Influence of Size-Sorting on Diversity Estimates from Tempestitic Shell Beds in the Middle Miocene of Austria
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142 RESEARCH REPORT In¯uence of Size-sorting on Diversity Estimates from Tempestitic Shell Beds in the Middle Miocene of Austria MARTIN ZUSCHIN Institut fuÈ r PalaÈontologie, UniversitaÈ t Wien, Althanstrasse 14, A-1090 Vienna, Austria, Email: [email protected] MATHIAS HARZHAUSER Naturhistorisches Museum Wien, Burgring 7, A-1014 Vienna, Austria OLEG MANDIC Institut fuÈ r PalaÈontologie, UniversitaÈ t Wien, Althanstrasse 14, A-1090 Vienna, Austria PALAIOS, 2005, V. 20, p. 142±158 DOI 10.2210/palo.2003.p03-87 INTRODUCTION Taphonomic factors set limits to the resolution of diver- Paleontological data frequently are extracted from geneti- sity patterns in the fossil record. The most important bias cally and stratigraphically complex shell beds. It is there- is the typical loss of taxa without a mineralized skeleton fore important to recognize taphonomic biases that can lead (for review, see Kidwell and Flessa, 1995). Usually, the to major errors in paleoecological interpretations (e.g., on shelly remains of the original community then are affected ancient local biodiversity). The strong in¯uence of trans- by disintegrative processes such as disarticulation, frag- port-related shell-size sorting on diversity estimates from mentation, abrasion, and dissolution (for reviews, see single samples was studied in a transect of the middle Mio- Powell et al., 1989; Kidwell and Bosence, 1991), in addi- cene Grund Formation (Lower Austria), which contains al- tion to time-averaging (e.g., FuÈ rsich and Aberhan, 1990; lochthonous, psammitic event beds with channel structures, Olszewski, 1999; Kidwell, 2002). Among disintegrative sharp erosional bases, and graded bedding. These event processes, for example, selective dissolution of aragonite beds are interpreted as proximal tempestites, and contain can alter molluscan diversities strongly (e.g., Brachert densely packed, polytaxic molluscan assemblages. The fau- and Dullo, 2000; Wright et al., 2003), and the degree of time-averaging will in¯uence the species richness of a par- nal composition and taphonomic features of shells indicate ticular fossil assemblage greatly (e.g., Staff and Powell, that transport occurred from wave- or current-agitated 1988; Kidwell, 2002). Another very important taphonomic nearshore habitats into a pelitic, inner-shelf environment. process that potentially in¯uences diversity estimates is The different skeletal concentrations contain a highly di- shell transport, which inevitably will in¯uence the size verse molluscan fauna with 130 species identi®ed from frequency distribution of a paleocommunity (e.g., Menard more than 4200 individuals. Although the quantitatively and Boucot, 1951; Boucot, 1953; Olson, 1957; Veevers, most-important species are the same in standardized sam- 1959; Johnson, 1960; Fagerstrom, 1964; Shimoyama, ples from ®ve different shell beds, species richness differs 1985). Although out-of-habitat transport is limited to few signi®cantly among the three samples from the base of the settings and to particular sets of organisms only (for re- transect and the two samples from its top. Diversity de- views, see Kidwell and Bosence, 1991; Behrensmeyer et pends on size-sorting and therefore re¯ects the transport al., 2000), transport and size-sorting frequently are re- history of the individual tempestites, not the species rich- sponsible for species abundance patterns in modern death ness of the original paleocommunity. Poorly sorted samples assemblages and fossil assemblages (Wolff, 1973; Cum- (indicating relatively minor transport) approximate the di- mins et al., 1986a; Westrop, 1986; Blob and Fiorillo, 1996; versity of single samples of that environment better than Olszewski and West, 1997). Storm-in¯uenced macrofau- well-sorted samples (which indicate stronger transport). Di- nal death assemblages and fossil assemblages can retain versities of shelly assemblages from parautochthonous and their spatial zonation patterns (Miller et al., 1992; Miller, allochthonous assemblages cannot be compared directly. 1997), but limited, lateral transport of skeletal material by Even comparisons among tempestites are problematic be- storms can in¯uence species richness nevertheless (Miller cause transport intensity governs diversity. The intensity of et al., 1992), and tempestitic transport can determine the any taphonomic process, however, is dif®cult to predict proportions of the most abundant species in relatively uni- without detailed investigations. The use of samples from ta- form environments (Westrop, 1986). phonomically complex shell beds for diversity comparisons The present study quantitatively evaluates the in¯u- can bias results, especially on the ®ne-scale level of local di- ence of transport on diversity measurements (here mea- versity patterns. Studies at such ®ne scales of resolution sured as species richness and as evenness) of fossil allo- should consider the taphonomic framework of assemblages, chthonous tempestitic shell beds. For this purpose, the re- which is necessary to recognize the dominant taphonomic lationship of the diversity measurements of ®ve samples, factors and their intensities. each taken from a different tempestitic shell bed at the same locality, to their shell-size frequency distribution is Copyright Q 2005, SEPM (Society for Sedimentary Geology) 0883-1351/05/0020-0142/$3.00 TAPHONOMY AND DIVERSITY 143 FIGURE 1ÐStudy area and studied arti®cial trenches (log A±E) in farmland between the villages of Grund and Guntersdorf (modi®ed after Zuschin et al., 2001). investigated. The quantitatively most important species ed by RoÈgl et al. (2002), who correlated the deposits with are the same in these standardized samples, but species plankton zone M5b and nannoplankton zone NN5. The richness and evenness of these samples vary between in- characteristic benthic foraminiferal assemblage allows dividual shell beds and depend on the sorting parameter correlation with the lower Lagenidae Zone of the Vienna of the shell-size frequency distribution. Diversity, there- Basin ecostratigraphic zonation (sensu Grill 1941; 1943). fore, re¯ects the transport history of the individual tem- pestites, as well as the species richness of the original pa- FACIES AND PALEOENVIRONMENT OF THE leocommunity and other post-mortem factors altering spe- STUDIED SECTION cies abundances. These ®ndings are of interest because transported assemblages are widespread in the fossil re- The studied section of the Grund Formation shows a to- cord (e.g., Dominici, 2004), and they are used frequently to tal thickness of approximately 9.5 meters, and is charac- study diversity patterns (e.g., Patzkowsky and Holland, terized by interbedded, allochthonous-psammitic and au- 2003). tochthonous-pelitic sediments (Fig. 2). The sandy layers, especially in the lower part of the section, show abundant PALEOGEOGRAPHY AND STRATIGRAPHY channel structures, and consist predominantly of thick po- lytaxic skeletal concentrations (sensu Kidwell et al., 1986) The studied samples derive from the type locality of the with sharp erosional bases, graded bedding, and a densely middle Miocene Grund Formation in the Austrian part of packed (bioclast-supported) biofabric (Fig. 3). Channel the Molasse Basin (Fig. 1; Roetzel et al., 1999a). Paleogeo- structures, sharp erosional bases, and graded bedding graphically, this basin was part of the Central Paratethys identify the shell-rich psammitic layers as the products of SeaÐa northern satellite sea of the Western Tethys (5 high-energy, short-term events, which have been inter- Proto-Mediterranean) that was formed in the early Oligo- preted as proximal tempestites (Zuschin et al., 2001; cene by the rising Alpine island chain, which acted as a Roetzel and Pervesler, 2004). geographic barrier (RoÈgl, 1998). Due to strong tectonic Towards the top of the section, the polytaxic skeletal control, subsequent evolution of the Paratethys differed concentrations are distinctly thinner and characterized by from that of the adjacent Mediterranean considerably, tabular beds and low-angle cross-bedding instead of chan- and is re¯ected in the regional stratigraphic stage system nel structures (Fig. 3; Roetzel et al., 1999b). The interca- for the Paratethyan sedimentary succession (RoÈgl, 1998; lated pelitic layers increase in thickness towards the top of Steininger and Wessely, 2000; Harzhauser et al., 2002). the section, and are characterized by intensive bioturba- According to this regional stage system, the Grund For- tion and the in-situ occurrence of the bivalve Thyasira mation is part of the lower Badenian stage, which is cor- michelottii (Roetzel and Pervesler, 2004; Zuschin et al., related with the Langhian and early Serravallian of the 2001; Pervesler and Zuschin, 2004). These features indi- Mediterranean standard scale. An early middle Miocene cate decreasing hydrodynamic energy towards the top of age, based on the occurrence of the planktonic foraminif- the section (Roetzel and Pervesler, 2004). eran Praeorbulina glomerosa circularis, was demonstrat- The short distance to the paleo-coastline (estimated 5± 144 ZUSCHIN ET AL. FIGURE 3ÐTempestitic shell beds. (A) Lower portion of transect; channel structure with sharp erosional base (lower arrows), densely packed (bioclast-supported) biofabric of skeletal concentrations, and mud clast (upper arrow) from log B; scale is divided into 1-cm incre- ments. (B) Upper portion of transect; tabular shell bed with sharp ero- sional base (arrows), bioclast-supported biofabric of skeletal concen- trations,