The palaeolatitudinal distribution of fossil wood genera as a proxy for European Jurassic terrestrial climate
Résumé
The potential of using the latitudinal distributions of fossil wood genera as a proxy for Jurassic terrestrial climate conditions in western Laurasia was tested using a database of 386 entries. The fossil wood genera Brachyoxylon, Protaxodioxylon and Simplicioxylon are southern taxa, which have only occasionally extended north of 45°N (all latitude references are to palaeolatitudes). Conversely, Protocedroxylon and Xenoxylon are mostly found at high latitudes. The standard deviation of the data on the palaeolatitudes of the northern genera is, however, much larger than that of the southern genera. This is interpreted as resulting from brief periodic southward extensions of the Protocedroxylon and Xenoxylon distributions, while the southern genera remained confined south of 40°N. The interval during which the northern genera replaced the southern genera at mid-latitudes can be hypothesized to be times of global terrestrial climate cooling or increased rainfall. Comparisons between the spatiotemporal distributions of these ‘northern’ taxa and independent climate-sensitive indicators (clay mineral assemblages, oxygen isotopes, and neritic carbonate distribution) suggest that these genera were adapted to more humid conditions characterizing the mid-latitudes of the Northern Hemisphere. Accordingly, southward migrations of this climatic zone occurred during the Early and Late Pliensbachian, Early Toarcian, and Early Middle Oxfordian time intervals. These reconstructions are limited by the low resolution and restricted spatial coverage of the wood record. Therefore, further sampling at the ammonite zone level is needed. Nevertheless, the present study suggests that wood taxa distribution represents an underexploited but potentially powerful record of terrestrial environmental changes during the Jurassic Period.