The initial radiation of vascular land plants, evidenced by increases in both diversity and morphological disparity during the Silurian and Devonian periods, is considered plant terrestrialization, which can be seen as the terrestrial equivalent of the Cambrian explosion of marine animals. During this period, novel structures such as tracheids, stomata, leaves, roots, and secondary xylem evolved. However, the evolution of life-history strategies in early land plants remains poorly understood.
As a predominant and typical component of Early Devonian floras, Zosterophyllopsida has a long history of research and has been widely documented in palaeontology textbooks. This group appeared in the late Silurian and persisted into the Late Devonian. Some species, such as Z. shengfengense, are preserved as complete or nearly complete plants. Thus, zosterophyllopsids provide important material for studying the life-history strategies of early land plants.
Recently, Dr. HUANG Pu from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences, in collaboration with other researchers, reported a new species of Zosterophyllum, Z. baoyangense, from the Lower Devonian of Guizhou Province. They also compiled a multi-morphological-character dataset of zosterophyllopsids from the late Silurian to Early Devonian and revealed divergent life-history strategies within this group. The study was published in Proceedings of the Royal Society B.
Unlike previous Zosterophyllum species, the plant body of the newly discovered Zosterophyllum baoyangense is approximately 45.4 mm in total length, with spikes ranging from 5.8 to 10.8 mm in height. Its smaller body size and tiny spikes represent one of the smallest extremes in early land plants.
The researchers collected morphological traits of Zosterophyllum species and zosterophyllopsids, including the width and length of axes, the width and length of spikes, and the height and width of sporangia. They also proposed a new descriptor, total sporangial accommodation (TSA), to evaluate the mass or energy investment required for spore production in each plant.
The researchers found that the length and width of axes generally follow the same trend as the diversity of Zosterophyllum species, increasing from the Ludlow to the Early Devonian, with the maximum range reached in the Early Devonian. The TSA values and sporangial size of Zosterophyllum also show a similar pattern, with a significant difference between vegetative and reproductive organs in the Early Devonian. Z. baoyangense represents the minimum TSA value, ranging from 4.3 to 16.8 mm³, while the TSA of Z. australianum ranges from 29.6 to 499.0 mm³, observed in the same bed of the Baoyang section. In addition, TSA appears to be closely related to axial width. For members of Lycophytina sensu Kenrick & Crane and Zosterophyllopsida sensu Hao & Xue, the length and width of axes follow a similar evolutionary trend.
In this study, the researchers considered that the highly morphologically diverse zosterophyllopsids in the Pragian represent two groups with different life-history strategies. One group, similar to r-selected species, is characterized by smaller sizes, smaller TSA, shorter lifespans, and lower vegetative and reproductive output. These species, such as Z. baoyangense, appear to be adapted to turbulent environments. The other group, such as Z. australianum, in contrast, resembles K-selected species. The researchers argued that the transition from the Rhyniophytic Flora to the Eophytic Flora was likely driven by the evolution of divergent life-history strategies within Zosterophyllum and more broadly in zosterophyllopsids.
This work was financially supported by the National Key R&D Program of China and the National Natural Science Foundation of China.
Figure 1. Zosterophyllum baoyangense sp. nov., showing a fertile axis with K-shaped branching and a terminal spike. (Image by HUANG Pu, LIU Lu and XUE Jinzhuang in 2017)
Figure 2.Diversity and morphology of Zosterophyllum species through the late Silurian to Early Devonian. (a) Species richness;(b) Maximum length of axes; (c) Width of axes; (d) TSA; (e) Crossplot of sporangial width and height of different time bins; (f, g) Crossplot of TSA and axial width for all sampled Zosterophyllum species. (Image by HUANG Pu, WANG Yiling, WANG Jiashu)
Figure 3. Artist's restoration of part of the Early Devonian Mangshan flora, with plant communities of Zosterophyllum baoyangense at the front, and Z. australianum and an unnamed plant to the back. (Image by HUANG Pu, TAN Chao)
