Phytogeography of Late Silurian macrofloras
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Cluster analysis and correspondence analysis of Late Silurian (Ludlovian and Pridolian) land-plant macrofossil genera and morphological traits yielded four phytogeographic units: 1) a North Laurussian unit (Bathurst Island), which lay near the palaeoequator; 2) a South Laurussian-Northwest Gondwanan unit (Great Britain, Podolia, and Bolivia), which stretched from 18 S palaeolatitude to 60-75 S palaeolatitude; 3) a Kazakhstanian unit, which probably lay north of the palaeoequator; and 4) a Northeast Gondwanan (Australian) unit, at 10 S palaeolatitude. Phytogeographic analyses based on both genera and morphological traits yielded the same phytogeographic units, suggesting that the observed differentiation does not result from regional taxonomic usage. A data set composed of Pridolian assemblages and 'range-through' assemblages (i.e. assemblages composed of taxa that crossed the Siluro-Devonian boundary in each region lacking unequivocal Pridolian [late Late Silurian] assemblages) yielded the same four phytogeographic units, as did a data set composed of range-through assemblages from all six regions having Late Silurian land-plant macrofossils. Thus, the observed phytogeographic differentiation does not result from regional differences in the age of floral assemblages. Zosterophylls (Zosterophyllum, Distichophytum and Bathurstia) characterize assemblages from the equatorial North Laurussian unit. The near-equatorial East Gondwanan (Australian) unit contains Zosterophyllum, an undescribed endemic zosterophyll genus, Hedeia, the lycopsid Baragwanathia and one additional undescribed endemic genus, which is probably a lycopsid [Tims, J.D.J., 1980. The Early Land Flora of Victoria. PhD Thesis, University of Melbourne, Melbourne, Australia.]. Thus, in the Late Silurian, zosterophylls and large lycopsids were most common in equatorial assemblages. The endemic genera Jugumella (a possible zosterophyll) and Cooksonella (a possible rhyniophyte) characterize the Kazakhstanian unit. Rhyniophytes and rhyniophytoids (Steganotheca, Pertonella, and the form genus, Tarrantia) characterize the wide-ranging South Laurussian-Northwest Gondwanan unit. This unit may reflect the ecological constraints of Late Silurian land plants. If many Late Silurian land plants belonged to ruderal species with wide dispersal capability that flourished only during the growing season, plants growing in seasonally dry and seasonally cold regions may have been quite similar. However, the morphological simplicity of Late Silurian land plants could cause assemblages from seasonally dry and seasonally cold regions to appear similar, even though they shared few biological genera. 2006 Elsevier B.V. All rights reserved.
