The Chicanery of the Isthmian Links Model

Two papers, ‘Gondwana land bridges’ by Charles Schuchert and ‘Isthmian links’ by Bailey Willis, were published together in 1932. They were apparently motivated by Schuchert's desire to defend his paleogeography of fixed continents against the threat of Alfred Wegener's continental mobilism. Schuchert and Willis both held to land-bridge theory but admitted that they could not accept each other's types of bridges. Schuchert insisted that some bridges had to be wide and of continental material, without explaining why he felt this was so. Willis insisted that wide continental bridges were isostatically and volumetrically impossible; so any ancient bridges that had sunk must have been narrow isthmuses of dense oceanic rocks. They wrote separate papers, but issued together, perhaps to lead readers to the impression that a compromise was possible; but it was not. They avoided alerting readers to fatal flaws in both their models, in part by limiting their discussion to the less familiar southern hemisphere (Gondwana) and never mentioning the continental connection between Europe and North America. Willis went further in his inventions than Schuchert, trying to explain the extremes of Permian climate. Fixed-continent paleogeography required glacial conditions at equatorial latitudes and tropical conditions at arctic latitudes. We now understand that these climate differences can only be explained by ‘continental drift’ (or plate tectonics), but in his valiant effort to support fixism, Willis postulated not only tectonic uplifts of oceanic isthmuses, but also uplifts in continental areas that were known to be stable.