Monday 27 May 2013

The first leaves; leafy plant fossils from the Early Devonian of South China.

Plant leaves are split into two groups by botanists; microphylls, which are simple plates of undifferentiated cells, as found in Mosses and Liverworts, and megaphylls, or true leaves, which have cellular differentiation and veins; such structures are not necessarily flat, Pine needles are true leaves. The earliest known fossil plants, such as the Silurian Cooksonia and Baragwanathia and the plants of the Early Devonian Rhynie Chert, lacked true leaves, but by the Middle Devonian plants with leaves are widespread.

In a paper published in the April 2013 edition of the Chinese Science Bulletin, Hao SouGang and Xue ZinZhuang of the Key Laboratory of Orogenic Belts and Crustal Evolution at the School of Earth and Space Sciences at Peking University describe a number of leafy fossil plants from the Earlt Devonian Posongchong Formation of Yunnan Province in southern China, which is thought to be roughly contemporaneous with the Rhynie Chert. These fossils have previously been described separately, but this paper provides a review of the flora in English.

The first plants discussed are the Trimerophytes Psilophyton and Pauthecophyton. Middle Devonian Trimerophytes are often thought to be the earliest true leafy plants, or at least the ancestors of them, since there is dispute as to weather their 'leaves' can be classed as true leaves, with some authors referring to them as 'incipient fronds' or 'proto-leaves'. Hao and Xue describe these plants as having 'branch-leaf complexes' comprising a vascular bundle, thin-walled cells, and epidermis. 

Trimerophytes from the Posongchong Formation. (a) Psilophyton dawsonii, has a lateral branching system with an outer cortex comprising a continuous layer of thick-walled sterome, a structure common in axes of many primitive tracheophytes but absent in Rhynie plants. Scale bar is 5 mm. (b) Pauthecophyton gracile has lateral dichotomous branching system with terminal sporangia. Scale bar is 2 mm. Hao & Xue (2013).

Estinnophyton yunnanense has clearly differentiated stems and leaves, though the leaves are simple lateral dichotomous branching systems. These are arranged in pseudowhorls with three to six leaves per  gyre. This plant has been theorized to be ancestral to the Sphenopsids (horsetails).

Estinnophyton yunnanense. (c) Partial reconstruction of a vegetative axis, with lateral, once or twice-bifurcated leaves arranged in low spirals or pseudowhorls. (d) Lateral forked leaves. Scale bar 1 mm. (e) Lateral forked leaves with sporangia. Scale bar 1 mm. Hao & Xue (2013).

Celatheca beckii has main axes and three-dimensionally arranged lateral branches. The branches undergo several rounds of division before ending in dichotomous appendages, recurved near the end with cylindrical tapering tips. Fertile structures superficially resembling a synangium (three-lobed spore bearing structures found in some fern-like plants) are found on the lateral branches.

Celatheca beckii. (f) Reconstruction of a branching system as a branch-leaf complex with more-or-less pinnate arrangement of ultimate dichotomous appendages. (g) Reconstruction of a vegetative specimen showing a branch-leaf complex with multi-ordered branching systems and ultimate appendages more or less pinnately arranged. Sometimes an ultimate appendage is borne below the branching point. (h) Fertile structures showing sporangia (right arrow) and incomplete leaf-like appendage (left arrow). (i) Fertile structure showing sporangia (arrow). (j) Fertile structures showing sporangia and complete leaf-like structure (arrow). All scale bars 2 mm. Hao & Xue (2013).

Polythecophyton demissum resembles Pertica spp., a genus of plants from the Early-to-Middle Devonian of northeastern North America. It has a clear architecture of fertile structures but lacks anatomy and vegetative organs. The fertile branches are terminated by pendulous umbrella-shaped fertile structures which may initially bifurcate, after which each component bears three to four alternately arranged, short axes, which divide into branchlets terminated by numerous slender fusiform sporangia in pairs or in groups of three or four.

Polythecophyton demissum. (k, l) Terminal portion of fertile branch showing umbrella-shaped fertile systems and pendulous sporangial clusters. Scale bars 5 mm. (m) Schematic reconstruction of branching of a fertile structure and reconstruction of attachment pattern of sporangia. Scale bar 4 mm. Hao & Xue (2013).

Eophyllophyton bellum has laminar (flat) leaf pairs bourn laterally or terminally on axes. These form a growth series with the youngest leaves at the tip. The leaves show clear venation, and have tips curving inwards towards each-other. Most of these leaves appear to bear spherical sporangia arranged in rows on the underside, in a similar manner to modern Ferns. 

Eophyllophyton bellum. (a) Lateral branch with small laterals terminating in leaf pairs, either vegetative or fertile. Scale bar 3 mm. (b) Lateral branch showing an acropetally developmental transformation: the basal part has mature leaf pairs, and the distal part has younger ones (left arrow) and recurved sterile tips (right arrow). Scale bar 3 mm. (c) A branch-leaf complex with ultimate appendages (dichotomy with recurved tips) borne alternately along the axes, showing a roughly pinnate arrangement. Scale bar 3 mm. (d) A leaf pair (two leaves, arrow indicates the faint appearance of bases of a leaf pair). The leaf margins of the laminate divisions are deeply incised and curved. A pinnately divided, laminar leaf reflects a branching system as shown in (c). Scale bar 1 mm. (e) A fertile pair with two leaves shows a reduced, expanded branching system with weak lamination. The lower arrow points to common base of the two leaves, and upper arrow points to a sporangium. Scale bar 1 mm. (f) A fan-shaped leaf with more conspicuous lamination. Note that the margins of the laminar divisions are deeply incised (arrows). Scale bar 1 mm. (g) Fertile leaf cluster with numerous sporangia. Scale bar 1 mm. Hao & Xue (2013).

Eophyllophyton bellum. (a) Reconstruction of a leaf showing that laminar divisions are not held in one plane. (b)–(e) Transverse sections at different levels of the structurally preserved lamina, showing veins and mesophyll cells. (b) Through petiole of a leaf, note the main vein. Note main vein in (d) and observe that main vein in (c) and second-order veins in lateral divisions in (c) and (d) are missing. (e) Through distal region, note leaf vein. Scale bar is 0.5 mm compared to (a) and 250 μm compared to (b-e). Hao & Xue (2013).

Eophyllophyton bellum. Numerous isolated fertile leaves on the bedding surface. Scale bar is 3 mm. Hao & Xue (2013).

AdoketophytonStachyophyton and Dibracophyton have fertile leaf-like appendages (sporophylls or bracts) to constitute a strobilar structure with different sterile organs, but they are neither microphyllous
plants nor megaphyllous plants. Adoketophyton has vegetative axes which dichotomously divide in three dimensions with some laterals ending in terminal, circinately coiled tips, a form of branch-leaf complex unlike that in any more recent plant. It has been placed within several plant groups, but is now considered to be in a class of its own, though related to the earliest ancestors of several groups.

Adoketophyton subverticillatum. (a) Abaxial view of a sporophyll with a fan-shaped blade. Scale bar 2 mm. (b) Lateral view of a fertile unit, with a sporangium adaxially attached on the sporophyll base. Scale bar 1 mm. (c) Transverse section of a sporophyllous lamina, showing variation in cells. Arrow points to probable tracheids. Scale bar 60 μm. (d) Transverse section of distal part of a sporangium, showing structure of sporangial walls and marginal dehiscence grooves and thickenings (left arrow). Also note that the cells in the outermost layer are elliptical. Their long axes are perpendicular to the surface of the wall. Among them can be found prominent intercellular spaces (right arrow); dark material between two walls presumably represents a broken tapetal layer. Scale bar 60 μm. Hao & Xue (2013).

Reconstruction of Adoketophyton parvulum. Scale bar 10 mm. Hao & Xue (2013).

Stachyophyton yunnanense has lateral leaf-like branches arranged helically along main axes. The leaf-like branches generally show a fan-shaped form showing morphological similarities to leaves with many divisions expanding distally in one plane, but they are rigid and unreduced, departing from the main axes. They show great variability, from the branching in a plane to more “webbed”, fan-shaped patterns with planation, and their distal segments are highly dissected divisions with rounded or cuneate tips . These may retain an axial nature with a planate branching system but have distal foliar divisions and thus perform some photosynthetic functions.

Stachyophyton yunnanense. (a) Leaf-like branch with distal parts that dichotomize 4-8 times forming terminal branchlets, to which are attached strobili with helically arranged sporophylls (arrow). Scale bar 8 mm. (b) Lateral vegetative 'leaf-like branches'. Scale bar 15 mm. Hao & Xue (2013).

Reconstruction of Stachyophyton yunnanense. Scale bar 15 mm. Hao & Xue (2013).

Dibracophyton possesses terminal strobili. Each fertile unit comprises a stalked long-elliptical sporangium, with dehiscence into two equal valves, and two discrete longovate bracts covering the sporangium from above and below. The sterile axes of Dibracophyton bear helically dichotomous appendages with curved or round tips. Some dichotomous appendages are alternately borne at the basal areas of the fertile axes. Besides the independent vegetative axes, a few vegetative appendages are distributed along the area of the fertile axes, and that the long upper region of the fertile axes lacks any appendages.

Reconstruction of Dibracophyton acrovatum. Scale bar 10 mm. Hao & Xue (2013).

Hao & Xue suggest that rather that megaphylls originating only once and all higher plants sharing a common ancestry, leaves may in fact have had multiple origins, evolving on at least four separate occasions and possibly as many as nine times.

Proposed phylogeny for early leafy plants. plotted against geological time showing the appearance of major clades, including euphyllophytes in Late Silurian-Early Devonian. Hao & Xue (2013).

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