PHYTOGEOGRAPHY AND LATITUDINAL DIVERSITY GRADIENTS OF LATE SILURIAN LAND PLANTS

Late Silurian land plants have been found on three paleocontinents (Gondwana, Kazakhstan and Laurussia) and over a wide paleolatitudinal range (from Bathurst Island, which lay near the paleoequator, to Bolivia, which lay close to 60^oS). Nonetheless, the small number of terrestrial macrofloral assemblages and their low diversity complicate the study of Late Silurian phytogeography. Late Silurian land-plant assemblages span three geological stages (Wenlock, Ludlow and Pridoli) and 12 m.y. This interval is long enough for significant plant migration triggered by the appearance and spread of new taxa or by paleoclimatic change, which could blur phytogeographic boundaries. Although the ideal phytogeographic dataset would be restricted to one stage, no single Late Silurian stage has macrofloral assemblages from all key regions.

Cluster and reciprocal averaging analysis of Ludlovian and Pridolian macrofloral genera and morphological traits revealed four phytogeographic units: near-equatorial Laurussia (Bathurst Island), characterized by unique zosterophylls; near-equatorial Gondwana (Australia), characterized by unique zosterophylls and lycopsids; subtropical Kazakhstan characterized by unique rhyniophytes; and a wide-ranging unit composed of assemblages from subtropical Laurussia (Great Britain and Podolia) and temperate Gondwana (Bolivia), characterized by unique and diverse rhyniophytes. Blurring of phytogeographic boundaries due to plant migration apparently does not affect this pattern. A probable 'end-Silurian' phytogeographic data set derived using early Late Silurian (Ludlow) and earliest Devonian (Lochkovian) macrofloral assemblages from each region to determine the composition of the end-Silurian flora of that region yielded the same phytogeographic units.

In these data, sampling intensity may control taxonomic diversity: well-studied assemblages from subtropical Laurussia have higher generic diversity than less studied assemblages from near-equatorial Laurussia or Australia (8 vs 6 genera). However, near-equatorial assemblages have a higher diversity of commonly occurring morphological traits tied to reproduction than do subtropical and temperate assemblages, suggesting greater reproductive complexity for Late Silurian equatorial floras.