doi: 10.1186/s12862-017-0992-1.
Further study of Late Devonian seed plant Cosmosperma polyloba: its reconstruction and evolutionary significance
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- PMID:28651518
- PMCID: PMC5485708
- DOI: 10.1186/s12862-017-0992-1
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Further study of Late Devonian seed plant Cosmosperma polyloba: its reconstruction and evolutionary significance
Le Liu et al. BMC Evol Biol..
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Abstract
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.
Keywords: Cosmosperma polyloba; Frond; Late Devonian; Ovule; Pollen organ; Seed plant; South China; Wutong Formation.
Figures
Stems ofCosmosperma polyloba.a Stem with primary rachises. PKUB13501a. Scale bar = 20 mm.b Combined figure of two counterparts of specimen shown in Fig. 1a, exhibiting stem with primary rachises.Arrow indicating portion enlarged in Fig. 1c. PKUB13501b (part indashed box), PKUB13501c. Scale bar = 20 mm.c Enlargement of Fig. 1 (b,arrow), showing conical prickles (arrows) and parallel vertical striations on the stem. Scale bar = 2 mm.d Stem with the widest primary rachis. PKUB13502. Scale bar = 20 mm.e Stem and helically arranged primary rachises with swollen bases.Arrows indicating portions enlarged in Fig. 1f,g. PKUB13503. Scale bar = 20 mm.f Enlargement of Fig. 1 (e,left arrow), showing conical prickles (arrows). Scale bar = 2 mm.g Enlargement of Fig. 1 (e,right arrow), showing parallel vertical striations (white arrow) and conical prickles preserved as pit-like impressions (black arrows). Scale bar = 2 mm.h Combined figure of part and counterpart of one specimen, showing a stem with primary rachises arranged in irregular helix. PKUB13504a, PKUB13504b (dashed box). Scale bar = 20 mm
Stems ofCosmosperma polyloba.a-c Stems illustrating helically arranged primary rachises with swollen bases.Arrow in Fig. 2b indicating portion enlarged in Fig. 2e. PKUB13505-PKUB13507. Scale bars = 20 mm.d Stem with one dichotomous frond rachis. PKUB13508. Scale bar = 20 mm.e Enlargement of Fig. 2 (b,arrow), showing conical prickles (arrow) and parallel vertical striations. Scale bar = 5 mm
Interpretative diagram showing architecture of bifurcate (a) and trifurcate (b) fronds ofCosmosperma polyloba. The connection between trifurcate frond and fertile rachises bearing synangia (dashed lines with a question mark) is speculative. Abbreviations: pr, primary rachis; sr, secondary rachis; mr, median rachis; tr, tertiary rachis; qr., quaternary rachis; fr, fertile rachis; pn, pinnule
Bifurcate fronds ofCosmosperma polyloba.a Combined figure of part and counterpart of one specimen, exhibiting frond with primary rachis ending in a dichotomy (arrow 1, enlarged in Fig. 4b), two secondary rachises, alternately (pinnately) arranged tertiary rachises and an attached ultimate pinna (quaternary rachis and pinnules;arrow 2, enlarged in Fig. 4c). A cupulate ovule (arrow 3, enlarged in Fig. 6d) is associated with the frond. PKUB13509a, PKUB13509b (above thedashed line). Scale bar = 20 mm.b Enlargement of Fig. 4(a,arrow 1), showing the dichotomy of primary rachis. Note parallel vertical striations and conical prickles (arrows) along the primary rachis and basal part of secondary rachises. Scale bar = 5 mm.c Enlargement of Fig. 4 (a,arrow 2), showing one ultimate pinna with highly dissected, alternate and planate pinnules.Arrows indicating conical prickles along quaternary rachis. Scale bar = 5 mm.d Longest primary rachis with base but without distal portion preserved. PKUB13508. Scale bar = 20 mm
Trifurcate fronds ofCosmosperma polyloba.a Frond consisting of a primary rachis, a median and two subopposite secondary rachises and two tertiary rachises.Arrow indicating portion enlarged in Fig. 5(d). PKUB13510a. Scale bar = 20 mm.b Frond consisting of a primary rachis, a median and two subopposite secondary rachises and alternately borne tertiary rachises. One cupulate ovule (arrow, enlarged in Fig. 6e) associated with the frond. PKUB13511. Scale bar = 20 mm.c Frond consisting of a primary rachis, a median and two subopposite secondary rachises, a tertiary rachis and an ultimate pinna.Arrow indicating portion enlarged in Fig. 5e. PKUB13512. Scale bar = 20 mm.d Enlargement of Fig. 5 (a,arrow), showing attachment of a median and two secondary rachises. Parallel vertical striations distributed along primary rachis, basal parts of median and secondary rachises.Arrows indicating conical prickles. Scale bar = 5 mm.e Enlargement of Fig. 5 (c,arrow), showing an ultimate pinna with poorly preserved pinnules. Scale bar = 5 mm
Fronds and cupulate ovules ofCosmosperma polyloba.a Frond consisting of distal part of bifurcated primary rachis, two secondary rachises, and one tertiary rachis bearing 11 ultimate pinnae (arrows). PKUB13513. Scale bar = 20 mm.b Combined figure of part and counterpart of one specimen, showing vegetative frond with secondary rachis at possible distal portion and three alternate (pinnate) tertiary rachises bearing ultimate pinnae.Arrow indicating portion redrawn in Additional file 2: Figure S2(d). PKUB13514a, PKUB13514b (dashed polygon). Scale bar = 2 cm.c Frond segment including secondary rachis, alternately (pinnately) borne tertiary rachises with ultimate pinnae.Arrow indicating portions redrawn in Additional file 2: Figure S2(b, c). PKUB13515. Scale bar = 20 mm.d Enlargement of cupulate ovule in Fig. 4(a,arrow 3), showing prickles (arrow) on the outer surface. Scale bar = 2 mm.e Enlargement of cupulate ovule in Fig. 5(b,arrow), showing compression of ovule with a stalk connecting the cupule (lower arrow, enlarged in Fig. 6f) and four integumentary lobes (upper four arrows). Scale bar = 2 mm.f Enlargement of Fig. 6 (e,lower arrow), showing lower part of ovule with a short stalk. Scale bar = 0.5 mm.g Ovule in Fig. 6e, remnant after removal of integumentary lobes by dégagement.Arrow indicating portion enlarged in Fig. 6h. Scale bar = 2 mm.h Enlargement of Fig. 6 (g,arrow), showing cupule tips (white star) and remnant of ovule (black star). Scale bar = 0.5 mm
Synangiate pollen organs on fertile rachises ofCosmosperma polyloba.a Anisotomous fertile rachises with terminal pollen organs and prickles (black arrows). PKUB13516. Scale bar = 5 mm.b Two anisotomous fertile rachises with terminal pollen organs. Fertile rachises (arrow 1, enlarged in Fig. 7c displaying sparse prickles; pollen organ (arrow 2, 3, enlarged in Fig. 7d-f) born in pairs or singly. PKUB13517. Scale bar = 5 mm.c Enlargement of Fig. 7 (b,arrow 1), showing one fertile rachis bearing conical prickles (black arrows). Scale bar = 0.5 mm.d,e Enlargement of Fig. 7 (b,arrow 2). Two stages of dégagement showing pollen organs. Scale bar = 2 mm.f Enlargement of Fig. 7 (b,arrow 3). Pollen organs terminating bifurcated fertile rachises. Scale bar = 2 mm
Reconstruction ofCosmosperma 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
References
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