Reconstruction of Cosmosperma
Based on the specimens described above, Cosmosperma is characterized by unbranched stem with two types of fronds attached in irregular helices, alternately arranged tertiary and quaternary rachises, uniovulate cupules and synangiate pollen organs terminating anisotomous fertile rachises. We tentatively reconstructed Cosmosperma as shown in Fig. 8, and it is thus one of the best morphologically understood Late Devonian seed plants in the world.
Comparisons with other Devonian seed plants
The cupules and synangiate pollen organs of Cosmosperma have been compared with those of related seed plants [3], and this comparison section primarily focuses on frond morphology. Vegetative fronds have been reported in the Late Devonian seed plants, i.e., Elkinsia from USA [5], Laceya from Ireland [17, 18], Kongshania [8], Yiduxylon [13] and Telangiopsis [10] from China. Among them, Elkinsia, Kongshania and Telangiopsis are also known for fertile rachises with terminal pollen organs. Some selected morphological traits of these plants are listed and compared in Table 1. All of these taxa except Kongshania display bipartite fronds, while Elkinsia exhibits repeatedly bifurcated frond rachises exclusively. Yiduxylon, Telangiopsis and Cosmosperma have highly dissected, planated pinnules, differing from the laminate pinnules of Elkinsia and Kongshania. The fertile rachises bearing pollen organs are anisotomously divided in Cosmosperma, which enables them to be distinguished from the isotomously divided ones in Elkinsia, Kongshania and Telangiopsis. Prickles are extensively distributed on Cosmosperma, but are confined to the stems of Telangiopsis and absent from other coeval seed plants.
Table 1 Comparison of morphological traits among Late Devonian seed plants
Variations in fronds among early seed plants
Early seed plants are characterized by bipartite fronds with dichotomized primary rachises [19, 20], while diversified frond structures are evidenced in the Late Devonian taxa, such as variable dimensions of fronds, different branching manners and flexible locations of ultimate pinnae (Table 1). It has been shown that great morphological disparities have occurred among the Late Devonian spermatophytes. Lyginopterid seed plants in the Early/Late Carboniferous are thought to possess fronds with dichotomized/pinnate branching patterns, respectively [19]. Since Elkinsia is characterized by repeatedly dichotomized fronds [5], while Laceya [17], Yiduxylon [13], Telangiopsis [10] and Cosmosperma show pinnate fronds, it seems that both branching patterns have arisen in the Late Devonian spermatophytes.
The fertile fronds with terminal pollen organs often exhibit cruciate dividing patterns in the Late Devonian seed plants (e.g., Telangium schweitzeri [6] and Elkinsia [5]). Among the Early Carboniferous spermatophytes, the fertile fronds with terminal pollen organs containing trilete prepollen are divided into three types: Rhacopteris/Triphyllopteris-type, Diplopteridium-type and Rhodea-type [21]. The Diplopteridium-type illustrates a trifurcate frond rachis producing a median fertile rachis that is short and dichotomous [21,22,23]. The trifurcate fronds of Cosmosperma display a unique architecture among coeval seed plants. Such fronds, if connected to the fertile rachises bearing terminal pollen organs (Fig. 8), would greatly resemble the Diplopteridium-type fertile frond. In this case, Cosmosperma exemplifies the diversification of fertile fronds among Late Devonian seed plants, and suggests that some Carboniferous fertile frond types may be traced back to an earlier time.
Different dividing patterns of the fertile and vegetative fronds were present in Carboniferous spermatophytes [22, 23], which is also supported by the anatomical evidence [24, 25]. Both Elkinsia [5] and Cosmosperma indicate that the dimorphic fronds have occurred in the Late Devonian.
Implications from the ovule of Cosmosperma
Nearly all Late Devonian seed plants have cupulate ovules (ovules enclosed in cupules) [2], and the cupules are uniovulate or multiovulate [4]. The uniovulate cupules were considered to be derived from the multiovulate ones [26, 27]. The uniovulate cupule has been proposed [3] and is now confirmed in Cosmosperma. Other Devonian seed plants with uniovulate cupules include Dorinnotheca [27], Latisemenia [4], Condrusia [28] and Pseudosporognites [2]. Their traits are listed in Table 2. The cupule or integument of the early ovules is considered archaic with numerous, terete and little fused segments or lobes [27, 29, 30]. In this case, Cosmosperma appears primitive among the ovules with uniovulate cupules.
Table 2 Comparison of Late Devonian seeds with uniovulate cupules
One of the most obvious functions of cupules and integuments is protection for the ovule [1], and a more entire (large and/or widely fused) integument may provide additional protection against water loss [4, 30]. The cupules of Cosmosperma enclose the ovule, while those of Dorinnotheca, Pseudosporognites and Latisemenia are recurved or short to extensively expose the ovule. On the other hand, the integrity of the integument is the lowest in Cosmosperma, moderate in Dorinnotheca and Pseudosporognites, and the greatest in Latisemenia. Therefore, the protection is largely provided by the cupule in Cosmosperma, and by the integument in the other three plants. The evolutionarily primitive status of Cosmosperma suggests that the protective function of uniovulate cupules may be replaced by the increasingly developed integument.
Function of prickles and probable growth habit of Cosmosperma
The acute outgrowths of epidermis or both epidermis and cortex, without vascular tissues, are usually named prickles, while the sharp-pointed vascularized protuberances modified from axes and leaves are separately called thorns and spines [31, 32]. Commonly, the thorns and spines are only distributed along the axes and, owing to their internal vascular tissues, cannot be easily removed. However, in Cosmosperma, the tiny conical structures occur on stems, vegetative and fertile rachises and even cupules. They also present a highly variable density corresponding to loss in the transport and/or burial process. Therefore, we tentatively assign such structures to prickles.
The prickles are not common in the Late Devonian spermatophytes, but they have been reported in some later Paleozoic seed plants, including the Early Carboniferous Medullosa steinii and Late Permian gigantopterid Aculeovinea yunguiensis [33, 34]. It has been suggested that prickles on the cupule surface of Cosmosperma may serve as protection [3]. On the other hand, arthropod herbivory was recorded in some Late Devonian myriapods and apterygote hexapods [35], while the major plant defensive adaptations to such herbivory are considered chemical [36]. However, the terrestrial vertebrate herbivory did not occur until the Permian [34]. Since prickles are considered to provide mechanical attachments in other younger Paleozoic seed plants [33, 34], it is plausible that the prickles on the axes and leaves of the Late Devonian seed plants may largely function as supporting structures rather than defense structures against the herbivores.
Previous studies have suggested that the seed plants assigned to the Lyginopteridales are vines/lianas possessing stems generally less than 20 mm wide, and those to the Calamopityales are upright with stems usually over 20 mm wide [13, 25]. Other evidence that supports lyginopterids as vines/lianas includes stems bearing long internodes, the presence of adventitious roots, large fronds with swollen frond bases, wide angle of frond attachment and Dictyoxylon-type outer cortex [13, 37, 38]. Cosmosperma possesses relatively large fronds with pulvinus-shaped bases, which resemble those of lyginopterids. The extensively born prickles of Cosmosperma also remind us of the glands on Lagenostoma and Lyginodendron [39]. However, in Cosmosperma, the width of the stems reaches 22 mm, the internodes are relatively short, the adventitious root is absent, the fronds depart at 40–70° and the cortex is most likely Sparganum-type. These traits enable Cosmosperma to be tentatively reconstructed as an upright, probably semi-self-supporting plant (Fig. 8), which may support each other by entangled bushy fronds rather than scrambling or climbing. The hypothesis is supported by the preservation that many slabs exhibit pure and dense communities of Cosmosperma, without any other arborescent plants. The prickles may help anchor fronds of adjacent individuals. However, the anatomical information is needed to test the suggested growth habits of this plant.