Two specimens of M. epiphylla were collected during field surveys conducted in Shennongjia National Park, Hubei Province, China, in May and September 2023 and deposited in the Herbarium (PE), Institute of Botany, the Chinese Academy of Sciences, Beijing. Collected specimens were identified by Y. Jia and Q. He from the Institute of Botany, Chinese Academy of Sciences. Photographs were taken using Leica DM 4000 B LED microscope (Leica Microsystems, Wetzlar, Germany). In addition to the collected specimens, one specimen of Metzgeria consanguinea Schiffn (collected on 5 May 2009, with Collection Number 2576, by Dr. Ningning Yu from the Institute of Botany, Chinese Academy of Sciences, Beijing, from a branch at altitudes of 879-886 m in Huangjing Town, Gulin County, Luzhou City, Sichuan Province, China, at coordinates 28°17’48″N and 105°46’07″E) was also examined to facilitate the differentiation between the two species.

The identification of Metzgeria epiphylla in Shennongjia National Park represents a significant extension of this species’ known geographical range. Previously recorded only in regions such as South America, the South Atlantic and the South Pacific (Frey and Michael 2009), this discovery in China marks its first report in East Asia. The habitat of M. epiphylla in Shennongjia National Park is consistent with its typical occurrence in evergreen broadleaf forests, indicating its adaptability to montane environments in Asia. This finding not only contributes to the understanding of the distribution patterns of Metzgeria species, but also highlights the potential for undiscovered bryophyte diversity in Asia, particularly in montane evergreen forests.

The classification of M. epiphylla is a challenging endeavour due to its morphological diversity, particularly the presence of two distinct forms of leaf tips within various species. Bechteler et al. (2021) identified that several morpho-species of Metzgeria, including M. furcata (L.) Corda and M. leptoneura Spruce, exhibit polyphyletic characteristics. However, their study did not include M. epiphylla, leaving its specific phylogenetic relationships unexplored. Additionally, we have not yet discovered superior morphological traits for distinguishing the species, necessitating the use of the traits currently available in this study. The dimorphism, characterised by blunt and acutely pointed leaf tips, is a notable trait in several species distributed primarily in the tropical regions of South America. In Asia, similar dimorphic patterns can be observed in species such as Metzgeria acuta, M. consanguinea and M. darjeelingensis, M. fruticulosa, M. harae, M. macrocellulosa, M. macrospora, M. sinensis and M. temperata. Historically, many of these species, except M. temperata, have been synonymised under M. consanguinea without substantive justification (So 2003), prompting a re-evaluation of whether this amalgamation accurately reflects the morphological and ecological distinctions amongst these taxa. Morphological investigations reveal significant differences amongst these species, despite apparent superficial similarities. For instance, M. acuta displays conical leaves uniquely in male specimens, whereas female counterparts have blunt leaf tips. The absence of conical leaf edges in the original description of M. consanguinea and the lack of distinctive sporophytes in various specimens further complicate its classification. These findings highlight the importance of a thorough assessment of mature morphological structures rather than relying solely on leaf tip characteristics for taxonomic decision-making.

The historical classification practices by So (2002) have largely attempted to consolidate M. epiphylla within M. consanguinea based on leaf morphology and epidermal cell structures. However, this consolidation overlooks critical distinguishing features, such as sporophyte forms and leaf cell anatomy. For instance, Piippo (1991) noted that, while some vegetative thalli and all gemmiferous branches of M. epiphylla exhibit a more or less tapered morphology, its tapering is never as abrupt as that seen in M. consanguinea. Upon examining the specimen of M. consanguinea, we observed a distinctively tapered leaf tip (Fig. 3a-d), a feature markedly different from that of M. epiphylla (Fig. 2a-e), thereby corroborating the distinction. While the interpretations of So (2003) aimed to provide a comprehensive treatment of Asian Metzgeria species, the morphological and ecological specifics of these taxa suggest the need for separate recognition. Thus, a nuanced approach to classification that underscores the individual characteristics of each species is warranted, rather than broad categorisations that do not adequately capture their distinctions.

Figure 3.  

Metzgeria consanguinea Schiffn. a Plants; b-d Tapered apex of thallus; e-f Gemma; g Cross-Section of midrib; h Parts of thallus cells; i Hairs on the marginal and midrib of thallus. All from specimen YU Ningning 2576.

The distribution of M. epiphylla across different global regions remains an area of considerable uncertainty. Historical records by Kuwahara (1978b) and later clarifications by Piippo (1991) have acknowledged its presence beyond well-documented ranges. The supposed exclusions made by So (2002) regarding its presence in the Neotropics fail to provide definitive categorisations, necessitating further investigation and confirmation. Our findings align with earlier literature, advocating for a re-evaluation of its distribution and calling for more comprehensive studies to verify this species’ presence in regions such as El Salvador and Chile.

In conclusion, our detailed examination of morphological traits, along with an evaluation of historical taxonomic literature, comparisons between M. epiphylla and closely-related species underscore the genus’ morphological diversity, supporting the view that M. epiphylla represents a distinct species. The original figure by Evans (1923) for M. epiphylla emphasises key characteristics such as the cylindrical gemmae with long marginal spiny hairs, distinguishing it from similar species like M. decipiens and M. fruticulosa. Detailed morphological analyses, including features such as elongated sporophytes and leaf cross-sectional anatomy, further reinforce its unique status. For example, the characteristic blue-green dots on the leaf tips when dried differentiate M. epiphylla from species like M. fruticulosa. This examination advocates for a careful and nuanced classification approach within Metzgeria, emphasising the necessity for detailed morphological investigations to resolve taxonomic ambiguities effectively.