Non-technical Summary
About 435 million years ago, during the early Silurian Period, warm seas covered what is now South China, providing a habitat for brachiopods—small, clam-like sea creatures. Scientists long thought the “Xiushan Fauna,” a unique group of these fossils, thrived only in shallow, sunlit coastal waters stirred by waves. Our new study in Sichuan Province’s Bifengguan area challenges this idea. By examining thousands of brachiopod shells in dark limestone rocks, we found three distinct associations that show the seafloor gradually becoming shallower over time. The oldest group, dominated by the brachiopod Epitomyonia, lived in deeper, calmer waters. As the land slowly rose, the sea transitioned to mid-depth and then shallow waters, each stage hosting a different mix of brachiopod species. The unique habitat at Bifengguan likely acted as a refugium, preserving these deep-water creatures in the fossil record. This discovery expands the known habitat range of the Xiushan Fauna to deeper waters and underscores the critical role of local paleogeography in shaping benthic community structure.
Introduction
The Late Ordovician Mass Extinction (LOME) was succeeded by a major climatic amelioration and transgression during the Llandovery (Brenchley et al., Reference Brenchley, Carden, Hints, Kaljo, Marshall, Martma, Meidla and Nolvak2003; Finnegan et al., Reference Finnegan, Bergmann, Eiler, Jones, Fike, Eisenman, Hughes, Tripati and Fischer2011; Trotter et al., Reference Trotter, Williams, Barnes, Männik and Simpson2016; Melchin et al., Reference Melchin, Sadler, Cramer, Gradstein, Ogg, Schmitz and Ogg2020; Rong and Huang, Reference Rong and Huang2023; Harper, Reference Harper2024), which created extensive new epicontinental habitats and spurred a global radiation of brachiopods (e.g., Rong and Harper, Reference Rong and Harper1999; Rasmussen and Harper, Reference Rasmussen and Harper2011; Huang et al., Reference Huang, Jin and Rong2018; Cocks and Rong, Reference Cocks and Rong2019). Although Llandovery brachiopods in South China are generally well documented, previous research has largely concentrated on Rhuddanian and Aeronian faunas, with those of the Telychian remaining comparatively understudied, except for a few key studies (e.g., Yang and Rong, Reference Yang and Rong1982; Chen and Rong, Reference Chen and Rong1996).
The early to middle Telychian of South China is recognized as a critical interval for post-LOME brachiopod evolution (Cocks and Rong, Reference Cocks and Rong2019; Huang et al., Reference Huang, Chen and Shi2025). Faunas from this interval in the region are typically characterized by a two-stage succession. Early assemblages are generally of low diversity and abundance, a pattern linked to the prevalence of restricted, nearshore environments often associated with the deposition of shallow marine red beds (Rong et al., Reference Rong, Wang and Zhang2012). In contrast, subsequent faunas became significantly more abundant and diverse, reflecting a transition to more stable, normal marine conditions. However, these richer Telychian assemblages have, until now, been documented almost exclusively from shallow-water settings. The recent discovery of a deep-water brachiopod assemblage from the Ningqiang Formation in the Guangyuan area of Sichuan Province, located on the northern margin of the Yangtze Platform, therefore provides critical new data for understanding Telychian biofacies and community structure.
This study presents this unique deep-water fauna and investigates its paleoecological structure. We employ network analysis (NA) and non-metric multidimensional scaling (NMDS), based on the Raup–Crick (RC) similarity index, to characterize the community. The results are used to interpret the paleoecology and environmental setting of the assemblage and to discuss its significance for understanding the ecological turnover that occurred on the Yangtze Platform during the Telychian.
Materials and methods
Studied section and fossil materials
The brachiopod fossils for this study were collected from the Bifengguan section (105°58′47.17″E, 32°41′2.92″N), which is located along a roadside exposure near the Bifengguan Taoist temple in Xuanhe Town, Chaotian District, Guangyuan City, Sichuan Province, China (Fig. 1). It exposes the middle to lower part of the Shenxuanyi Member of the Ningqiang Formation, from which fossils were systematically collected from several measured horizons (BFG 01–05, SGQQ 07–14). The detailed stratigraphy and a comprehensive faunal list for the section are presented in the accompanying stratigraphic column (Fig. 2).
Figure 1. Map showing the geographical location of the Bifengguan section in Chaotian Town, Guangyuan City, northern Sichuan, South China.
Figure 2. Collections made in the Ningqiang Formation at Bifengguan section. Range chart shows brachiopod composition.
The fossiliferous strata at this section consist primarily of yellowish-brown, yellowish-green, and light-purple mudstones and shales. The collected assemblage is highly diverse, yielding a total of 1108 specimens. These were assigned to 21 species belonging to 20 genera: Aegiria shiqianensis Yang and Rong, Reference Yang and Rong1982; ‘Clorinda’ cf. C. globosa (Sowerby, Reference Sowerby and Murchison1839); Epitomyonia bifengguanensis Chen, Reference Chen2023; Fardenia sp.; Howellella shiqianensis (Rong et al., Reference Rong, Xu and Yang1974); Leptaena sp.; Linostrophomena convexa Rong et al., Reference Rong, Xu and Yang1974; Megaspinochonetes subrectangulatis Yang and Rong, Reference Yang and Rong1982; Mesopholidostrophia minor (Rong et al., Reference Rong, Xu and Yang1974); Nalivkinia elongata (Wang, Reference Wang1956); Nucleospira calypta Rong et al., Reference Rong, Xu and Yang1974; Paracraniops cf. P. ellipticus Goriansky, Reference Goriansky and Zanina1972; Pentlandina cf. P. lewisii (Davidson, Reference Davidson1847); Quangyuania ovalia Sheng, Reference Sheng1975; Salopinella minuta (Rong et al., Reference Rong, Xu and Yang1974); Spinochonetes notata Rong et al., Reference Rong, Xu and Yang1974; Eospirifer sp.; Striispirifer jingshanensis Wang, Reference Wang1984; Striispirifer sp.; Valdaria sp.; and Whitfieldella sp.
The assemblage is dominated by three species, which together account for over 85% of the total fauna: Aegiria shiqianensis (463 specimens; 41.8%), Megaspinochonetes subrectangulatis (307 specimens; 27.7%), and Spinochonetes notata (180 specimens; 16.2%). The presence of genera such as Linostrophomena and Megaspinochonetes, and particularly the endemic genus Quangyuania, allows the assemblage to be assigned to the upper Xiushan Fauna, indicating an early to middle Telychian age for these strata.
Methods
To investigate compositional similarities among the fossil assemblages, we performed non-metric multidimensional scaling (NMDS). This ordination was based on the Raup–Crick (RC) similarity index (Raup and Crick, Reference Raup and Crick1979), a probabilistic measure selected for its robustness in handling sparse paleontological data matrices (Huang, Reference Huang2011). The NMDS analysis was carried out in PAST v.4.01 (Hammer et al., Reference Hammer, Harper and Ryan2001).
To further explore the underlying ecological structure, we employed network analysis (NA), a powerful tool for visualizing relationships between taxa and sampling horizons (e.g., Sidor et al., Reference Sidor, Vilhena, Angielczyk, Huttenlocker, Nesbitt, Peecook, Steyer, Smith and Tsuji2013; Huang et al., Reference Huang, Zhan and Wang2016; Reference Huang, Jin and Rong2018; Kiel, Reference Kiel2017; Baarli et al., Reference Baarli, Huang and Johnson2024). A bipartite network based on the species-by-collection dataset was generated in Gephi v.0.10 (Bastian et al., Reference Bastian, Heymann and Jacomy2009). We calculated modularity to detect distinct faunal associations (communities) within the network. In the resulting graph, the size and grayscale intensity of the collection nodes were scaled by their average degree to emphasize their relative connectivity.
Finally, to visually summarize the distribution and relative abundance of taxa across the sampled horizons, a heatmap was generated using the pheatmap package (v.1.0.12; Kolde, Reference Kolde2025) in R (v.4.3.1). The authors used a large language model to improve the language in parts of the manuscript.
Repository and institutional abbreviation
All figured specimens are housed in the Nanjing Institute of Geology and Palaeontology (NIGP), Chinese Academy of Sciences.
Results
Our quantitative analyses reveal a clear ecological structure and succession within the Bifengguan section. Bipartite network analysis (NA) of the species-by-collection dataset groups the samples into three main clusters (visualized as green, purple, and orange nodes), with one collection appearing as an outlier (blue node). In the network graph, the sizes of collection nodes correspond to their degree, visually representing collection diversity. This tripartite division, identified through modularity, is broadly corroborated by the Non-Metric Multidimensional Scaling (NMDS) ordination based on the Raup–Crick similarity measure, which also separates the sample groups clearly in ordination space (Fig. 3).
Figure 3. Bipartite network diagram of NA (1) and NMDS results (2) for 10 collections showing brachiopod assemblage composition in the Bifengguan section. Different colors in the figure represent distinct associations identified by modularity analysis. Striispirifer 1 = Striispirifer jingshanensis; Striispirifer 2 = Striispirifer sp.
While the NA and NMDS analyses group the samples based on faunal composition, the heatmap provides a clearer view of relative abundances and species dominance (Fig. 4). The heatmap highlights two major faunal breaks: a distinct compositional shift at the uppermost horizon (SGQQ07) and a notable change in diversity and species dominance occurring above horizon BFG02. Based on the combined results of these analyses, three successive brachiopod associations are recognized in the Bifengguan section, each of which is described below in ascending stratigraphic order.
Figure 4. Heatmap showing the number of specimens per species in the Bifengguan section. Striispirifer 1 = Striispirifer jingshanensis; Striispirifer 2 = Striispirifer sp.
The Aegiria–Epitomyonia association
This lower association occurs in horizons BFG 03–05 and SGQQ 13–14. It comprises 160 specimens from four collections and contains 10 genera. The association is strongly dominated by Aegiria shiqianensis (56.3% of specimens, Fig. 5.3), with Epitomyonia bifengguanensis (Fig. 6.5–6.14) as the second-most abundant species (15.6%). The co-dominance of these two genera, both considered deep-water indicators, is significant. Notably, Epitomyonia is rare or absent in the well-documented shallow-water Telychian faunas of South China. The prominence of these taxa suggests a deep-water environment, corresponding to Benthic Assemblage 4 (BA4).
Figure 5. (1) Pentlandina cf. P. lewisii (Davidson): dorsal internal mold (NIGP 209277, sample SGQQ-07). (2) Leptaena (Septomena) sp.: dorsal internal mold and broken external mold (NIGP 209278, sample SGQQ-12). (3) Aegiria shiqianensis Yang & Rong: ventral internal mold (NIGP 209279, sample SGQQ-12). (4, 5) Linostrophomena convexa Rong: (4) dorsal internal mold (NIGP 209280, sample SGQQ-07); (5) dorsal external mold (NIGP 209281, sample SGQQ-07). (6–11) Megaspinochonetes subrectangulatis Yang & Rong: (6) ventral external mold (NIGP 209282, sample BFG-02); (7) ventral internal mold (NIGP 209283, sample SGQQ-04); (8, 9) dorsal internal mold and latex cast showing cardinal process and socket ridge (NIGP 209284, sample BFG-02); (10) dorsal internal mold, detail of cardinalia (NIGP 209285, sample BFG-01); (11) dorsal external mold (NIGP 209286, sample BFG-02). (12, 13) Spinochonetes notata Rong et al.: (12) ventral internal mold (NIGP 209287, sample SGQQ-12); (13) dorsal external mold (NIGP 209288, sample SGQQ-12). (14, 15) Valdaria sp.: dorsal internal mold and external mold (NIGP 209289, sample SGQQ-12). (16) Salopinella minuta Rong et al.: ventral internal mold (NIGP 209290, sample BFG-05). Scale bars represent 2 mm except where indicated otherwise.
Figure 6. (1–4) Fardenia sp.: (1) ventral internal mold (NIGP 209291, sample SGQQ-07); (2) dorsal internal mold (NIGP 209292, sample SGQQ-07); (3, 4) external mold and detail of ornamentation (NIGP 209293, sample SGQQ-07). (5–14) Epitomyonia bifengguanensis: (5) ventral internal mold (NIGP 209294, sample BFG-03); (6, 7) ventral internal mold and latex cast (NIGP 209295, sample BFG-03); (8–11) holotype (NIGP 209296, sample BFG-03): (8) dorsal internal mold; (9) cast; (10) cast in oblique anterolateral view; (11) cast in oblique lateral view; (12) dorsal internal mold (NIGP 209297, sample BFG-03); (13, 14) two dorsal external molds (NIGP 209298, NIGP 209299, sample BFG-03). (15–18) ‘Clorinda’ cf. C. globosa (Sowerby): (15, 16) ventral internal mold and posterior detail (NIGP 209300, sample SGQQ-07); (17, 18) dorsal internal mold and posterior detail (NIGP 209301, sample SGQQ-07). Scale bars represent 1 mm except where indicated otherwise.
The Aegiria–Megaspinochonetes association
This association is found in horizons BFG 01, 02, and SGQQ 10–12. It is the most diverse and abundant association in the section, with 884 specimens representing 12 genera collected from five horizons. While Aegiria shiqianensis remains the most abundant species (42.2%), the faunal composition is distinctly altered by the emergence of Megaspinochonetes subrectangulatis as a co-dominant element (34.6%). The continued dominance of Aegiria suggests an offshore setting, but the appearance of Megaspinochonetes (not a typical deep-water indicator) and the disappearance of the deep-water genus Epitomyonia signal a shift to a shallower environment. This association is therefore interpreted as representing a lower BA3 setting.
The Fardenia–Striispirifer association
The uppermost association is represented by a single collection from horizon SGQQ 07, containing 64 specimens. Its composition marks a radical departure from the underlying associations. It is co-dominated by Fardenia sp. (39.1%) and Striispirifer jingshanensis (35.9%) (Fig. 7.5–7.10). The complete turnover in dominant taxa and its distinct separation from the other assemblages in all quantitative analyses indicate a major faunal replacement event at the top of the studied interval. Meanwhile, the disappearance of deep-water taxa such as Aegiria in this association indicate a shallower environment that can be assigned to upper BA3 setting.
Figure 7. (1, 2) Whitfieldella sp.: (1) dorsal internal mold (NIGP 209302, sample BFG-05); (2) ventral internal mold (NIGP 209303, sample BFG-03). (3) Nucleospira calypta Rong et al.: ventral internal mold (NIGP 209304, sample SGQQ-07). (4) Striispirifer sp.: ventral internal mold (NIGP 209305, sample BFG-03). (5–10) Striispirifer jingshanensis Wang: (5) ventral internal mold (NIGP 209306, sample SGQQ-07); (6, 7) dorsal internal mold and external mold (NIGP 209307, sample SGQQ-07); (8, 9) dorsal internal mold and hairy cardinal process (NIGP 209308, sample SGQQ-07); (10) ventral external mold, detail (NIGP 209309, sample SGQQ-07). (11–17) Howellella shiqianensis Rong & Yang: (11–13) dorsal external mold and shell-surface details (NIGP 209310, sample SGQQ-07); (14) dorsal internal mold (NIGP 209311, sample SGQQ-07); (15, 16) dorsal internal mold and posterior view (NIGP 209312, sample SGQQ-07); (17) dorsal internal mold (NIGP 209313, sample SGQQ-07). Scale bars represent 2 mm except where indicated otherwise.
Discussion
Associations and environmental settings
The Telychian brachiopod faunas of South China are classically exemplified by the assemblages from the Xiushan Formation (Chen and Rong, Reference Chen and Rong1996). Early research established a tripartite succession for the upper Xiushan Formation, comprising the Nalivkinia–Nucleospira, Aegiria–Valdaria, and Linostrophomena–Xinanospirifer associations in ascending order (Yang and Rong, Reference Yang and Rong1982). Subsequent studies refined this framework, generally dividing the regional faunas into a lower (Nalivkinia–Nucleospira) association and an upper (Salopinella–Xinanospirifer) association. This upper fauna was further subdivided into facies-controlled biofacies (mud, marl, reef, and reef-flank), all of which were interpreted as representing various shallow-water environments of Benthic Assemblage 2–3 (BA2–BA3) (Chen and Rong, Reference Chen and Rong1996). The lower two associations identified in our study, the Aegiria–Epitomyonia and Aegiria–Megaspinochonetes associations, are broadly correlative with the widespread Aegiria–Valdaria association, while our uppermost Fardenia–Striispirifer association corresponds with the Linostrophomena–Xinanospirifer level.
The Bifengguan fauna, however, presents a significant departure from this established shallow-water paradigm. The fossils were recovered from yellowish-brown, yellowish-green, and light-purple mudstones and shales of the Shenxuanyi Member of the Ningqiang Formation. Crucially, no shallow-water sedimentary structures, such as wave ripples or cross-bedding, were observed throughout the section. This lithological evidence indicates a persistently low-energy setting. Our analysis indicates a depositional environment ranging from the lower part of BA3 to BA4. This represents a deeper and lower-energy setting than the previously documented clastic and marly facies of the Xiushan Fauna (cf., Rong et al., Reference Rong, Shen, Zhan, Qiao, Huang, Jin, Rong, Jin, Shen and Zhan2017).
The composition of the Bifengguan fauna shows both similarities to and marked differences from the previously described shallow-water facies. It is dominated by Aegiria shiqianensis, Megaspinochonetes subrectangulatis, and Spinochonetes notata. The interpretation of an extremely quiet-water environment is further supported by taphonomic evidence: specimens of Megaspinochonetes subrectangulatis found in the lower associations frequently preserve delicate, unbroken median spines exceeding twice the shell length (e.g., Fig. 5.7). While taxa common in other facies are present, their relative abundances are notably different. For instance, Nalivkinia magna Yang and Rong, Reference Yang and Rong1982, common in the clastic and marly facies, is present but subordinate. Conversely, dominant elements of the marly facies, such as Quangyuania ovalia and Septatrypa guangyuanensis (Sheng, Reference Sheng1975), are rare or absent at Bifengguan. Furthermore, the Bifengguan fauna contains several taxa that are either rare or absent in other facies, including Fardenia sp., Leptaena (Septomena) sp., and a distinct species of Striispirifer (S. jingshanensis). This distinct composition is further highlighted by horizons showing extreme dominance, such as BFG02, where Megaspinochonetes subrectangulatis constitutes 79% of the assemblage.
The most compelling paleoecological evidence for a deep-water setting, however, lies in the abundance of Epitomyonia. This genus is exceedingly rare throughout the rest of the Xiushan Fauna, with only two specimens previously reported from all other localities combined (Rong Jiayu, personal communication, Reference Rong and Huang2023). The recovery of 25 specimens of Epitomyonia bifengguanensis from the Bifengguan section alone, where it is a co-dominant element of the basal association, strongly supports the interpretation of a deep-water environment, comparable to other Silurian deep-water assemblages where Epitomyonia is also a key component (e.g., Chen et al., Reference Chen, Jin and Lenz2012). Therefore, this study presents the first robust evidence for a deep-water expression of the Xiushan Fauna, expanding our understanding of the ecological breadth and biogeographical complexity of brachiopod communities in South China during the early to middle Telychian.
Faunal turnover and paleogeographical implications
The faunal succession at Bifengguan documents a distinct environmental shift, progressing from the deep-water Aegiria–Epitomyonia association (BA4) to the shallower Aegiria–Megaspinochonetes association (lower BA3). This transition is marked by a significant increase in both species diversity and overall specimen abundance. Intriguingly, while the appearance of Megaspinochonetes and the disappearance of the key deep-water indicator Epitomyonia signal this shallowing, the primary dominant genus, Aegiria, not only persists but thrives, increasing its numerical dominance. This pattern, where diversity and abundance peak in an intermediate offshore environment, aligns well with the classic benthic assemblage model, which identifies BA3 as typically representing the most optimal conditions for many benthic communities (Boucot, Reference Boucot1975). The final turnover to the Fardenia–Striispirifer association at the top of the section represents a more profound ecological reorganization, likely driven by continued environmental change.
This shallowing-upward trend recorded in the Bifengguan section likely reflects a broader regional tectonic regime rather than just localized environmental fluctuations. The pattern is consistent with the progressive uplift of the South China Platform, a process that began with the Late Ordovician Kwangsian Orogeny and continued with the emergence of the Yangtze Platform during the early to middle Telychian (Chen et al., Reference Chen, Fan, Chen, Tang and Hou2014). While the ultimate drivers are debated, the continuous northwestern expansion of the Cathaysian Oldland is thought to have induced significant regional tectonic effects, leading to widespread regression (Rong et al., Reference Rong, Boucot, Harper, Zhan and Neuman2003, Reference Rong, Wang and Zhang2012). This regional shallowing can be superimposed upon global eustatic sea-level fluctuations during this interval (Haq and Schutter, Reference Haq and Schutter2008), resulting in the complex local environmental history observed at Bifengguan.
The general absence of globally recognized pentameride communities within the Xiushan Fauna warrants discussion, particularly because the Telychian is often considered a period of low global provinciality (Huang et al., Reference Huang, Chen and Shi2025). During this time, low-latitude platforms such as Laurentia, Baltica, and Avalonia, brought into proximity by the narrowing Iapetus Ocean, were characterized by widespread and cosmopolitan pentameride communities (e.g., Pentamerus, Stricklandia, and Clorinda) that occupied a range of environments from BA3 to BA5 (see Jin, Reference Jin2008). Their conspicuous rarity in South China can be attributed to a combination of strong regional provincialism and unfavorable environmental conditions. Biogeographically, South China exhibited significant endemism in the middle Telychian, with over 30% of its brachiopod genera being unique to the region, a figure much higher than on other paleoplates (Cocks and Rong, Reference Cocks and Rong2019). This isolation likely acted as a primary filter limiting the influx of cosmopolitan pentameride taxa. Environmentally, the dominant pentamerides were typically large, thick-shelled, sessile filter-feeders adapted to clearer waters and relatively firm, often carbonate, substrates (e.g., Jin, Reference Jin2008; Huang et al., Reference Huang, Jin and Rong2018). In contrast, while South China did have carbonate facies during this interval, brachiopod diversity was predominantly sourced from siliciclastic settings (mudstones and shales). The ongoing tectonic influence of the Kwangsian Orogeny resulted in widespread clastic deposition and likely higher sedimentation rates, creating soft and turbid substrates unsuitable for the establishment of large pentameride populations. This marks a notable departure from earlier Silurian intervals in South China, such as the Aeronian, where pentameride-dominated shell beds (Paraconchidium shiqianensis Rong et al., Reference Rong, Xu and Yang1974) did flourish (Li and Rong, Reference Li and Rong2007), and the upper Rhuddanian, where assemblages dominated by Sinokulumbella transversa (Grabau, Reference Grabau1925) have recently been reported (Huang et al., Reference Huang, Candela, Shi and Rong2024). The failure of such communities to establish in the Telychian of South China suggests that even where suitable carbonate environments developed, they may have been too ephemeral or localized to support pentameride brachiopods.
The other key question arising from this study is why a deep-water BA4 fauna of the Xiushan type has, until now, only been identified at this specific locality. The answer may lie in its unique paleogeographical position. The Bifengguan section is situated on the northern margin of the Yangtze Platform, a transitional area that likely functioned as a deeper ramp or slope environment while much of the platform interior was characterized by shallower epicontinental seas. This specific setting probably provided a habitat that was sufficiently deep to support stenotopic deep-water taxa like Epitomyonia and a thriving population of Aegiria yet remained connected enough to the broader marine system to maintain favorable conditions, such as adequate nutrient supply and oxygenation, which are necessary for supporting a diverse benthic community. The discovery of this fauna, therefore, not only fills a critical gap in our knowledge of Telychian biofacies but also underscores the importance of a paleogeographic setting in controlling the distribution of benthic life during a key phase of the post-extinction recovery.
Conclusion
This study describes and analyzes a brachiopod fauna from the Telychian Ningqiang Formation at the Bifengguan section, on the northern margin of the Yangtze Platform. Through quantitative analysis of sample collections, we recognize three successive brachiopod associations. In ascending stratigraphic order, these are the Aegiria–Epitomyonia, Aegiria–Megaspinochonetes, and Fardenia–Striispirifer associations.
The composition of these associations suggests a shallowing-upward environmental trend, from a deeper-water (BA4) to a shallower-water (lower BA3) setting, culminating in a significant faunal turnover. The primary contribution of this study is the documentation of the Aegiria–Epitomyonia association. The notable abundance of the deep-water indicator genus Epitomyonia in this assemblage provides the first clear evidence for a BA4 facies within the Xiushan Fauna, which previously has been characterized as a predominantly shallow-water fauna.
The observed shallowing trend is consistent with the proposed effects of regional uplift on the South China Platform during this period. The specific location of the Bifengguan section may have provided the necessary environmental conditions for a deep-water community to develop, a setting not widely preserved or identified elsewhere on the platform. These findings contribute new data to our understanding of Telychian biofacies in South China and illustrate the influence of local paleogeography on community structure.
Acknowledgments
We sincerely thank J.Y. Rong, R.B. Zhan, Y. Wang, H.H. Xu, P. Tang, K. Yan, R.C. Wu, G.X. Wang, X. Wei, X.C. Luan, X.L. Zhang, and Q. Wang for their help during the field work, especially for their assistance when we first conducted field investigations in this area. The comments of C. Rasmussen and an anonymous referee together with those of the editor C. Sproat greatly improved the clarity of the manuscript. This research was supported by the National Natural Science Foundation of China (NSFC 42272007) and Ministry of Science and Technology of the People’s Republic of China (2023YFF0803703).
Competing interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
