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Dating of Cambrian–Ordovician Boundary Strata in Northernmost Vietnam and Methodological Aspects of Evolutionary Biostratigraphic Inference

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Dating of Cambrian–Ordovician Boundary Strata in Northernmost Vietnam and Methodological Aspects of Evolutionary Biostratigraphic Inference Dating of Cambrian–Ordovician boundary strata in northernmost Vietnam and methodological aspects of evolutionary biostratigraphic inference Jerzy Dzik1,2 and Nguyen Duc Phong3 1Institute of Paleobiology, Polish Academy of Sciences, Twarda 51/55, 00-818 Warszawa, Poland 2Faculty of Biology, Biological and Chemical Research Centre, University of Warsaw, Aleja ¨wirki i Wigury 101, Warszawa 02-096, Poland 3Vietnamese Institute of Geosciences and Mineral Resources, Thanh Xuan, Hanoi, Vietnam e-mail:[email protected] ABSTRACT: Unrepeatability of evolution and the correspondence of the fossil record to ancestor-descendant successions of species are the unavoidable, although usually hidden, assumptions in any reliable age determination based on fossils. We expose these assumptions while dating early Paleozoic carbonate rock deposits in the Lung Cu section at the Vietnamese–Chinese border. The best-preserved and most abundant fossils in this section are shumardiid trilobites. The succession of shumardiid species, based on data from elsewhere, pro- vides an evolutionary reference standard. The shumardiid record is not sufficiently complete to verify hypotheses of ancestor-descendant relationships but enables estimation of the ‘degree of evolutionary advancement’of the Vietnamese species. This suggests an age close to the Cambrian–Ordovician boundary. Although considered non-scientific by cladists, such inferences are testable. Support for a late Furongian or early Tremadocian age is provided by the occurrence of Cordylodus conodonts in strata above the trilobite-bearing bed. The conodont evolution has a good fossil record interpreted in population terms in the Baltic region and Australia, including the lineage repre- sented in Vietnam. Age determination based on such evolutionary reasoning is reliable but of a relatively low resolution, because the rate of morphological evolution is generally low. Generally, more precise dating is offered by distribution of fossils controlled by ecological factors, which are repeatable and mostly diachronous over large geographic distances, but they may have happened relatively rapidly. The appearance of the Iapetognathus-Chosonodina-bearing conodont assemblage in the Lung Cu area, as suggested by its occurrences elsewhere, was probably due to abrupt faunal migration into the region. history of the region, its geological mapping and stratigraphy INTRODUCTION has remained at a rather basic level for decades (e.g., Deprat Temporal calibration of phylogenies requires evidence from the 1915a, 1915b, 1916; Mansuy 1915a, 1915b, 1916; Kobayashi fossil record. This results in a significant, although rarely ac- 1944; Dovzhikov 1965) and has only recently been re-evaluated knowledged, conflict between the methodological approaches (Tinh 1976; Tinh et al. 2001; Ngân and Huöc 1996; Ngân 2008; of biostratigraphy and phylogenetics that has implications also Huyen et al. 2007; Thanh & Khuc 2011). Here we use evidence for related research in biology. Cladistic analysis simply evalu- mainly from trilobite and conodont assemblages to resolve the ates relationships between taxa independent of time, whereas age of the strata at Lung Cu and expose some methodological biostratigraphy commonly infers ancestor-descendant relation- problems with the dating of evolutionary events and sedimen- ships (discussed, e.g., by Dzik 2005, 2015; Wickström and tary strata using fossils. Donoghue 2005). Owing to a similar methodological discrep- ancy, the nature of correspondence between the succession of GEOLOGICAL SETTING fossils and definitions of geochronological units is among the The Lung Cu section is adjacent to the Vietnamese–Chinese most controversial issues in geology. Contemporaneity of strata border at Lung Cu village, 23 km north of Dong Van District from multiple sites is usually inferred by identifying ranges of Town, at the northernmost tip of Vietnam (text-fig. 1; 23°21’ the same species (or higher rank taxa). The crucial point is the 15”N; 105°17’38”E). This region belongs to the South China meaning of correlation lines that are results of such infer- terrane (Stokes 2008; Krobicki et al. 2008), which was part of ences—do they represent unrepeatable evolution of organisms the Yangtse continent. The strata exposed in this section are or are they just an expression of repeatable and possibly composed mostly of steeply dipping carbonate beds (i.e., diachronous control by local ecological factors? micritic and oolitic limestone, locally dolomitic) interbedded with shale, marlstone, siltstone and sandstone. These fundamental aspects of biostratigraphic methodology are of minor importance in everyday practice and are rarely consid- Two samples near the top of the succession mapped as Ordovi- ered, so much so that in today’s reality of geological research cian yielded a few conodont elements and one contained numer- any proposal of correlation is entangled in countless facts and ous silicified trilobite exoskeletons. The conodonts derive from interpretations compiled by earlier students of the same subject. a coquina of orthoid brachiopods (text-fig. 1B) that are too frag- Discussed in this study, pristine sedimentary successions at the mentary and diagenetically altered for taxonomic identification. northernmost tip of Vietnam host important assemblages of The coquina also yielded crudely silicified unidentifiable trilo- early Palaeozoic invertebrates. Because of the complex political bite exoskeleton fragments, phosphatic linguliform brachiopods Stratigraphy, vol. 13, no. 2, text-figures 1–5, pages 83–93, 2016 83 Jerzy Dzik and Nguyen Duc Phong: Dating of Cambrian–Ordovician boundary strata in northernmost Vietnam A B 5 km Lung Cu coquina * SS. 125/2 shumardiid SS. 1080 trilobites C d i i d s r e a t i m b u o l h i r s t a n i u q o c D E shumardiid trilobites 1 m 10 cm TEXT-FIGURE 1 A, Location of the Lung Cu section (asterisk) on the geological map of the Dong Van area in northernmost Vietnam. Abbreviations: e2-e3 cp, Chang Pung Formation (Upper Cambrian); O1 lx, Lutxia Formation (Lower Ordovician); D1 sk, Si Ka Formation (Lochkovian); D1 bb, Bac Bun Formation (Lochkovian–Pragian); D1ml, Mia Le Formation (Pragian); D1-3 sp, Si Phai Formation (Pragian–Frasnian); D3 tt, Toc Tat Formation (Frasnian–Famennian); C-Pbs, Bac Son Formation (Carboniferous–Permian); P3dd, Dong Dang Formation (Upper Permian) T1 hn, Hong Ngai Forma- tion (Lower Triassic). B, Photograph of the outcrop with location of productive samples. C, View of the abandoned quarry showing relationship between the brachiopod coquina and limestone concretions with the shumardiiid trilobites. D, Position of the fossiliferous beds on the diagrammatic rock column. E, Enlargement of the bed with concretions containing silicified shumardiid trilobites. and internal moulds of probable bellerophontid gastropod larval resenting all segments of the thorax, and 54 pygidia. Twenty shells. one specimens represent the protaspis stage, without the distinc- tion between cephalon and pygidium yet developed. The sample Relatively well preserved but low-diversity silicified trilobites is monospecific (with the exception of the fragmentary speci- were extracted from flat limestone concretions occurring in a men in text-fig. 2C). stratigraphically lower marl bed. Trilobites are of great impor- tance in discussions on the palaeogeography of Indochina Only two samples (2 kg) were productive for conodonts: five el- (Gonzalo et al. 2003; Stokes 2008) and we focus our research ements were found in the brachiopod coquina (one represents on them. Teridontus, others Cordylodus), four in sample SS 1080, taken about 30 m above the coquina (Cordylodus, Semiacontiodus, Iapetognathus, and Chosonodina). MATERIAL The shumardiid trilobite material consists of 98 cranidia, 48 The materials discussed in this paper are housed at the Institute librigenae, two of them still attached to cranidia and 6 of Paleobiology, Polish Academy of Sciences, in Warsaw (ab- hypostomes (two attached), 169 thoracic tergites probably rep- breviated ZPAL). 84 Stratigraphy, vol. 13, no. 2, 2016 A 1 B A3 C1 C2 A4 A2 D1 D 2 E1 E2 E3 H2 I I J J J F F F G1 G2 1 2 1 2 3 1 2 3 H1 K1 K 2 K3 M 2 P1 P2 L1 L 2 M1 N1 N 3 Q1 Q2 N 2 R1 R 2 S S X X O1 O2 1mm 1 2 1 2 X3 V1 V 2 T T T U1 U2 1 1 3 W1 W2 Y1 Y2 TEXT-FIGURE 2 Silicified exoskeleton elements of the shumardiid Conophrys sp. extracted from limestone concretions (A, B, D–Y ) and probable partial cranidium or hypostome of unidentified trilobite ZPAL V.47/5 (C). A Right half of fused free cheeks (librigenae) ZPAL L Two first thoracic tergites ZPAL V.47/53 in dorsal and V.47/23 in ventral (A1), dorsal (A2), inner lateral (A3) lateral views. and oblique outer lateral (A4) views. M, N Macropleural third tergites ZPAL V.47/21 and 33 in B More complete cheeks ZPAL V.47/22 in dorsal view. ventral, posterior, and dorsal views. D–G Cranidia ZPAL V.47/69, 3, 64, and 62 of various O Tergites 3–5 ZPAL V.47/20 preserved together in dor- ontogenetic ages in dorsal, ventral and lateral views. sal and ventral views. H Coiled complete early meraspis ZPAL V.47/15 seen P, X Pygidia and associated thoracic segments ZPAL from its cephalon and pygidium. V.47/14 and 16 of early meraspides. I, J Protaspides ZPAL V.47/17 and 18 in dorsal, ventral Q, R Spinose macropleural tergites ZPAL V.47/19 and 8 of and lateral views. late meraspides. S. Right pleura ZPAL V.47/7 of post-macropleural tergite. K Hypostome ZPAL V.47/6 in ventral, lateral and dorsal views. T–Y Pygidia ZPAL V.47/10, 9, 11, 12 and 13 of various stages in dorsal, lateral and ventral views. 85 Jerzy Dzik and Nguyen Duc Phong: Dating of Cambrian–Ordovician boundary strata in northernmost Vietnam QUANTITATIVE EVOLUTIONARY PALAEOBIOLOGY alone may cause controversies, but probably even more intrigu- VS. BIOSTRATIGRAPHY ing is the way in which temporal data used to calibrate Modern biostratigraphy and quantitative evolutionary palaeobi- chronograms are obtained.
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