ULTRASTRUCTURE OF TRANSITIONAL DISPERSED MEGASPORES FROM THE MIDDLE DEVONIAN OF NEW YORK

摘要

The Ashokan Formation of New York contains a rich assemblage of Middle Devonian spores whose average size straddles the traditional threshold that defines the megaspore (200 μm). This assemblage provides the opportunity to examine how spore wall ultrastructure changed during the evolution of the heterosporous reproductive syndrome. The spores range in diameter from 139 to 220 μm (average, 177 μm), lack proximal sculpture except for ～10% nearest the equator, and bear spines distally. When viewed with the TEM, wall thicknesses range from 4 to 9 μm. The outer half of the wall appears homogeneous. In several cases, there is evidence for an inner separable layer (ISL) and associated laminae, like those seen in many extant lycophyte megaspores. Though near the diameter cutoff that defines a megaspore, there is little evidence of extensive laminae or sponginess (in the exospore that lies outside of the ISL) so common among both fossil (coeval and younger) and extant megaspores. It is possible that the plant(s) that produced these spores did not possess the developmental mechanisms to construct spore walls capable of expanding to sizes well above 200 μm as some other plants of a similar age did. Alternatively, perhaps early-evolving large spores that did not produce a microspore did not require a laminate/granular/spongy outer exospore, either because of size requirements or for some other undiscovered reason.