Laminae in palynomorph walls from the Middle Cambrian–Early Devonian

This study was undertaken to examine the wall ultrastructure of widely dispersed, mainly lower Paleozoic palynomorphs/cryptospores. Many of these palynomorphs have walls that are at least partially composed of laminae. The walls and their constituent laminae are variable in their construction, but show certain similarities to one another and to some extant land plants (e.g., sphaerocarpalean liverworts). Careful examination with transmission electron microscopy reveals that: 1) Cambrian–Early Devonian laminate palynomorphs have laminae that are fairly uniform in their thickness, 2) most contemporaneous acritarchs have walls that are thicker than individual laminae in these putatively land-derived palynomorphs and cryptospores, 3) extant algae have lamellae, not laminae, in their cyst walls, and in smaller numbers than the laminae in these palynomorphs/cryptospores, and 4) extant liverworts have laminae whose thickness overlaps that of these palynomorphs/cryptospores, but also have some laminae that are much thicker. The antiquity of these palynomorphs clearly establishes the plesiomorphic state of laminate wall construction among land plants. While it is not possible to assign the producers of these palynomorphs to any specific group of organisms based on the ultrastructure of the wall, they clearly produced spores with thicker, more complex walls than any fossil or extant alga known to date.