Omniastrobus gen. nov., an Emsian Plant with Implications for the Evolution of Heterospory in the Early Devonian

Premise of research. Heterospory, or the production of different spore size classes, is a key aspect of advanced vascular plant reproductive strategies. Heterospory is widespread across plant lineages, but early stages in its evolution have rarely been observed. Here we describe a new Devonian plant, Omniastrobus dawsonii, which may show an intermediate step in the emergence of heterospory.Methodology. We examined Early Devonian (early Emsian) specimens from the Campbellton Formation of New Brunswick containing impressions of strobili with numerous carbonized spore masses. We used standard paleobotanical maceration and microscopy techniques to isolate spores and characterize their size distribution.Pivotal results. Strobili in Omniastrobus are similar to those of the enigmatic order Barinophytales and consist of two rows of lateral sporangia. Unlike in other Barinophytales, however, sporangia within the strobili show a continuous gradient in average spore size, from those producing smaller spores (∼70–150 μm) to those producing larger spores (>200 μm), as well as some with a bimodal mix of sizes.Conclusions. Omniastrobus is coeval with the earliest previously known heterosporous plant, and it represents the oldest lycophyte that is not strictly homosporous. The spore size distribution of Omniastrobus is unlike that of nearly all other known plants, although similar patterns of high spore size variation are present in a few unrelated Paleozoic taxa, suggesting that this may be a common early step in the evolution of heterospory.