Further study of Late Devonian seed plant Cosmosperma polyloba: its reconstruction and evolutionary significance

Background The earliest seed plants in the Late Devonian (Famennian) are abundant and well known. However, most of them lack information regarding the frond system and reconstruction. Cosmosperma polyloba represents the first Devonian ovule in China and East Asia, and its cupules, isolated synangiate pollen organs and pinnules have been studied in the preceding years. Results New fossils of Cosmosperma were obtained from the type locality, i.e. the Leigutai Member of the Wutong Formation in Fanwan Village, Changxing County, Zhejiang Province, South China. The collection illustrates stems and fronds extensively covered in prickles, as well as fertile portions including uniovulate cupules and anisotomous branches bearing synangiate pollen organs. The stems are unbranched and bear fronds helically. Fronds are dimorphic, displaying bifurcate and trifurcate types, with the latter possibly connected to fertile rachises terminated by pollen organs. Tertiary and quaternary rachises possessing pinnules are arranged alternately (pinnately). The cupule is uniovulate and the ovule has four linear integumentary lobes fused in basal 1/3. The striations on the stems and rachises may indicate a Sparganum-type cortex. Conclusions Cosmosperma further demonstrates diversification of frond branching patterns in the earliest seed plants. The less-fused cupule and integument of this plant are considered primitive among Devonian spermatophytes with uniovulate cupules. We tentatively reconstructed Cosmosperma with an upright, semi-self-supporting habit, and the prickles along stems and frond rachises were interpreted as characteristics facilitating supporting rather than defensive structures. Electronic supplementary material The online version of this article (doi:10.1186/s12862-017-0992-1) contains supplementary material, which is available to authorized users.

architecture. The entire plant is reconstructed and its evolutionary significance is discussed.

Material and Methods
Over 100 new specimens of Cosmosperma polyloba were obtained from the Wutong (Wutung) Formation in a quarry near Fanwan Village, Hongqiao Town, Changxing County, Zhejiang Province, China. The information regarding the locality and stratigraphy has been provided in recent studies [3,14,15]. At the Fanwan section, the Wutong Formation is divided into the Guanshan Member, with quartz sandstone and conglomerate, and the overlying Leigutai Member, with interbedded quartz sandstone and mudstone. The fossil plant occurs at the 13th bed of the Wutong Formation (in the Leigutai Member), i.e. the same bed from which former specimens of Cosmosperma and strobili of lycopsid Changxingia sp. were collected [3,15]. The LC (Knoxisporites literatus-Reticulatisporites cancellatus) spore assemblage suggests that the upper part of the Leigutai Member is of the latest Famennian age [16].
In siltstone with tiny crystals of quartz and white micas, the plant is preserved as dark-brown compressions and impressions, displaying great contrast to the yellowish matrix. Steel needles were applied to expose the plant morphology and a digital camera and a stereoscope were used for photographs. All the specimens are housed at the Department of Geology, Peking University, Beijing, China.
[Seed plant with unbranched stems bearing dimorphic fronds, dichotomized fertile rachises terminated by synangiate pollen organs, and cupulate ovules. Fronds with a swollen pulvinus-shaped base. Majority of fronds bifurcate, with primary rachis dichotomizing into two secondary rachises. The other fronds trifurcate, with primary rachis ended by two subopposite secondary rachises and one median rachis. Tertiary rachises and ultimate pinnae (with quaternary rachis) borne alternately and pinnately.] Nonlaminate pinnules planate, highly dissected and alternately arranged on [quaternary rachis]. Pollen organs synangiate, with each terminating a stalk and consisting of [

Cupulate ovules
Cupules are isolated, 5.3-7.7 mm long and 3.0-5.1 mm at the maximum width (Fig. 6d, e). The cupules display minute conical prickles on the outer surface (Fig. 6d, arrow) that are ca. 0.2 mm long and ca. 0.3 mm wide at the base. Each cupule possesses segments with multiple tips that are about half of the total cupule length and are ca. 0.5 mm wide. One specimen illustrates that the cupule is uniovulate (Fig. 6e). The upper part of the ovule (Fig. 6g, arrow) is dégaged to expose several cupule tips (Fig. 6h, white star), which are beneath the ovule remnant (Fig. 6h, black star). Before the dégagement, this ovule was 4.7 mm long and 2.2 mm wide, and connected to the cupule by a short stalk ca. 0.2 mm long and 0.5 mm wide (Fig. 6e, lower arrow, f). Four integumentary lobes are linear and straight (Fig. 6e, black arrows), ca. 3.8 mm long and ca. 0.5 mm wide, and fused to each other in the basal 1/3 of the ovule.

Fertile rachises with terminal pollen organs
The fertile rachises are anisotomous and terminate in pollen organs (Fig. 7a, b, d-f; Additional file 3: Figure  S3). These rachises dichotomize 3-6 times and at an angle of 50-120°, with the intervals between two adjacent bifurcating points being 1.4-19.3 mm long and 0.3-1.2 mm wide. Both length and width of the intervals reduce acropetally. Conical prickles are sparse on the branches and ca. 0.2 mm long and 0.3 mm wide at the base (Fig. 7a, c, Additional file 3: Figure S3a). Pollen organs, ca. 2.5 mm long and 2.0 mm wide, are born mainly in pairs, but sometimes singly or incompletely preserved ( Fig. 7d-f; Additional file 3: Figure S3). Individual pollen organs are synangiate with basally fused microsporangia. Each synangium consists of 4-8 elongate microsporangia, which are ca. 2.3 mm long and 0.3-0.4 mm wide.

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.

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. Fig. 8 Reconstruction of Cosmosperma polyloba. The plant is considered to possess an upright, probably semi-self-supporting habit, with adjacent individuals entangled by their bushy, prickle-bearing fronds. Dimorphic fronds are helically arranged along stem, with bifurcate fronds in the majority, and scattered trifurcate fronds displaying median rachises; the connections between trifurcate fronds and fertile parts are speculative 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   [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-selfsupporting 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.

Conclusions
We further studied the seed plant Cosmosperma polyloba from the Upper Devonian of South China, and its stems, fronds, cupulate ovules and fertile rachises bearing pollen organs are now known in detail. Based on the morphological evidence mentioned above, we tentatively reconstructed the whole plant with an upright, semi-self-supporting habit. The prickles on stems and rachises may facilitate supporting. The fronds of Cosmosperma show bifurcated or trifurcated primary rachises, which further add to the diversity and demonstrate dimorphism of the early spermatophyte fronds. The less-fused cupules and integuments suggest that Cosmosperma is primitive among Late Devonian seed plants with uniovulate cupules.