A New Fern-like Plant Xinhangia spina Gen. et sp. Nov. from the Upper Devonian of China

Abstract

Palaeozoic fern-like plants show great diversity in their morphology and/or anatomy. Within this group, a novel taxon, Xinhangia spina gen. et sp. nov., is now reported from the Upper Devonian (Famennian) Wutong Formation of Anhui Province, China. The primary and secondary branches are borne alternately and sometimes in a triseriate pattern. Spines are evident on the main axes or stems and on the primary branches. Vegetative ultimate appendages with recurved tips are alternate, usually dichotomous 1–2 times, and sometimes as an aphlebia located at the base of primary or secondary branches. Fertile ultimate appendages are alternate, usually dichotomous 1–2 times, and terminate in elongated and paired sporangia. The stele has a clepsydroid-like primary xylem with each end bearing a protoxylem strand. The secondary xylem surrounding the primary xylem illustrates uniseriate rays. With rare divisions in both the vegetative and fertile ultimate appendages, Xinhangia represents a morphologically primitive plant. It is of uncertain affinity at the class or order level. The stelar architecture suggests that the clepsydroid stele may not be emphasized in discussing the relationship among fern-like plants such as rhacophytaleans.

Fern-like Plants Establishing the Understory of the Late Devonian Xinhang Lycopsid Forest

“…The fern-like plants in this research were found from several beds in the Leigutai Member (Figure 1c), suggesting their existence for a long time. Materials from the previous study only contain shoot parts, and elaborate structures such as complete fertile organs attached to branches are preserved [16]. Thus, Xinhangia in both previous and present studies, as well as some other fern-like plants (Figures 6 and S2) from the same bed (Figure 1c, the two lowest vertical red lines), are considered as being autochthonous or parautochthonous for the preservation of roots and/or well-preserved aerial branches.…”

“…As previously studied, Xinhangia spina possesses characters including primary and secondary branches arranged in alternate and sometimes triseriate patterns, dichotomous vegetative and fertile ultimate appendages with rare divisions, and stele in a clepsydroid shape [16]. The current plants have characteristics including (1) procumbent, spiny, long and sometimes bifurcated stems, (2) primary branches arranged in alternate and sometimes triseriate pattern (Figure 2a-c,e,g), (3) single dichotomized aphlebia inserted at the bases of primary branches (Figure 4e, arrow 3; Figure 4f, arrow 4; Figure 4h, arrow 2), (4) secondary branches borne alternately (Figure 2a, arrows 3-5; Figure 2g, arrow 7), ( 5) dichotomous vegetative ultimate appendages with recurved tips (Figure 5a-c), ( 6) dichotomous fertile organs terminating in paired sporangia (Figure 5d-g), and (7) clepsydroid-shaped primary xylem surrounded by secondary xylem (Figure 5h,i).…”

“…Similarly, Flabellopteris [30] is interpreted with an erect habit connected to a horizontal rhizome, though evidence of roots is deficient. In the previous study, procumbent stems and roots are undetected in Xinhangia [16]. In this study, slender root systems are attached to stems (Figures 3a,b and 4a,b) or found isolated (Figure 6), and some of them have bifurcation (Figure 3b, arrow 4; Figure 3h, arrows 3,4; Figure 4f, arrow 1) or one order of lateral roots (Figure 6c, arrows 1-6), which is rare among Devonian fern-like plants.…”

“…The trees of G. micrum grew in a littoral habitat and were prone to be submerged by the frequent coastal flooding [14]. Xinhangia is considered as a primitive taxon with simple vegetative and fertile organs [16], which could be related to the disturbed environment for faster growth. As the biggest Devonian forest in the world, Xinhang forest may represent a universal forest structure at that time.…”

“…With continual excavation, new fossil plants other than lycopsids were recently discovered from this forest. Among them, Xinhangia spina is a fern-like plant characterized by an aerial stem bearing alternate to triseriate primary branches, simple ultimate appendages and clepsydroid-shaped stele [16]. By studying more specimens, this article focuses on X. spina with procumbent stems Life 2024, 14, 602 2 of 16 and other fern-like plants in Xinhang forest, whose habits are discussed in relation to the palaeoenvironment.…”