a, Sketch of O. giraldae, displaying the main features of the microfossil. b–h, Unornamented terminal sphere (spore). b–g, Transmitted light microscopy images that show specimens with secondary branching at a right angle (b, d–g), with terminal spheres connected together (c), with a bulbous connection (e) and with tertiary branching (d, f, g). Arrows show septate connections. Details of sample and specimen numbers have previously been published14. h, SEM image shows the compressed vesicle. Inset 1 shows the two layers of the vesicle wall, with an amorphous outer layer (AOL) and a fibrillary inner layer (FIL). Inset 2 shows the intertwined microfibrils of the microfibrillary wall structure. i, TEM images show the compressed vesicles of the bilayer wall structure in ultra-thin section, with magnification in insets 1 and 2. The outer layer (OL), the inner layer (IL) and intracellular space (ICS) are indicated in both insets. Scale bars, 20 μm (h), 200 nm (h inset 1), 300 nm (i insets 1 and 2), 400 nm (h inset 2), 1,600 nm (i), 30 µm (b–g). Images b–g are modified from a previous publication14.