Tasas evolutivas de algunos subórdenes de foraminíferos triásicos del área occidental del Tethys

Resumen

In this work are approached some aspects on the evolution of a whole of species of some suborders of triassic foraminifera by means of the study of evolutionary rates and polycohort analysis. The achieved results have allowed to show high speciation rates during Griesbachian, Nammalian, Spathian and Anisian, as a result of the gradual occupation of the ecological niches that were empties after the great extinction in the Upper Permian. During the same period, the extinction rates are also high. It is due to the persistence of unstable environmental conditions and to the appearance of a lot of new species with low evolutionary success. During Ladinian, Carnian and Norian, the environments become stabilized and appear abundant and diversified faunas with low speciation and extinction rates. In the Rhaetian occurs a new important crisis that produces the extinction of nearly a half of the species of foraminifera that are present at this period. Analysis for each one of different suborders have been carried out, also. The results show significative differences between them, specially interesting in the case of suborders with aragonitic test as Involutinina and Robertinina. The polycohort analysis has allowed to indentify three important extinction events. One of them is in the Lower Anisian, another in the Upper Anisian and the last one in the Rhaetian. Furthermore, a less strong extinction event appears in the Carnian-Norian boundary. On the other hand, the cohorts show a similar patterns so that the inflections are simultaneous. It means that the extinctions, in triassic foraminifera, were produced by unfavourable environmental changes that affected all cohorts at the same moments. Finally, cohorts show a near constant extinction probabilities (Van Valen model) during periods of environmental stability (Ladinian, Norian and Carnian). In relation to the causes of extinctions, it is suggested that were produced, at least in part, by the important changes in the ocean chemistry that usually occur during the regresive moments. This hypothesis is supported by the fact that forms with aragonitic test, as Involutinina and Robertinina, are the most affected during the extinction events.