Mediterranean mammals migrated prior to the Messinian Salinity Crisis

by Jeffrey Knox
Friday, July 19, 2013

The people of the Iberian Peninsula and North Africa share a long and complicated history, evident in culinary and genetic similarities, due in large part to their close proximity. Now it appears that the animals of the region have shared an even longer history. Researchers studying mammal fossils in Spain and Morocco recently determined that a migration event between the Iberian Peninsula and North Africa occurred more than 6 million years ago — more than half a million years earlier than previously thought.

The fossils include paracamelus (an ancestor of modern camels), certain species of rodents and canines, and other mammals, all of which were endemic to North Africa but can be found at a handful of sites in Spain. Likewise, European immigrants were also found in Morocco, indicating a two-way exchange.

Past paleontological studies had dated the fossils to 5.6 million years ago, suggesting they lived during the time of the Messinian Salinity Crisis (MSC), when low sea levels cut off the connection between the Mediterranean Sea and the Atlantic Ocean. The subsequent desiccation of the Mediterranean lasted about 300,000 years, until 5.33 million years ago when the Zanclean Flood refilled the empty sea in a matter of years. Because of the previous dating of these fossils, scientists have long thought that a singular migration event — in which animals freely moved between Iberia and North Africa across the land bridge spanning the modern Strait of Gibraltar — occurred during the MSC.

But in a recent study in Geology, Luis Gibert of the University of Barcelona and his colleagues redated the fossils from sites in southeastern Spain and northern Morocco using a variety of stratigraphic, paleontologic and paleomagnetic techniques, and found them to be significantly older.

Through paleomagnetic analysis of the sedimentary strata in which the fossils were found, the team identified the orientation of the iron-rich magnetic grains, which align themselves with the direction of the contemporaneous magnetic field during deposition. Using the Geomagnetic Polarity Time Scale (GPTS), which is constructed from analysis of anomalies in Earth’s magnetic field recorded in independently dated ocean sediments, and a recalibrated Astronomically Tuned Neogene Time Scale (ATNTS), which links cyclic variation in climate records to changes in solar radiation associated with well-known cycles in Earth’s orbit, the researchers were able to date the fossil samples with unprecedented precision.

All of the study sites in Spain and Morocco yielded concordant ages of 6.23 million years ago. Thus, the researchers concluded that the mammal fossils must be associated with a separate and distinct migration event that took place more than half a million years prior to the onset of the MSC. Gibert suggests the animals were able to cross because the onset of Miocene glaciations lowered sea level and caused the closure of the Betic seaway — a passage between Europe and a group of islands that would eventually form the Spanish side of Gibraltar — which created an ephemeral land bridge about 6.26 million years ago.

Pablo Peláez-Campomanes, a paleobiologist with the National Museum of Natural History in Madrid who was not involved with the study, says that the study’s “conclusions are appropriate and based on well-verified dates.” The improved magnetostratigraphy has “significant paleobiogeographic and evolutionary implications because it sets the foundation for precise correlations between periMediterranean areas,” especially “when dating the phases of faunal dispersion,” he says.

The team’s findings have certainly created a “more coherent sequence of events for the paleobiologic evolution of the western Mediterranean,” says co-author Jorge Morales, also of the National Museum of Natural History. But knowledge of Mediterranean fauna from this time period is still limited, notes co-author Plinio Montoya Bello, a paleontologist at the University of Valencia. Next, he says, the team will focus on other sites in Spain to gather information on this newly discovered migration event and how it relates to the MSC.

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