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Cambrian and earliest Ordovician fauna and geology of the Sông Đà and adjacent terranes in Việt Nam (Vietnam)Abstract Later Cambrian and earliest Ordovician trilobites and brachiopods spanning eight horizons from five localities within the Sông Mã, Hàm Rồng and Đông Sơn formations of the Thanh Hóa province of Việt Nam, constrain the age and faunal affinities of rocks within the Sông Đà terrane, one of several suture/fault-bounded units situated between South China to the north and Indochina to the south. ‘Ghost-like’ preservation in dolomite coupled with tectonic deformation leaves many of the fossils poorly preserved, and poor exposure precludes collecting within continuously exposed stratigraphic successions. Cambrian carbonate facies pass conformably into Lower Ordovician carbonate-rich strata that also include minor siliciclastic facies, and the recovered fauna spans several uppermost Cambrian and Lower Ordovician biozones. The fauna is of equatorial Gondwanan affinity, and comparable to that from South China, North China, Sibumasu and Australia. A new species of Miaolingian ‘ptychopariid’ trilobite, Kaotaia xuanensis , is described. Detrital zircon samples from Cambrian–Ordovician rocks of the North Việt Nam and Sông Đà terranes, and from Palaeozoic samples from the Trường Sơn sector of Indochina immediately to the south, contain a predominance of ages spanning the Neoproterozoic period and have a typical equatorial Gondwanan signature. We associate the Cambrian and Tremadocian ofmore »
Development of the early Cambrian oryctocephalid trilobite Oryctocarella duyunensis from western Hunan, ChinaAbstract Abundant articulated specimens of the oryctocarine trilobite Oryctocarella duyunensis from the lower Cambrian (Stage 4, Series 2) Balang Formation at the Bulin section in western Hunan Province, South China, permit the description of all meraspid degrees. The maximum number of thoracic segments observed in this collection is 11. Meraspid growth was accompanied by progressive and gradual change in overall form, and this animal showed an homonymously segmented trunk with variation in the number of pygidial segments during ontogeny. Such variation permits a variety of plausible explanations, but a model of successive instars defined by the number of thoracic segments, and in suborder by the number of pygidial segments, is highly unlikely to explain the growth pattern because it would result in the loss of trunk segments between some instars. Degree-based ontogenetic staging is compatible with the variation observed.