NONMARINE PALYNOLOGY OF THE ARISAIG GROUP

The main emphasis of the Arisaig study was on nonmarine palynology. "Sporomorphs" were divided into plant spores and problematic cryptospores. Banded tubes and cuticles (but no whole plants) were also recovered, but not formally described. Taken together, the variation in structural and sculptural details and the consecution of such events in time provides a detailed look at the primary adaptive radiation of embryophytes to a terrestrial lifestyle in Western Avalonia. This radiation, which resembles simultaneous events in east Avalonia (Anglo-Welsh Basin) and east Laurentia (preliminary work at Allenport, Pennsylvania), begins in the Homerian (425 Ma) when the number of sporomorph species approximately doubles to eleven (see adjacent chart). New forms are added continuously up through the middle Pridoli (410 Ma), with the total number of species identified in all samples approaching forty. The Ludfordian drop-off in diversity at Arisaig and in the Anglo-Welsh Basin is attributed to sampling bias and relatively deeper water deposits, respectively. Likewise,the higher species counts during the Homerian at Allenport and in the Anglo-Welsh Basin is attributable to relative depositional conditions.

A "range-through" plotting method is used to show the distribution of spores and cryptospores in the Arisaig Group.

Photomicrographs of all the spores and cryptospores observed in the Arisaig Group are presented in Plates 1 to 11. Eight(?) new species have been found in the Arisaig Group.

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