Compositional variations and diversity patterns of brachiopods and bivalves along the Late Paleozoic carbonate platform of western South America: a preliminary study

STERREN, A.F.; HALPERN, K.; BALSEIRO D.; CISTERNA, G.A.

Concepción

Congreso; XV Congreso Geológico Chileno; 2018

The widespread distribution of the Late Paleozoic carbonate successions along the western margin of South America suggests an extensive shallow marine platform developed during a climatic amelioration period. Important invertebrate faunal assemblages have been recorded at different latitudes of this platform in the Pennsylvanian-Cisuralian interval. Although there are few recent taxonomic studies focusing on these faunas, the available databases seemed to suggest significant variations, in terms of composition and diversity of brachiopods and bivalves, from low-latitude basins (Copacabana Formation in Amazonas region in Peru, Palmarito Formation in Venezuela and Chochal Formation in Guatemala) to relatively intermediate (Copacabana Formation in Bolivia, Juan de Morales, Cerros de Cuevitas and Cerro El Arbol formations in northern Chile, Taciba Formation in southern Brazil and Arizaro Formation in northern Argentina), and the southernmost records in central Chile (Huentelauquén Formation). However, at the present, there are no quantitative studies to understand the diversity patterns along the latitude gradient in this carbonate platform.Two hundred and five samples with 1181 occurrences of 184 genera of brachiopods and bivalves from Western South America in the Bashkirian-Kungurian interval have been herein considered for the database. It includes data entries from the PBDB (Clapham et al., 2018) plus several localities ranging from Guatemala to northwestern Argentina, southern Brazil and northern Chile (Branisa, 1965; Aceñolaza et al., 1972; Benedetto, 1980; Sanchez, 1984; Cisterna et al., 2014). Each sample was assigned with its paleogeographic location. In the absence of this parameter, its paleolatitudinal distribution was estimated by transforming the GPS coordinates into their respective time setting using Paleolatitude Calculator 2.1 (Douwe et al., 2015). Samples were grouped by stages and then separated in 3 bins of 20° of latitudinal distribution. The alpha and beta diversity in each temporal and latitudinal bin was estimated by using vegan package (Oksaken et al., 2018). In order to complete the diversity latitudinal gradient along the western basins of Gondwana, the same parameters were estimated for the western-central basins of Argentina using recently published data (Balseiro, 2016).Hierarchical diversity partitioning has shown to be a powerful tool for unraveling the principal drivers of biodiversification (Hautmann, 2014). Therefore, patterns of alpha (collection scale) and beta (among collection turnover) diversity are studied in this contribution.Although the fragmentary stratigraphic data from Western South America does not allow us to describe the entire latitudinal gradient from the Pennsylvanian to the mid Permian, a few tendencies are possible to recognize. In relation to alpha diversity (Fig. 1), along the equatorial region (0° to 20°S), it exhibits a general increase from Asselian to Kungurian, reaching values four times larger in the Cisuralian than in the Pennsylvanian. In mid-latitudes (20° to 40°S) alpha diversity is severely reduced from the Pennsylvanian to the Cisuralian times: it is stable during the lower Pennsylvanian, shows a decrease of 35% from Moscovian to Gzhelian, and drops steadily from Gzhelian on, being four times smaller in the Artinskian. In higher latitudes (~45°S), the alpha diversity diversity increases slightly from Bashkirian to Moscovian and shows a decrease of the 33% to Kasimovian. The Sakmarian is very poor in diversity, but it is probably because it is represented by only two samples.To further understand diversity structure, the compositional variation among collections at each time interval and latitudinal bin are also studied. A preliminary analysis showed that all time intervals had many compositionally identical collections. Hence, we used a simple index to measure compositional variation (i.e., beta diversity) through time and among intervals (β= 1 ? number collections with identical composition / total number of collections). Beta diversity (Fig. 1) results to low latitudes (from 0° to 20° S) indicate a marked increase of compositional variability among the Pennsylvanian/Cisuralian interval with an unusual low value in the Artinskian, probably being related to the low number of samples included in the analysis. At middle latitudes (from 20° to 40°S), Bashkirian exhibits the lowest values of beta diversity while the following time intervals show intermediate values of compositional variability. Finally, towards high latitudes (~45°S), the available data comes exclusively from the west central Argentina and beta diversity values recorded therein are intermediate. Although beta diversity mainly tends to remain stable, an increase of 22.5% from Bashkirian to Moscovian, and a slight decrease (-14.5%) to Kasimovian have been identified.In summary, the patterns of diversity partitioning evidence variations along the latitudinal gradient during the Pennsylvanian/Cisuralian times. In one hand, alpha diversity shows a general pattern of increase in equatorial regions (Guatemala, Venezuela and northern Peru), a steady decrease in template regions (Bolivia, northern Chile and northwestern Argentina) and, even though small variations can be detected, it rather remains stable in higher latitudes. On the other hand, beta diversity markedly increases in low latitudes and, on the contrary, minor variations were identified in intermediate and high latitudes.The Late Paleozoic Ice Age (LPIA) had profound consequences on the global marine biota (Stanley and Powell, 2003; Powell, 2005). At the onset of this event, a severe extinction hit the tropical biota (Powell, 2005; McGhee et al., 2010), that recovered only at the end of this major global ice age. However, its effect on diversity ecological dynamics of extratropical biota was minimal (Balseiro, 2016; Balseiro and Halpern, 2016) and it also agrees with the preliminary results herein provided. As the LPIA ended towards middle Cisuralian, alpha diversity recovered in tropical environments but remained either stable or even decreased in extratropical regions. The drastic loss recorded in the benthic fauna, at intermediate latitudes, would represent the extinction of glacial taxa (Waterhouse and Shi, 2010). Thus, these results support the return of the steep latitudinal diversity gradient in the oceans (Powell, 2007).Moreover, the beta diversity rise found in the tropics agrees with previously reported diversity patterns for west central Argentinian basins (Balseiro, 2016, Balseiro and Halpern, 2016) together with other macroecological patterns recorded for the LPIA (Powell, 2005, 2007). In particular, at the beginning of the LPIA, the Serpukhovian biodiversity crisis was characterized by the extirpation of mainly tropical and small range taxa. The recovery after the LPIA witnessed the increase in small range taxa in the tropics. Hence, an increase in beta diversity is expected in this region, but not in extratropical regions. Results herein provided, once again, agree with this dynamics, reinforcing the idea that the LPIA had a much more profound effect on tropical regions than on actual glaciated ones (Balseiro, 2016; Balseiro and Halpern, 2016).In equatorial latitudes, a sustained increase of both alpha and beta diversity point to a gamma diversity rise. On the other hand, the stability in beta diversity and the severe loss in alpha diversity in mid-latitudes suggest a decrease in gamma diversity. Both parameters exhibit a similar pattern in high latitudes, showing probable small variations in regional diversity. These preliminary results show a distinctive latitudinal diversity gradient that could be explained by differential intensity of the consequences of environmental changes related to the LPIA.Although current results are promising for understanding the response of western Gondwanan marine faunas to the LPIA, further analysis are needed in order to confirm this patterns in the Southern Hemisphere. Also, more data from the intermediate and higher latitudes are necessary to achieve a better understanding of regional diversity variations and their causes.