Curious fossils in continental sedimentary strata that range from about a millimetre in diameter up to the size of a fingernail and appear to have a net-like coating on the surface have reported for over 150 years and have been variously interpreted as the eggs of insects, parts of lichens, the food-catching devices of ancient invertebrates, the membranous coatings of seeds, or the linings of clubmoss sporangia. Many early palaeobiologists simply labelled them as ‘red eggs’ and avoided assigning them to any particular biological group. However, these fossils match the characteristics of the egg-bearing cocoons of modern leeches and their relatives. During cocoon secretion, micro-organisms from the surrounding environment can become entrapped and entombed in the sticky threads of the cocoon wall, thus escaping decay, and ultimately becoming part of the fossil record.