trend observed from their origin in the Late Silurian and through the Early Devonian.
Zosterophyllum baoyangense, however, does not comply with this trend, being much smaller than known contemporary species, and even most Silurian specimens. Huang et al. suggest that this may be a sign of a much shorter life-cycle than other species. This in turn could be an adaptation to an unstable environment, or one with very limited resources, showing that early Vascular Plants had begun to show adaptation to different environments by the Early Devonian.
The history of Vascular Plants can be divided into five evolutionary floras, the Rhyniophytic Flora, dominated by Rhyniophytes and Cryptospore producers, the Eophytic Flora, dominated by Zosterophyllopsids, followed by the Palaeophytic Flora, the Mesophytic Flora and the (modern0 Cainophytic Flora. The Plants of the Rhyniophytic Flora were typically very small, and it is thought that the (first) terrestrial habitats that they inhabited were probably very ephemeral. The Plants of the Eophytic Flora, while still very simple, are generally much larger, which has been interpreted as a sign of more stable environments developing. In South China, however, a strongly seasonal wet-dry climate is known to have developed in the Early Devonian, which may have made it harder for early Plants to stabilize environments, producing dwarfed species such as Zosterophyllum baoyangense, which could have completed their entire life-cycles in a shorter period of time.
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