Predictable structure of Miocene insectivore (Lipotyphla) faunas in Western Europe along a latitudinal gradient

The insectivores (Grandorder Lipotyphla) have been seldom used in mammal-based paleoenvironmental inferences because this group is generally considered insufficiently represented or studied. Nevertheless, a recent compilation on the insectivore record of Western Europe has shown that this is not the case. From the insectivore occurrences themselves a major paleobiogeographical pattern seems to be present: the faunas of the Mediterranean regions of Western Europe are poorer and seem to be a subset of those occurring at higher latitudes. This kind of pattern, where smaller assemblages seem to be a ‘nested subset’ of the richer ones, is known as nestedness. In this work we analyze the prevalence of nestedness in the Miocene insectivore record of western Europe. The calculations are carried separately for each MN (Mammal Neogene) zone and a measure of nestedness or order is given. The probability that a similarly nested pattern could be obtained at random for each MN is assessed by permutation. Our results show the prevalence of nestedness in all the MN zones except the MN 1. In the second step, we study the causes of nestedness. A significant correlation of the nested structure with latitude is found in 8 out of 12 matrices, thus confirming that the southern assemblages are a subset of the northern ones. This must be related to some underlying latitudinal gradient that already existed during the Miocene. Latitudinal gradients in temperature seem to have been unimportant until the Late Miocene yet the nested pattern existed as early as the Early Miocene. A precipitation gradient, which was already present by the Early Miocene, seems to be the most important structuring factor. Independent evidence has confirmed that the Central European regions were more humid than the Mediterranean ones thus explaining the higher diversity of insectivores in that area as this group is more diverse in humid climates. The structure of the southern assemblages is a highly predictable subset of that of the northern ones. In the south the species that seem to have required higher levels of environmental moisture are missing. This applies for instance to many talpids and dimylids which never reached the Iberian Peninsula.