Spore size data for free-sporing extant and fossil plants

The evolution of different spore size classes, or heterospory, is a
fundamental reproductive innovation in land plants. The appearance of
heterospory is particularly notable during the Devonian, when most known
origins of the trait occur. Here we provide a perspective on the evolution
of heterospory during this time interval, particularly from the late Early
Devonian through the Middle Devonian (Emsian to Givetian stages; 408-383
Ma), which shows an unusually high concentration of heterospory origins.
We use theoretical considerations and compilations of fossil and extant
spore sizes to suggest that the basic features of most heterosporous
lineages, large spores and gametophytes that mature within the spore wall,
are difficult to evolve in combination because large spores disperse
poorly but small spores cannot support a functional gametophyte developing
within their walls; evolving spores between 100-200 microns in diameter
appears to represent a particularly important barrier for the evolution of
heterospory. We then discuss why this barrier may have been lower in the
Devonian, noting that the appearance and spread of heterospory is
coincident with the emergence of peat accumulating wetland habitats. We
suggest that more widespread wetland habitats would have generally lowered
barriers to the evolution of heterospory by reducing dispersal limitation
in larger spores. Ultimately, we suggest that the initial evolution of
heterospory may be explained by major changes in sedimentology, thought to
have been driven by plant evolution itself, that increased the diversity
of terrestrial depositional environments and led to a greater number of
habitats where large spores could be successful.