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The paper describes the brachiopod fauna recovered from the outer-ramp deposits developed …


Biology Articles » Paleobiology » Paleoecology » An outer-ramp brachiopod assemblage from the uppermost San Juan Formation (middle Arenig), northern Argentine Precordillera: paleoecologic and biogeographic implications » Paleoecology

Paleoecology
- An outer-ramp brachiopod assemblage from the uppermost San Juan Formation (middle Arenig), northern Argentine Precordillera: paleoecologic and biogeographic implications

As stated above, the studied brachiopod assemblage occurs in dark gray nodular wackestones-mudstones which are absent at the type section of Cerro La Silla (central Precordillera). This lithofacies is interpreted as having been deposited in an outer ramp environment, mostly below storm wave-base (Cañas, 1999). The fauna is dominated by trilobites and brachiopods, with hexactinellid spicules as minor components.
For paleoecologic analysis, the faunal content was estimated by counting identifiable silicified brachiopod remains in the etched residue as well as molds in the insoluble muddy matrix. The relative abundance of taxa in terms of biovolume was calculated for each species following the procedure proposed by Sánchez et al . (1993), which consists in estimating the biovolume by submerging in water a series of conjoined specimens of different size and measuring the volume displaced. On the basis of these measurements a curve representing the ?shell volume/shell width' ratio was constructed for each species. On the basis of shell morphology, which reflects particular life strategies, brachiopod taxa were ascribed to different guilds in order to evaluate the ecospace occupation. (Bambach, 1983; Thayer, 1983; Waisfeld et al ., 2003) (table 1). Diversity was estimated according to the Shannon index (D = - ?" pi ln [ pi ]), where pi = ni /N, where ni = biovolume of the i'th species, and N = total biovolume), and richness by counting the total number of species in the sampled level. The studied assemblage from Quebrada Potrerillos is characterized by low values of richness (6 taxa) and diversity (0.9). The latter is due to the dominance of Tritoechia sp. (76.52%) respect to the other components (figure 2.A). The six brachiopod taxa present in the assemblage were referred to four guilds, of which the semiinfaunal one is by far the more abundant in terms of percentage of total biovolume (76.52%). The low attached forms were divided in biconvex shells, umbo down (9.74%) and transverse shells with strong fold and sulcus (1.88%). The liberosessiles represent 11.86% of the total (figure 2.B).

A comparison of outer shelf assemblages with coeval communities developed in the shallower water facies of the central Precordillera basin studied by Cech and Carrera (2002) and Cech (2004) reveals that richness values are similar but diversity is higher in the central Precordillera assemblages (1.59). The latter value indicates high equitability, reflecting that none taxon clearly dominates respect to the others (figure 3.A). The most abundant taxon is Niquivilia extensa (36.5%) followed by Leptella ( Leptella ) spp. (22.86%), Tritoechia sp. (19.9%), Huacoella radiata (10.95%), Hesperonomia sp. (4.37%), Paralenorthis sp. (4.13%) and unidentified orthids (1.29%). On the other hand, the mid-ramp facies contains only three guilds, of which the liberosessiles are largely dominant, while the semi-infaunals and pedunculate lowattached are less represented in the communities (figure 3.B). Although a quantitative analysis of components other than the brachiopods is beyond the scope of this study, it should be noted that sponges and gastropods, which are among the main components of the central Precordillera communities, are nearly absent in the outer-ramp deposits.
Among physical parameters, substrate is considered a key factor controlling the brachiopod distribution, while other variables as temperature, salinity, light, current strength, seem to have played a secondary role in the bathymetric distribution, at least in the living rhynchonelliform brachiopods (Richardson, 1997). The dominance in the studied brachiopod association of the semi-infaunal life-style, followed in importance by the free-lying forms, is consistent with the expected reduction of the pedicle system in the low-energy muddy substrates prevailing in outer shelf environments. According to Sánchez et al . (1996) such a strategy became frequent in stable and relatively soft substrates. The semi-infaunal mode of life of Tritoechia has been inferred from the relatively small foramen piercing apically the pseudodeltidium. Sánchez and Tóffolo (1996) considered that such a reduced, probably rachet-like pedicle, played a secondary role in the attachment. In their reconstruction, Tritoechia is oriented with the apical region of its pyramidal ventral valve sunk in the bottom and the anterior commissure well above the sediment-water interface. For this reason it is considered as a semi-infaunal form.
The free-lying plectambonitoids (the liberosessile guild) are characterized by concavoconvex or planoconvex, transverse, often alate shells having small apical foramen or lacking them. This life strategy, as stated above, dominates the mid-ramp communities of the Central Precordillera. According to Sánchez and Tóffolo (1996) and Sánchez et al. (1996) the liberosessile brachiopods flourished in relatively stable carbonate substrates affected occasionally by storm waves, the stability on the substrate being achieved by mean of their reduced, probably functional pedicle. It is uncertain, however, to what extent such a pedicle was capable to elevate the shell above the substrate ( cf . Rong and Cocks, 1989). It seems likely that adults of Taffia and Leptella ( Petroria ) reclinated on the bottom, with the thickened umbonal region of ventral valve slightly buried. In the Precodilleran mid-ramp deposits this guild was occupied by three genera ( Leptella , Huacoella and Niquivilia ) and six or more species. The fact that in the Precordillera basin the liberosessiles were more diversified in shallower than in deeper water carbonates seems to be in contradiction with trends observed in other regions and ages where plectambonitoids attain higher biovolume values in deep water settings (Potter and Boucot, 1992; Patzkowsky, 1995; Sánchez et al , 1996; Benedetto, 2001b). The deviation from this general pattern is due to the great expansion, in terms of biomass, of Tritoechia (low value of equitability). It is interesting to note that in the lower Llanvirn limestone beds of the uppermost San Juan Formation exposed at Talacasto, plectambonitoideans also tend to be associated with tritoechiids, as cluster analysis (R-mode) of brachiopod taxa showed (Sánchez and Tóffolo, 1996), though biovolume values in the former are higher. Such a link suggests that both life strategies were equally advantageous for inhabiting muddy soft substrates. Tritoechiids, however, are absent in the overlying deeper water parted limestones (base of the Gualcamayo Formation) in which only monospecific populations of Nanorthis fragilis have been recovered. This small thin-shelled species may be interpreted as an opportunistic rstrategist able to tolerate lowered oxygen values. Associated forms are trilobites of the nileid biofacies and numerous linguliformean brachiopods, mainly acrotretids (Benedetto et al ., 1986). Because these assemblages are absent in the interbedded black shales, the brachiopod-bearing levels may be interpreted as periods of colonization triggered by brief oxygenation events (Wignall, 1990). The organic-rich graptolitic black shales forming the bulk of the Gualcamayo Formation have yielded only few valves of a plectambonitoid referred herein to Pelonomia ? sp. Clearly, the dysaerobic conditions of the bottom inhibited the colonization not only of most suspensionfeeding rhynchonelliformean brachiopods but also of deposit-feeders, as the absence of bioturbation indicates.
In summary, the nature of substrate is interpreted as the main controlling factor in the distribution of brachiopods within the carbonate ramp. The midramp settings are characterized by low sedimentation rates sporadically affected by storm events, with recurrent development of firmgrounds and hardgrounds covered by bioclastic debris. Level bottom communities are dominated by encrusting organisms (receptaculitids), sponges, bryozoans, gastropods, crinoideans and brachiopods (Carrera, 2001), the latter forming relatively diverse and equitable assemblages. The muddy substrates developed in the outer-ramp areas were colonized mainly by semi-infaunal brachiopods (tritoechiids) and subsidiarily by liberosessile forms (plectambonitoids), other suspension feeders becoming almost absent. In the ramp-to-basin transition the decreasing of the oxygen level was probably the main barrier for a permanent colonization of rhynchonelliformean brachiopods.


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