UCLA

The recent discovery of a fossil of Enhydritherium terraenovae in upper Miocene fluvial deposits in Juchipila (Mexico), nearly 200 km away from the nearest coast, together with other known occurrences of the same species in Florida and California, made possible to envision an alternative to the Panamanian and Polar routes of migration through fluvial systems in Mexico. In order to cross from one ocean to the other, individuals of E. terraenovae must have passed the continental divide, which is a physiographic feature that separates surface waters that flow into the Atlantic and Pacific versants. Two vast endorheic regions, which together span more than 400,000 km2 in area, currently dominate drainage systems in northern and central Mexico. The endorheic regions are broadly bounded by two mountain ranges and coincide with the arid and semi-arid regions of the Chihuahuan desert. These closed basins are an additional obstacle for migration. However, drainage systems are constantly varying and adjusting to changing conditions imposed by climate, tectonic activity, volcanism, and pronounced asymmetries in topography and rainfall distribution. The migration route across Mexico for Enhydritherium terraenovae in the late Miocene (≥6 Ma) could have been facilitated by one or more river captures that inverted the flow direction near the headwaters of a drainage system that debouched either into the Gulf of Mexico or the Pacific coast. Biologists studying fresh water fish faunas in the southern part of the United States and in northern and central Mexico have documented several living species that occur in both the Rio Grande and in the Mezquital rivers, two drainages that are not presently connected, drain in opposite directions (i.e. towards the Gulf of Mexico and the Gulf of California, respectively) and are separated by the endorheic regions. Furthermore, systematic studies of fresh water fish faunas in the region has numerous examples of endemicity and allopatric speciations that are interpreted as evidence of several events of drainage system rearrangement either by stream capture, beheading, or river diversion or by fragmentation caused by formation of closed basins by faulting, volcanic activity or isostatic rebound in the footwall block of normal faults. The western boundaries of the extensive endorheic regions in Mexico are controlled mainly by a highland produced by intense felsic volcanism during the Sierra Madre Occidental activity (Eocene – Miocene), and the eastern hydrological divide coincides with mountains associated with the fold and thrust belt produced by the Hidalgoan orogeny in early Cenozoic time. Normal faulting associated with Basin and Range extension formed additional closed basins. Another major factor in the formation of the endorheic regions is the terminal Miocene climate change, which caused runoff reduction and lowered the rivers capacity of erosion. Opening of the Gulf of California changed the overall slope of rivers draining into the Pacific in northwestern Mexico triggering the capture of rivers formerly confined to the closed basins or draining to the Gulf of Mexico. Quaternary normal faults have been documented a few tens of kilometers from the hydrological divides of the endorheic regions, suggesting that in some regions there is an active tectonic component in their formation. The location of the crossing point for the otters is uncertain as the fossil record is scant and investigation on the evolution of drainage systems through time in Mexico is in the early stages. However, it is argued that there is a fair chance that the crossing occurred via the Mezquital river capture near Durango city. Isotopic age of the sediments where the Enhydritherium terraenovae fossil was collected, imply that the capture must have occurred in the late Miocene.