H alffter's M exican transition zone (1962–2014), cenocrons and evolutionary biogeography

The M exican transition zone is the complex and varied area in which the N eotropical and N earctic biotas overlap. In a series of contributions, G onzalo H alffter provided a coherent theory that explains how sets of taxa that evolved in different geographical areas assembled in this transition zone. H alffter's theory developed gradually, being refined and clarified in successive contributions from him and other authors. After a review of the historical development of the M exican transition zone, including the characterization of the dispersal or distributional patterns recognized by H alffter, its relevance for evolutionary biogeography is discussed briefly. The M exican transition zone in the strict sense includes the highlands of M exico and G uatemala ( S ierra M adre O ccidental, S ierra M adre O riental, T ransmexican V olcanic B elt, S ierra M adre del S ur and C hiapas H ighlands provinces), whereas northern M exico and the southern U nited S tates are clearly N earctic, and the lowlands of southern M exico and C entral A merica are clearly N eotropical. The distributional patterns recognized by H alffter are considered to represent cenocrons (sets of taxa that share the same biogeographical history, constituting identifiable subsets within a biota by their common biotic origin and evolutionary history). The development of the M exican transition zone is summarized into the following stages: (1) J urassic– C retaceous: the four P aleoamerican cenocrons extend in M exico; (2) L ate C retaceous– P alaeocene: dispersal from S outh A merica of the P lateau cenocron; (3) O ligocene– M iocene: dispersal from the C entral A merican N ucleus of the M ountain M esoamerican cenocron; (4) Miocene–Pliocene: dispersal from N orth A merica of the N earctic cenocron; and (5) P leistocene: dispersal from S outh A merica of the Typical N eotropical cenocron.