Several million years ago, a land bridge between the two American continents started to emerge. The appearance and final closure of this Isthmus resulted in a terrestrial connection of North- and South America and the separation of the western Atlantic and eastern Pacific oceans. The emergence of the Isthmus had considerable consequences on oceanographic, environmental, and faunistic conditions on a global as well as on a regional scale. Recently conducted studies challenge the widely accepted assumption that the rise of the Isthmus and its final closure occurred in Late Pliocene time (i.e. around 3 4 million years ago (Ma) with potential breaching of the Isthmus until about 1.8 Ma; common Pliocene model ) and allocate this event much earlier, at around 15 Ma ( new Miocene model ). Due to the emergence and closure of the Isthmus, transisthmian sister species (TSS) originated. TSS are defined as species that have diverged due to the closure of the Isthmus and are each other s closest relatives on opposite sides of the barrier. However, the TSS concept (i.e. the definition of the term TSS and the fulfillment of five criteria regarding biogeographic distributions, morphological similarities, and molecular characteristics) is often inconsistently used in biogeographical research. Consequently, some studies suffer from an ambiguous and confusing TSS terminology, as well as misidentified TSS pairs. However, TSS pairs and the controversially discussed closure of the Isthmus of Panama play, among others, a key role in molecular clock calibrations. The inconsistency of the TSS concept, the complex and long lasting geological history of the Isthmus itself, as well as difficulties in molecular clock approaches may be the reasons why previously estimated divergence times for TSS pairs are not conclusive as to the time of final Isthmus closure.
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