ABSTRACT. A floral gradient for the Early Mesozoic has been reconstructed from localities that ranged from the subtropics to the polar region of the northern hemisphere, and encompassed climates interpreted as ranging from the warm dry subtropical to the cool wet temperate regime. Our previous ordination studies on the floras demonstrated a gradual replacement of morphological types; from coniferophytes and cycadophytes with thick cuticles and small leaves in low latitudes, through broader-leaved forms of cycadophytes with filicopsids, to broad-leaved deciduous ginkgophytes and coniferophytes in near polar positions. Parallels with the Recent tropical and subtropical distributions of the cycads, and with the late Cenozoic temperate distribution of Ginkgo may be drawn.
Floral lists were assembled from eight exceptionally well sampled regions in Northern Eurasia ranging in age from Late Triassic through Late Jurassic, and were used to determine the correlation of the floral gradient with paleolatitude. Floral lists were also assembled from basins associated with the Chinese microcontinents of South China, North China and Tarim, which were converging with Northern Eurasia at the time. Their positions are therefore less well known, and our purpose is to show that the floral gradient is sensitive enough to be used as a check on the tectonic and paleomagnetic based reconstructions currently available.
South and North China were in the warm temperate zone in the Late Triassic and Early Jurassic and collision with the southward moving Eurasia was complete by the Late Jurassic. During the Jurassic, the complex was moving equatorward into the dry subtropical zone. These conclusions accord with current tectonic interpretations, but the available paleomagnetic data seem to underestimate significantly the paleolatitude of these blocks during some time intervals.