王德明
论文题目：云南曲靖下泥盆统徐家冲组植物研究
作者简介：王德明
，男,1970年03月出生，1997年09月师从于北京大学郝守刚教授，于2002年01月获博士学位。
摘
要
过去二十多年,中国早期陆生维管植物的研究主要集中在云南东南部的文山和蒙自地区的下泥盆统
(布拉格阶Pragian)
坡松冲组,
先后描述了20余种植物。坡松冲植物群以丰富的内容和高度进化的营养、生殖器官组合特征为认识陆生植物多样性的起源和早期演化,
恢复古植物地理,
提供了极为珍贵的科学资料
(Edwards, 1997; Hao and
Gensel, 2001)。云南东部曲靖地区的徐家冲剖面是华南乃至中国早泥盆世非海相地层的典型剖面,
徐家冲组代表海陆过渡相沉积,
也是研究早期陆生维管植物的理想场所。自从丁文江于1914年在该剖面采集植物和鱼化石以来, 引起了广泛关注,
很多学者在此进行了古生物学和地层学方面的研究。然而,
和取得重要成果并被国内外广泛重视的坡松冲植物群的研究相比,
徐家冲植物群的面貌尚待深入认识。该组的时代尚存有争议,
还需要进一步论证。
本文研究了曲靖下泥盆统徐家冲组的4种化石植物的形态特征和解剖结构, 建立1个新属、种—楔形广南蕨Guangnania cuneata
gen. et sp. nov., 2个新种—纤细先骕蕨Huia
gracilis sp. nov. 和回弯徐氏蕨Hsüa deflexa
sp. nov., 发现并描述Hedeia
sinica Hao et Gensel
1998, 报导3种植物
(澳大利亚工蕨Zosterophyllum
australianum, 云南工蕨Z.
yunnanicum和曲靖镰蕨Drepanophycus
qujingensis)，从而加深了对该地区植物群的认识,
丰富了我国早期陆生维管植物群的组成。新植物的研究完善并深化了对其属性的认识。其中,
Guangnania也出现于滇东南广南地区的达莲塘剖面,
是在坡松冲组建立的第15个新属植物。
新属种楔形广南蕨Guangnania
cuneata gen. et sp. nov. 茎轴光滑,
二叉分枝,
直立的长楔形孢子囊侧生,
松散螺旋排列,
具长柄,
由较小的近轴瓣和较大的远轴瓣组成, 两瓣凹凸,
开裂缝具明显的边缘加厚带。根据孢子囊的着生位置和边缘加厚带,
该植物被归入工蕨类。
先骕蕨属Huia
Geng 1985由耿宝印建立,
正型种H.
recurvata产自文山纸厂剖面的坡松冲组。深入研究表明,
新种Huia
gracilis sp. nov. 的匍匐茎为K、H型,
气生茎二叉分枝,
有时着生腋生瘤,
顶生和侧生孢子囊螺旋排列,
组成松散的穗,
孢子囊纵向开裂;
心始式木质部由G-型输水管胞组成,
环纹次生加厚带之间分布不规则的简单穿孔。H.
gracilis不同于H.
recurvata的分枝方式,
茎轴较细,
孢子囊较小,
孢子囊柄较短。
徐氏蕨属Hsüa Li
1982由李承森建立,
正型种H.
robusta产自徐家冲剖面东北部约6
km的沾益县龙华山剖面的徐家冲组。新种Hsüa
deflexa sp. nov. 具有根状茎,
气生茎上着生有稀疏的刺状突起,
假单轴分枝,
侧枝等二叉分枝。一级侧枝大多与主茎轴近乎垂直着生,
成熟的末级侧枝反向回弯,
顶生孢子囊。它与H.
robusta形态上的区别主要在于茎轴具刺,
侧枝水平延伸,
末级侧枝顶端回弯。深入的研究和比较表明H.
deflexa的木质部除为心始式外,
管胞为G-型,
木质化次生壁具有不规则的简单穿孔,
穿孔直径小,
密度大,
分布在环纹次生加厚带和其间,
兼有S-型管胞穿孔的特征;
粗壮的木质部束和密集的穿孔表明较强的输导能力。
早期陆生维管植物的输水管胞分为S-型,
G-型和P-型三种
(Kenrick and Crane, 1991, 1997; Kenrick et al., 1991)。以往由于研究手段的限制,
管胞壁层和穿孔的超微结构尚待深入认识。本文第一次在扫描电子显微镜
(SEM) 下对比研究经化学浸解获得的丝碳化管胞和矿化的管胞腔铸体,
明确揭示出了G-型管胞壁层和穿孔的超微结构: 1) Hsüa
deflexa木质化次生壁由不等厚、异质的3层（S1、S2和S3层）组成;
穿孔腔为哑铃形,
腔壁光滑;
2) Huia gracilis的穿孔腔收缩,
腔壁上发育次级穿孔,
孔口具有边缘加厚。
解剖学研究方法的创新导致认识的深入:
1) 验证了G-型管胞次生壁分两个阶段形成的理论
(Bierhorst, 1960), 即先后形成不连续的纤维质
(cellulose) 层和连续的木质化
(lignified) 层,
前者易被分解,
后者抗侵蚀性强;
2) 证明管胞纤维质的初生壁和次生壁不具穿孔,
Hsüa管胞的木质化次生壁具有不规则的简单穿孔,
而不是纵列的纹孔;
3) Hsüa deflexa木质化次生壁的三分性表明细胞次生壁的化学分异始于早泥盆世;
4) 第一次提出G-型管胞在属一级水平上存在的性状差异：穿孔的孔腔形状和微细结构;
5) Hsüa deflexa 的G-型管胞兼有S-型管胞的原始性状（细胞壁穿孔的分布、密度和孔径），证明了管胞在单系演化序列中的有机联系，从细胞水平上为早期陆生维管植物类群的演化分异提供了新证据。
早期陆生维管植物的分类系统和演化谱系是植物研究的核心问题。Banks
(1968) 将它们划分为三大类群—莱尼蕨类
(rhyniophytes), 工蕨类
(zosterophyllophytes) 和三枝蕨类
(trimerophytes)。在此基础上,
Kenrick 和Crane
(1997) 根据分支系统学的分析将维管植物(包括化石和现生植物)
划分为三个谱系
(lineages): 石松植物亚门
(Lycophytina), 真叶植物亚门
(Euphyllophytina) 和莱尼蕨纲
(Rhyniopsida)。通过本文的研究,
取得了以下认识:
1. 根据Guangnania的研究和对比,
对工蕨类植物的特征进行了扩展,
认为孢子囊存在形状
(肾形―圆形/球形―纵向伸长)、开裂方式
(横向或纵向)
和两瓣
(双凸或凹凸)
的分异。2.
通过对Hsüa和Huia的研究,
认为存在以Hsüa为代表的徐氏蕨类群
(Hsüopsida)。该类群包括Hsüa,
Renalia, Nothia,
Sartilmania和Huia,
其主要特征为:
1) 具有明显的主茎轴及侧出的能育枝系/孢子囊柄;
2) 孢子囊顶生于侧枝上或侧生的长柄上;
3) 孢子囊性状存在变异,
为肾形、圆形或纵向伸长,
具明显的开裂机制;
4)初生木质部发生顺序为心始式;
5) G-型输水管胞。徐氏蕨类群与上述早泥盆世的三大类群并行,
它不同于三枝蕨类或者是真叶植物基部类群的长纺锤形、成簇顶生的孢子囊和P-型管胞;
不同于工蕨类或石松植物基部类群的外始式木质部;
不同于莱尼蕨类植物简单的等二叉分枝和S-型管胞。
根据共有的植物属种及其层位，论文第一次提出徐家冲中下部植物群同早泥盆世布拉格期的坡松冲植物群和澳大利亚上Baragwanathia植物群具有可比性。基于此,
并结合植物定量分析法
(Gerrienne, 1996a) 和层序,
认为徐家冲组的中下部地层
(第1-6层)
属于
(晚)
布拉格期;
依据较进化的植物、双壳类组合、鱼化石、地层层序和岩性的综合分析,
认为该组上部地层
(第7-8层)
当属早埃姆斯期。
上述三个植物群的可比性进一步证实了早泥盆世东北冈瓦那古植物地理区和华南古植物地理亚区
(Hao and Gensel, 1998) 的存在。以Hsüa为代表的华南早埃姆斯期植物群与该古植物地理区内
(晚)
布拉格期的坡松冲植物群有着演化上的继承性,
仍表现出鲜明的地方性特征;
该植物群虽然缺少在劳亚大陆中占主导地位的早埃姆斯期三枝蕨类植物,
但它们明显的主茎轴,
复杂的侧枝系和超微的输导组织结构显示出相同的进化水平和演化阶段。
另一方面, 徐家冲植物群在组成上与坡松冲植物群存在一定的差异, 这是由不同的生活环境和保存环境所致。这两个植物群分别代表河口三角洲环境和滨湖/泻湖环境。
关键词 云南
曲靖 早泥盆世
布拉格期-早埃姆斯期 徐家冲组植物群
Guangnania
cuneata
gen. et sp. nov. Huia
gracilis sp. nov.
Hsüa deflexa sp. nov.
输水管胞
分类系统和演化谱系 Hsüopsida 古植物地理 古环境
Abstract
During the past twenty years, the research of the early land vascular plants of China was focused in the Lower Devonian (Pragian) Posongchong Formation of Wenshan and Mengzi Districts, southeastern Yunnan. As the result, more than twenty species have been described. Characterized by the diverse assemblages and combining the highly developed sterile and fertile organs, the Posongchong flora has provided the invaluable scientific data to the understanding of the origin of diversity and early evolution of land plants, as well as to the reconstruction of palaeophytogeography (Edwards, 1997; Hao and Gensel, 2001). The Xujiachong Section in Qujing District, eastern Yunnan is the Early Devonian nonmarine type section of South China or even China. The Xujiachong Formation represents the marine-continent transitional sediments and also gives the ideal locality for the study of early land vascular plants. Since Ding Wenjiang collected the fossil plants and fishes in 1914, wide attention has been paid and many workers have undertaken the palaeontology and stratigraphy researches here. However, compared with the research of the Posongchong flora, which is of important achievement and world-wide attention, the Xujiachong flora still requires deep understanding. In view of the disputes, the geological age of the Xujiachong Formation needs further discussion.
In this morphological and anatomical study of four fossil plants documented from the Lower Devonian Xujiachong Formation of Qujing District, new plants Guangnania cuneata gen. et sp. nov., Huia gracilis sp. nov. and Hsüa deflexa sp. nov. were erected; Hedeia sinica Hao et Gensel 1998 was found and described. Zosterophyllum australianum, Z. yunnanicum and Drepanophycus qujingensis were reported. These plants contribute to the deep understanding of flora in this area and to the composition of early land vascular plants in China. The researches of new plants give complete and deep knowledge of their general characters. Among them, Guangnania also occurs in the Daliantang Section of Guangnan District, southeastern Yunnan. It is the fifteenth new genus described from the Posongchong Formation.
Guangnania cuneata gen. et sp. nov. has dichotomous smooth axes. With long stalks, the upright elongate cuneate sporangia are lateral and loosely, helically attached. Each sporangium consists of a smaller adaxial valve and a larger abaxial one, which are respectively concave and convex. The dehiscence line is conspicuously bordered by the valve marginal thickening. In view of the sporangial position and marginal thickening, this plant is allied to the zosterophyllophytes.
Huia was erected by Geng (1985) and the type species H. recurvata occurred in the Posongchong Formation of Zhichang Section, Wenshan District. Detailed research shows that Huia gracilis sp. nov. has K- or H-type branching in the rhizome. Erect axes dichotomize, at the branching position of which an axillary tubercle is sometimes present. Loose spike comprises terminal and lateral sporangia. The sporangium dehisces longitudinally. Centrarch xylem consists of G-type conducting tracheids bearing annular secondary thickenings, between which are irregular simple perforations. H. gracilis differs from H. recurvata in the branching pattern, the narrower stem, smaller sporangium and longer stalk.
Hsüa was established by Li (1982) and the type species H. robusta occurred in the Xujiachong Formation of Longhuashan Section, ca. 6 km northeast of the Xujiachong Section, in Zhanyi County. Hsüa deflexa sp. nov. possesses root-like axes. Upright stems bear sparse emergences and divide pseudomonopodially with the isotomous lateral branches. In many cases, the first order lateral branches depart at nearly right angles from the main axis. Mature ultimate laterals reflex in two opposite directions and each terminates in a sporangium. It morphologically differs from H. robusta mainly in the presence of axial emergences, horizontal extending of lateral branches and reflexing of ultimate laterals. Deep research and comparison show that, besides the centrarch xylem, H. deflexa has G-type tracheids; upon and between the annular secondary thickenings, the lignified secondary cell wall bears irregular simple perforations of small diameter and high density. So these G-type perforations also have some characters of S-type tracheids. The robust xylem strand and dense perforations indicate the strengthened water-conducting capacity.
In the early land vascular plants, the conducting cells are divided into S-, G- and P-types (Kenrick and Crane, 1991, 1997; Kenrick et al., 1991). Limited by the previous research methods, the ultrastructures of their cell wall and perforations need further study. Under the scanning electron microscope (SEM), for the first time we compare and study both the fusainized tracheids obtained through chemical maceration and permineralized tracheid lumen casts. This method reveals the ultrastructures of wall and perforations of G-type tracheids: 1) The lignified secondary wall of Hsüa deflexa is composed of three layers (S1, S2, S3) that are of unequal thickness and heterogeneous; with the smooth enclosing wall, the perforation is internally dumbbell-shaped; 2) In Huia gracilis, the enclosing wall of the tracheidal perforation contracts and possesses secondary perforations; the perforation has thickened border facing the tracheid lumen.
Innovation in the anatomical research method results in the deep understandings: 1) It proves the theory of two depositional process for the secondary wall of G-type tracheids (Bierhorst, 1960): the discontinuous cellulose part is laid down before the continuous lignified part; the former is prone to be broken down, while the latter is highly decay-resistant; 2) It suggests that the cellulose primary and secondary walls of tracheids are not perforated. Instead of longitudinal pits, the lignified tracheid secondary wall of Hsüa possesses irregular simple perforations; 3) The three layers of lignified secondary wall in Hsüa deflexa indicate that the chemical diversification of the secondary cell wall has occurred in the Early Devonian; 4) For the first time it is noted that, at the genus level of plants, G-type tracheids differentiate in the internal shape and minute structure of the perforations; 5) G-type tracheids of Hsüa deflexa combine the plesiomorphic characters of S-type tracheids (distribution, density and diameter of cell wall perforations). This phenomenon therefore demonstrates the linkage between tracheids in their monophyletic evolution sequence; at cell level, it also provides new evidence for the evolution and diversification of early land vascular plants.
The taxonomy and evolution lineage of the early land vascular plants are the key points to the plant research. Banks (1968) applied three categories to early land plants—the rhyniophytes, zosterophyllophytes and trimerophytes. On this basis, the cladistic analysis (Kenrick and Crane, 1997) recognized three lineages among the vascular plants (including the fossil and living plants): the Lycophytina, Euphyllophytina and Rhyniopsida. Through this study, following conclusions are drawn: 1. According to the comparative research of Guangnania, the diagnosis of the zosterophyllophytes is expanded. It is suggested that their sporangia have the differentiation in the shape (reniform, rounded/globose to vertically elongate), dehiscence (transverse or longitudinal) and valves (biconvex or concave/convex). 2. Through the study of Hsüa and Huia, a new classification Hsüopsida represented by Hsüa, is proposed to include Hsüa, Renalia, Nothia, Sartilmania and Huia. The main characters of this category are: 1) The plants possess clear main axes and lateral fertile branches/sporangial stalks; 2) Sporangia terminate the lateral branches or long stalks; 3) With the distinct dehiscing mechanism, the sporangia have the variation in shape, i.e. reniform, rounded or vertically elongate; 4) Primary xylem is of centrarch maturation; 5) Conducting tracheids are G-type. Hsüopsida is parallel to the above three plant categories in Early Devonian and differs from them in that: the trimerophytes or basal members of the Euphyllophytina have terminal trusses of long fusiform sporangia and P-type tracheids; the zosterophyllophytes or basal members of the Lycophytina are characterized by the exarch xylem; the rhyniophytes bear simple isotomous branches and S-type tracheids.
In view of the common genus, species and their horizon, for the first time it is indicated that the mid-lower assemblage of the Xujiachong flora is comparable with the Early Devonian (Pragian) Posongchong flora and Australia upper Baragwanathia flora. Based on this fact, plus the plant quantification method (Gerrienne, 1996a) and stratigraphic sequence, we speculate that the mid-lower part of the Xujiachong Formation (1-6 units) dates (late) Pragian age. According to some highly developed plants, bivalve assemblages, fishes, stratigraphic sequence and lithology, it is suggested that the upper part of the Xujiachong Formation (7-8 units) is early Emsian age.
The similarity of the above three floras further proves the presence of the northeastern Gondwana palaeophytogeographic unit and South China palaeophytogeographic subunit during the Early Devonian (sensu Hao and Gensel, 1998). In this palaeophytogeographic unit, the early Emsian flora of South China, represented by Hsüa, evolutionarily succeeded the (late Pragian) Posongchong flora and also exhibited the obvious endemic characters. This early Emsian flora is short of the coeval prevailing trimerophytes of Laurussia palaeocontinent. However, as the trimerophytes, the plants show the equal developing level and same evolving stage in the distinct main axes, complex lateral branching system and ultrastructure of conducting tissues.
On the other hand, the Xujiachong flora differs from the Posongchong flora in composition. This phenomenon results from different living and preserving conditions. These two floras respectively represent the estuarine delta and near-shore lake/lagoon environments.
Keywords
Yunnan
Qujing Early Devonian
Pragian-early Emsian Xujiachong
flora Guangnania
cuneata
gen. et sp. nov. Huia
gracilis sp. nov.
Hsüa deflexa sp. nov. Conducting
tracheid
Taxonomy and evolution lineage Hsüopsida Palaeophytogeography Palaeoenvironment
