华南与缅甸奥陶纪末赫南特贝动物群中的非铰合类腕足动物

LATEST ORDOVICIAN NON-ARTICULATED BRACHIOPODS IN THE HIRNANTIA FAUNA OF SOUTH CHINA AND MYANMAR

在奥陶纪末生物大灭绝的两幕之间,海洋底域繁盛着赫南特贝腕足动物群(Hirnantia Fauna)。它数量丰富,分布广泛,历程短暂。以往国内外学者研究这个动物群时,常把铰合类腕足动物作为重点,而非铰合类只被简单描述或列出名单,整体面貌不明。非铰合类化石尽管材料有限、研究基础薄弱,但只就属的数目而言,占据了全球赫南特贝动物群总属数的近1/5,其群落、演化和环境意义不可小觑。文中专门记述上扬子区(华南古板块)观音桥层(赫南特早中期)与缅甸曼德勒地区(滇缅马苏古地体)Hwe Mawng紫色页岩段(赫南特中期)所产赫南特贝动物群的非铰合类化石,计有3目、4超科[Linguloidea(舌形贝超科)、Discinoidea(平圆贝超科)、Craniopsoidea(似髑髅贝超科)和Cranioidea(髑髅贝超科)]的10属、12种,包括5个命名属种[Plectoglossa cf.davidsoni(Barrande),Schizotretinia cf.euxina(Havlicek),Pseudopholidops partibilis(Rong),Petrocrania cribrum(Temple),Xenocrania haimei(Reed)]和7个未定种(Trematis sp.,Paracraniops sp.,Acanthocrania sp.,Petrocrania?sp.1,Petrocrania?sp.2,Pseudolingula?sp.和Orbiculoidea sp.),其中,Pseudolingula?sp.和Orbiculoidea sp.两个未定名种因标本不佳,未正式描述。还有,以Pseudopholidops最为常见,其次是Xenocrania和Petrocrania。前两属是华南、滇缅马苏、波罗的卡、阿瓦隆尼亚、佩鲁尼卡等古板块或地体赫南特贝动物群的常见分子。它们已知限于南、北纬30°之间的低纬度地区,这可能与化石采集和研究程度有关,但更可能反映奥陶纪末全球气候仍存在分异现象。研究识别了奥陶纪晚期非铰合类腕足动物7个目,可归为3个类群:灭绝目(仅神父贝目Paterinida)、消减目(多样性与丰度大幅衰减,尤其是三分贝目Trimerellida和乳孔贝目Acrotretida在Hirnantian销声匿迹)和延续目(适应能力强、忍耐阈值高,成功穿越大灾难的首幕)。这些分类单元对研究奥陶纪-志留纪交界期腕足动物群的多样性、群落生态、生物地理及宏演化等有重要意义。

During the latest Ordovician mass extinction, there were two episodes and in between there was the widespread development of the Hirnantia brachiopod Fauna. The fauna is rich in quantity and short in history, and plays an important role in stratigraphic correlation. In the Hirnantia Fauna, articulated brachiopods were investigated well, but non-articulated ones have had a weak foundation due to limited material and insufficient research. Since non-articulated genera accounts for nearly 1/5 of the total number of the fauna, their research values should not be underestimated. The fossils described in this paper were collected by the present authors from the Kuanyinchiao bed(lower-middle Hirnantian) in the Upper Yangtze Region(South China Palaeoplate) and the Hwe Mawng purple shales(middle Hirnantian) in Mandalay Division of Myanmar(Sibumasu Terrane). The authors recognized representatives of four superfamilies, namely, Linguloidea, Discinoidea, Craniopsoidea and Cranioidea, including Plectoglossa cf. davidsoni(Barrande), Schizotretinia cf. euxina(Havlícek), Pseudopholidops partibilis(Rong), Petrocrania cribrum Temple, and Xenocrania haimei(Reed), associated with five undetermined taxa(Trematis sp., Paracraniops sp., Acanthocrania sp., Petrocrania? sp. 1, and Petrocrania? sp. 2). In addition, there occur Pseudolingula? and Orbiculoidea?, which are not described and illustrated herein due to the poor preservation state of specimens. The two widespread species Pseudopholidops partibilis(Rong) and Xenocrania haimei(Reed) are considered the characteristic elements of the Hirnantia Fauna, being of great value in biostratigraphy, and are recorded mostly in lower BA3 and rarely in BA2, upper BA3 and BA4, indicating their synecological significance. Palaeogeographically, it is interesting to mention that they are known to be confined to Avalonia, Baltica, Sibumasu, South China and Perunica which were located between 30o north and south latitudes during the Hirnantian, latest Ordovician. This phenomenon may have been related to the degree of fossil collection and research, but more likely reflecting that global climate may have differentiated to some extent during the development of the Gondwana glaciation at the end Ordovician. There occur 7 orders of non-articulated brachiopods in Late Ordovician, which can be recognized to 3 groups: 1) extinct: Paterinida only;2) greatly declining in diversity and abundance, especially Trimerellida and Acrotretida as Lazarus taxa that disappeared in the Hirnantian;3) extended(Lingulida, Craniopsida, and Craniida): successfully survived the first episode of the crisis due to adaptability and higher tolerance threshold. This indicates that the records of non-articulated brachiopods are of special significance for study of biodiversity, synecology, biogeography and macroevolution of brachiopods during the Hirnantian crisis.