How The Messinian Salinity Crisis And Changes To The Mediterranean Sea Impacted The Myocricetodon Gerbil

Nowadays, European and African mammals and other fauna are rather different, mainly due to the separation of these two continents by the Mediterranean Sea, which precludes the migration of terrestrial mammals between these land masses. Nevertheless, in some moments of the (more or less recent) history of the Earth, this situation changed radically.

Between six and five million years ago, one of the more impressive geological events ever known took place, the so-called Messinian Salinity Crisis. The convergence between Africa and the Iberian Peninsula (still active), together with climatic factors and changes in the sea level, led to the progressive closure of the sea corridors that connected the Atlantic Ocean and the Mediterranean Sea through the areas that currently occupy southern Spain and northern Morocco. This event culminated with the nearly complete desiccation of the Mediterranean and the sedimentation of huge amount of gypsum and salt due to the intense evaporation. This crisis finished just at the Miocene-Pliocene boundary (5.33 million years ago) with the so-called “Zanclean flood”: the Strait of Gibraltar opened, the Atlantic and the Mediterranean became connected again, and this latter sea filled up, acquiring the configuration that we can see today.

Obviously, the Messinian Salinity Crisis had terrific consequences in the aquatic fauna of the Mediterranean, which turned on a lake of variable extension and changing salinity conditions. But this event also affected the distribution of terrestrial animals, since the particular configuration of the continents during this time interval implied the occurrence of land connections between Africa and Europe, allowing the migration of continental faunas from North Africa to Spain and vice versa.

These continental corridors explain the finding of typical African mammals like camels and hippopotamus in some Spanish fossil sites of late Miocene age. In that context, not only large mammals crossed from Africa to Europe, but also small ones, particularly rodents, which constitute the more diverse and abundant mammal group at present and are also the best-represented group in the fossil record. Rodent fossils typically consist of teeth, which are the best-preserved body elements (enamel is the hardest and most resistant tissue of vertebrates) and also the most useful for identifying fossil mammals, because their morphology is much different from one species to another.

Fossil rodents with African affinities were first identified in several late Miocene Spanish localities during the 1970s. One of the most striking discoveries was that of the genus Myocricetodon, which belongs to the group of gerbils, subdesertic rodents that today only inhabit arid landscapes of northern Africa and southwestern Asia. This small rodent was found in a few fossil sites from southern and eastern Spain and southernmost France, all of them relatively close to the Mediterranean coast. For long, the provenance and the way of entry of Myocricetodon into the Iberian Peninsula was a matter of discussion: did it come from Northern Africa by a land connection close to the current Strait of Gibraltar, or did it arrive from Asia, by means of a long East-West migration through the desiccated Mediterranean?

The main problem to solve this question was the lack of a proper “candidate” for being on the origin of the Spanish populations of Myocricetodon: the Asian members of the genus were rather different from the Iberian forms, while only some of the African species of Myocricetodon known at that time resembled the Spanish fossils, but were much older (the African species M. parvus is around 6 million years older than the Spanish localities yielding remains of this genus). These doubts began to be cleared in 2009, when a team of Spanish paleontologists headed by Dr. Raef Minwer-Barakat ascribed for the first time a European sample of Myocricetodon (that from Negratín-1, in the Granada province) to an African species: Myocricetodon jaegeri, known from similar-aged fossil sites in Morocco. However, the specific determination of the rest of the European findings of Myocricetodon, as well as their origin, remained unknown.

This year, the same team, which includes researchers from different Spanish institutions (Universidad de Granada, Institut Català de Paleontologia Miquel Crusafont, Institut Català de Paleoecologia Humana I Evolució Social) studied for the first time all the material of Myocricetodon recovered from European sites so far in detail, paying particular attention to the rich sample from Almenara-M (Castellón province, Eastern Spain), consisting of up to 40 teeth. The first conclusion of this work is that all the European material of  Myocricetodon, including the populations from Almenara-M, Negratín-1, Salobreña and Pino Mojón in Spain and Castelnou-3 in France, can be confidently assigned to the species M. jaegeri, originally described from Morocco. Such an assignation confirms the African origin of Myocricetodon, which would have colonized Europe in a single migratory event from northern Africa in the context of the Messinian Salinity Crisis, probably between 5.52 and 5.33 Ma.

This work further analyzes the temporal and geographical distribution of Myocricetodon in the European continent. Its record is limited to the latest Miocene, in contrast to other African immigrants that were able to persist in Spain during the Pliocene. Moreover, its geographical range is restricted to the Mediterranean coast and the Guadix Basin, whereas it is absent in other Spanish inner basins (such as the Granada, Cabriel, and Teruel basins) despite their richness of late Miocene terrestrial mammals. In fact, in those areas where Myocricetodon is missing, latest Miocene faunas include indicators of humid conditions like beavers and water moles. All this information allows interpreting that Myocricetodon was extremely sensitive to climatic and environmental factors. Thus, this small rodent inhabited the European continent during a short interval of the arid climate, and only was able to occupy a limited area where climatic conditions were particularly warm and dry.