A new species of Estinnophyton from the Lower Devonian Posongchong Formation, Yunnan, China; its phylogenetic and palaeophytogeographical significance

Abstract

A new species of early vascular land plant, Estinnophyton yunnanense, is described from the Lower Devonian (upper Pragian) Posongchong Formation at Zhichang village, Gumu Town, Wenshan District, Yunnan Province, China. This plant possesses aerial stems with isotomous branching. The leaves are slender, once-or twice-bifurcated, and arranged in low spirals or pseudowhorls. Fertile leaves are morphologically identical to the vegetative ones, clustered on the axis, forming a loose strobilus-like structure. There are three to six fertile leaves in each gyre. Two ovoidelongate sporangia are singly attached to the adaxial surface each subtended by a recurved stalk, either at the base of each segment in a once-bifurcate fertile leaf or below the points of successive divisions in a twice-bifurcate fertile leaf. Estinnophyton yunnanense differs from two other species described previously from Europe, E. gracile and E. wahnbachense, mainly in possessing two single stalked sporangia on each fertile leaf rather than two pairs of sporangia, as well as in the dimensions of its leaves. In view of the complex taxonomy and nomenclature surrounding the type material of Estinnophyton, we considered possible emendation of the generic diagnosis based on the new Chinese material. Following comparison with typical members of the Protolepidodendrales and other taxa, Estinnophyton is considered to have had a closer affinity with primitive sphenopsids than the protolepidodendralean lycopsids. A comparison between Chinese and European floras containing Estinnophyton indicates that South China and Western Europe had floristic elements in common although they were in different phytogeographical units during the Early Devonian epoch.

Earliest record of megaphylls and leafy structures, and their initial diversification

“…The characteristics of the fertile leaves of Estinnophyton, such as recurved sporangia, sterile filiform extensions, and pseudowhorled arrangement, are similar to those of Calamophyton [33]. These two taxa may have a close phylogenetic relationship, considering that reproductive characters should outweigh those from anatomy in early vascular plants [18]. The ancient origin of the sphenopsids and simple leaves can be traced to the Early Devonian Estinnophyton.…”

“…They are planated but unwebbed and once-or twice-bifurcated at the middle part with two or four straight, tapering segments [18]. These leaves are arranged nearly in pseudowhorls, three to six per gyre (Figure 1(c), (d)).…”

“…Estinnophyton has been considered a precursor of the sphenopsids in that its leaves have a pseudowhorled arrangement and the fertile leaves have recurved sporangia and sterile filiform extensions [18,28]. Typical sphenopsids with whorled simple leaves appeared in the Late Devonian [29,30].…”

“…They have paired elongate or fusiform sporangia borne at the ends of synchronous isotomies ( Figure 5(e)) [19][20][21][22][23]; (2) Estinnophyton-sphenopsids. They share fertile structures with recurved sporangia and simple leaves in pseudowhorls ( Figure 5(f)) [18,28]; (3) Polythecophytonaneurophytaleans. Their fusiform sporangia are in clusters borne along the inner sides of the fertile structures ( Figure 5(h)) [34]; (4) Celatheca-pteridosperms (seed ferns).…”

“…This paper is a part of the summary of the Posongchong Flora [11,12]. Evidence from this indicates that by the Early Devonian megaphylls had evolved in several different groups within the euphyllophyte clade and already showed wide variance of morphologies [15][16][17][18].…”