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The trans-Pacific zipper effect: disjunct sister taxa and matching geological outlines that link the Pacific margins


*Dennis McCarthy, 21 Kiwanis Beach Road, Upton, MA 01568, USA. E-mail:


Aim To combine analyses of trans-Pacific sister taxa with geological evidence in order to test the hypothesis of the existence of a Panthalassa superocean.

Location The study is concerned with taxa, both fossil and extant, from East Asia, Australia, New Zealand, South America and North America.

Methods Phylogenetic and distributional analyses of trans-Pacific biota were integrated with geological evidence from the Pacific and circum-Pacific regions.

Results A series of recent biogeographical analyses delineates a zipper-like system of sister areas running up both margins of the Pacific, with each section of western North and South America corresponding to a particular section from East Asia/Australia/New Zealand. These sister areas coincide neatly with a jigsaw-like fit provided by the matching Mesozoic coastlines that bracket the Pacific.

Main conclusions The young age (<200 Myr) of oceanic crust, the matching Mesozoic circum-Pacific outlines, and a corresponding system of interlocking biogeographical sister areas provide three independent avenues of support for a closed Pacific in the Upper Triassic–Lower Jurassic. The hypothesis of the existence and subsequent subduction of the pre-Pacific superocean Panthalassa is not only unnecessary, it conflicts with this evidence. Panthalassa-based paleomaps necessitate the invention of dozens of additional hypotheses of species-dependent, trans-oceanic dispersal events, often involving narrow-range taxa of notoriously limited vagility, in order to explain repeated examples of the same biogeographical pattern. Removing the vanished-superocean hypothesis reunites both the matching geological outlines and all the disjunct sister taxa. In brief, what appears to be a multi-era tangle of convoluted, trans-oceanic distributions on Panthalassa-based paleomaps is actually a relatively simple biogeographical pattern that is explainable by a single vicariant event: the opening and expansion of the Pacific.