Morphology and wall ultrastructure of the Devonian spore Acinosporites macrospinosus Richardson 1965 and its bearing on the origin of the megaspore apical prominence

The spore Acinosporites macrospinosus Richardson 1965 is common in exceptionally well-preserved dispersed spore assemblages recovered from the Middle Devonian (Eifelian) Middle Old Red Sandstone deposits in the Orcadian Basin, Scotland. This article reports on a detailed LM, SEM, and TEM analysis of these spores. The spores are large and spinose with an apical prominence associated with the trilete mark. TEM analysis reveals that the wall consists of four layers that were all formed by white-line-centered lamellae that develop into laminae. The spore wall ultrastructure is most similar to that of extant and fossil lycopsids, and a lycopsid affinity is proposed for the parent plant. On the basis of this interpretation, a mechanism for spore wall development is presented. The nature and ecology of the parent lycopsid plant are unclear. However, on the basis of similarities to younger bona fide megaspores, it is suggested that A. macrospinosus was probably an incipient megaspore produced by one of the first groups of lycopsids to have experimented with a heterosporous reproductive strategy. This is the earliest report of an apical prominence on a lycopsid spore, a feature that went on to characterize many later lycopsid megaspores, leading to the development of extensive gulas and massas.