statistical analysis of middle jurassic bivalve assemblages from

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V. BARBU & I. LAZĂR considering a few species of bivalves as Middle Jurassic in age. Later, Stanoiu (1967) dated the sedimentary deposits as Bathonian-Lower Callovian (reddish, sandy and oolithic limestones, with brachiopods, ammonites and pectinids). Turculet (1998, 1999-2000) described and illustrated a rich bivalve fauna belonging to the families Posidoniidae, Oxytomidae, Chlamyidae, Entoliidae collected from calcareous sandstones and limestones from the Tarnita – Rarau – Breaza area. Turculet (1968) also studied the Middle Jurassic deposits of the Câmpulung - Pojorata area, re-finding the fossiliferous site described previously by other researchers. This outcrop is located by this author on the left shore of the of the Fundul Pojoratei creek, 400 m upstream of the confluence with Moldova creek. Turculet dated these deposits (calcareous, grey, compact, very fossiliferous sandstones overlained by tough, grey marls with belemnites) based on the molluscan fauna as Lower Bajocian (humphersianum zone). Turculet described and illustrated only a part of the collected fauna with belemnites, brachyopods, bivalves, and ammonites. The species of bivalves are common with Bucegi associations, and they were collected from deposits with similar facies. It is very possible that the bivalve associations will be higher, but this comparison is biased by the published number of species from this area. Rosia (Padurea Craiului Mountains) Preda (1962, 1968) described the most abundant Lower and Middle Jurassic bivalve species from the Rosia zone, from the following sites: Strungarului creek, Lazuri creek, Gorunaş Hill, Cuţilor creek. This association shares only few species with that of Bucegi Mountains: Entolium (E.) corneolum Young & Bird, Placunopsis fibrosa Laube, Trigonia costata Parkinson, Astarte modiolaris Lamarck, Pholadomya murchisoni Sowerby, Chlamys (C.) textoria (Schlotheim). These species were collected from oolithic, ferruginous limestones. BUCEGI CENTRAL CODLEA HAGHIMAS RARAU PADUREA ANINA DOBROGEA CRAIULUI BUCEGI 0.50000 0.30088 0.36364 0.20619 0.11429 0.26772 CENTRAL 0.50000 0.33043 0.32143 0.18182 0.16822 0.26357 DOBROGEA CODLEA 0.30088 0.33043 0.32787 0.20833 0.25000 0.33333 HAGHIMAS 0.36364 0.32143 0.32787 0.31111 0.18868 0.26667 RARAU 0.20619 0.18182 0.20833 0.31111 0.20000 0.22581 PADUREA 0.11429 0.16822 0.25000 0.18868 0.20000 0.17143 CRAIULUI ANINA 0.26772 0.26357 0.33333 0.26667 0.22581 0.17143 Table 1. Dice coefficient - The similarity matrix BUCEGI CENTRAL CODLEA HAGHIMAS RARAU PADUREA ANINA DOBROGEA CRAIULUI BUCEGI 0.50007 0.37056 0.46828 0.37423 0.16204 0.28972 CENTRAL 0.50007 0.41133 0.41878 0.33547 0.24172 0.28719 DOBROGEA CODLEA 0.37056 0.41133 0.32866 0.23438 0.25521 0.34443 HAGHIMAS 0.46828 0.41878 0.32866 0.33944 0.19037 0.28111 RARAU 0.37423 0.33547 0.23438 0.33944 0.20833 0.29484 PADUREA 0.16204 0.24172 0.25521 0.19037 0.20833 0.19022 CRAIULUI ANINA 0.28972 0.28719 0.34443 0.28111 0.29484 0.19022 Table 2. Kulczynski II coefficient - The similarity matrix BUCEGI CENTRAL CODLEA HAGHIMAS RARAU PADUREA ANINA DOBROGEA CRAIULUI BUCEGI 0.33333 0.17708 0.22222 0.11494 0.06061 0.15455 CENTRAL 0.33333 0.19792 0.19149 0.10000 0.09184 0.15179 DOBROGEA CODLEA 0.17708 0.19792 0.19608 0.11628 0.14286 0.20000 HAGHIMAS 0.22222 0.19149 0.19608 0.18421 0.10417 0.15385 RARAU 0.11494 0.10000 0.11628 0.18421 0.11111 0.12727 PADUREA 0.06061 0.09184 0.14286 0.10417 0.11111 0.09375 CRAIULUI ANINA 0.15455 0.15179 0.20000 0.15385 0.12727 0.09375 Table 3. Jaccard coefficient - The similarity matrix The Jaccard, Dice, Kulczynski II coefficients point to the following conclusions: - the similarity between bivalve associations from Bucegi and Central Dobrogea with Dice (Tab. 1, Fig. 4
STATISTICAL ANALYSIS OF MIDDLE JURASSIC BIVALVE ASSEMBLAGES FROM ROMANIA: A PRELIMINARY REPORT WITH PALEOECOLOGICAL SIGNIFICANCE 1A) and Kulczynski II (Tab. 2, Fig. 1B) coefficients of 0.51 obtained from the similarity matrix is influenced by the facies in which these associations occur (calcareous sandstones interlayered with biocalcarenites). Ecological groups that dominate these associations point out this aspect. These ecological groups are infaunal and semi-infaunal species belonging to Pleuromyidae, Pholadomyidae, Ceratomyidae, Mytilidae, Pinnidae). The similarity between the two associations is confirmed also by the low value of the Jaccard coefficient (0.44) (Tab. 3, Fig. 1C), this coefficient showing the differences between the assemblages. This difference was triggered by the byssate infaunal species of families Arcidae, Cucullidae, Paralellodontidae and Bakewellidae, occurring in a pellitic facies of the western Bucegi Mountains, lower part of the Strungulita Formation respectively. - the Haghimas associations are similar to the Bucegi and Central Dobrogea associations due to the same type of facies, sharing 14 common species belonging to the families Mytilidae, Oxytomidae, Rhombopteriidae, Entoliidae, Pectinidae, Pholadomyidae, Pleuromyidae. The Jaccard coefficient value (0.18), indicating the minimal differences between the three associations, is influenced by the low number of species recorded until now from Haghimas area. Figure 1. Rectangular cladograms using the Dice (A), Kulczynski II (B) and Jaccard (C) coefficients - the associations from Rarau and Rosia zones (Padurea Craiului Mountains) have the lowest values of similarity coefficients, due to the low number of species described in the literature, with only two common species. - the associations from Anina and Codlea have a high similarity rank (according to the Dice coefficient), with little differences (according to the Jaccard coefficient). The similarity of these two assemblages is evident when considering the relative dominance related to the species number, of Bivalvia Class' subclasses. In this way, the subclasses Pteriomorphia, Anomalodesmata and Heterodonta show a high number of common species. The results obtained in this study are very useful for paleoecological and paleobiogeographical reconstructions. However, these results are preliminary, as the primary data are significant only for the Anina, Bucegi, Codlea, Central Dobrogea areas, while for other areas such as Rarau, Haghimas, Rosia – Padurea Craiului Mountains, the data were provided only from previous literature, without quantity information. Completing these data may lead to high similarity coefficient values between these associations and those studied in detail by the authors. Acknowledgements The authors thank to Daniel Scradeanu, who kindly indicated the Free Tree and Tree View software and to Mihai E. Popa for the English translation of this paper. 5
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