Going south : latitudinal change in mammalian biodiversity in Miocene Eurasia

For palaeontologists, the challenge is to reconstruct biodiversity patterns of the past. Mammal richness in grids is used to assess the stability of biodiversity hotspots and document changes over time in Europe for Mammal Neogene units 3 to 11 (19.5 to 7.6 Ma), early to late Miocene. The maps clearly show the patchiness of the fossil record. As the Miocene was an eventful epoch with severe environmental changes, Europe slowly became drier, and more seasonal, both in temperature and precipitation. From the early to middle Miocene an area of high biodiversity moved from higher to lower latitudes, culminating in one of the most remarkable hotspots in the history of mammals: the early late Miocene (Vallesian mammal stage) faunas from the Vallès-Penedès (Catalonia, Spain). Remarkably, the surrounding areas did not exhibit similar richness. During the subsequent Vallesian turnover event (~ 9.7 Ma), the large and small mammal distribution became more equitable and the hotspots less prominent. The richest area was found in the periphery of the humid Miocene ecosystem, which experienced species influx from the drier south. The southward shift was a result of the expansion of the humid area with subsequent closed environments and related mixing of ecosystems, coming to a halt in the late Miocene, when all of Europe became equally open

The range and extent of the Vallesian Crisis (Late Miocene) : new prospects based on the micromammal record from the Vallès-Penedès basin (Catalonia, Spain)

The Vallesian Crisis was initially recognized as a local event, which implied the extinction of certain rodent and artiodactyl genera coinciding with the early/late Vallesian boundary (at 9.7 Ma). Following works increased the range and extent of this event to encompass all Europe and involve a great number of mammal taxa. Here, we analyze the Vallesian rodent and insectivore record of the Vallès-Penedès basin (Catalonia, Spain), where the crisis was first recognized. We show that the quality of the record before the crisis is comparatively much better than afterwards so diversity appears inflated and extinction rates are overrated. Accordingly, we used inferred taxon ranges and rarefaction to calculate new diversity measures independent of sample size. These measures virtually eliminate the Vallesian Crisis, showing that diversity somewhat decreased during the earliest late Vallesian and soon recovered afterwards. This is because it cannot be discarded that several rare taxa, customarily said to have disappeared during the crisis, are in fact present. Amongst the rodents and insectivores, these taxa include genera that are generally rare and show a discontinuous record during the early Vallesian. These are presumed specialists adapted to humid forested environments such as flying squirrels, beavers or certain dormice, most of them being only recorded when the sample size is large enough. Alternatively, these genera may have been associated to very specific habitats which, for an unknown reason, are not sampled during the late Vallesian. Our results cast serious doubts on the very existence of the Vallesian Crisis suggesting that rather than an abrupt event a series of extinctions occurred during a longer time span. It has not been evaluated whether the same pattern is observed in the case of large mammals and in other areas. However, our results show that biases introduced by the quality of the record need to be taken into account when assessing the extent of the event.La Crisis Vallesiense fue inicialmente definida como un fenómeno local que implicó la desaparición de ciertos géneros de roedores y artiodáctilos coincidiendo con el límite entre Vallesiense inferior y superior (hace 9.7 Ma). Trabajos posteriores ampliaron el ámbito y alcance de este evento hasta incluir toda Europa e implicar un gran número de taxones de mamíferos. En este trabajo analizamos el registro Vallesiense de roedores e insectívoros de la cuenca del Vallès-Penedès (Cataluña, España), donde la crisis fue reconocida por primera vez. Se muestra que la calidad del registro fósil con anterioridad a la crisis es comparativamente mucho mejor que la de después de modo que se magnifica la diversidad y se exageran las tasas de extinción. Por lo tanto, se ha utilizado rarefacción y se han inferido rangos estratigráficos a fin de calcular nuevas medidas de diversidad independientes del tamaño de la muestra. Estas medidas eliminan virtualmente la Crisis Vallesiense, mostrando que la diversidad disminuyó a principios del Vallesiense superior para recuperarse poco después a finales de esta edad. Esto es debido a que no podemos descartar que diversos taxones poco abundantes, normalmente citados como víctimas de la crisis, estuvieran de hecho presentes. Entre los roedores e insectívoros dichos taxones incluyen géneros que son generalmente raros y que muestran un registro discontinuo durante el Vallesiense inferior. Se trata de supuestos especialistas adaptados a ambientes boscosos húmedos tales como ardillas voladoras, castores o ciertos lirones, la mayoría de los cuales sólo se encuentran cuando el tamaño de la muestra es lo suficientemente grande. Alternativamente, estos géneros podrían haber estado asociados a hábitats muy específicos que por algún motivo no están siendo muestreados durante el Vallesiense superior. Nuestros resultados cuestionan seriamente la existencia de la Crisis Vallesiense y sugieren que en lugar de un evento abrupto podría tratarse de una serie de extinciones que habrían sucedido durante un período más largo. Queda pendiente de evaluación si el mismo patrón se observa en el caso de los grandes mamíferos y en otras áreas. No obstante nuestros resultados muestran que se deberían tomar en consideración las desviaciones introducidas por la calidad del registro al analizar el alcance de este evento

Going south : latitudinal change in mammalian biodiversity in Miocene Eurasia

For palaeontologists, the challenge is to reconstruct biodiversity patterns of the past. Mammal richness in grids is used to assess the stability of biodiversity hotspots and document changes over time in Europe for Mammal Neogene units 3 to 11 (19.5 to 7.6 Ma), early to late Miocene. The maps clearly show the patchiness of the fossil record. As the Miocene was an eventful epoch with severe environmental changes, Europe slowly became drier, and more seasonal, both in temperature and precipitation. From the early to middle Miocene an area of high biodiversity moved from higher to lower latitudes, culminating in one of the most remarkable hotspots in the history of mammals: the early late Miocene (Vallesian mammal stage) faunas from the Vallès-Penedès (Catalonia, Spain). Remarkably, the surrounding areas did not exhibit similar richness. During the subsequent Vallesian turnover event (~ 9.7 Ma), the large and small mammal distribution became more equitable and the hotspots less prominent. The richest area was found in the periphery of the humid Miocene ecosystem, which experienced species influx from the drier south. The southward shift was a result of the expansion of the humid area with subsequent closed environments and related mixing of ecosystems, coming to a halt in the late Miocene, when all of Europe became equally open