>>2126041They are iron reduction spots. I'm sorry to disappoint you but they weren't aliens.
>'Measurements of the size and shape of the grey reduction spots and of the variation of iron content across the spot–host rock interface have been made on samples from the Enville Group. The red coloration due to Fe3+ is thought to reside mainly in the matrix material cementing the mineral grains of the rock. The reduction to Fe2+ is ascribed to carbonaceous particles included in the sediment. To account for the sharp boundary of the spots it is suggested that the outward movement of the boundary is due to concentration dependent inward diffusion of oxygen which converts the carbonaceous material to carbonate.'https://www.cambridge.org/core/journals/geological-magazine/article/on-the-mechanism-of-formation-of-reduction-spots-in-the-carboniferouspermian-red-beds-of-warwickshire/2DA422F954ECEEF15DD3D42B14927BC4 