LATEST ORDOVICIAN NON-ARTICULATED BRACHIOPODS IN THE HIRNANTIA FAUNA OF SOUTH CHINA AND MYANMAR
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Abstract:
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 District 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 (Havlí?ek), 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 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 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.
