Fossil hunters occasionally chance upon small glossy red to amber-coloured, roughly circular objects on bedding planes, when they crack open shales that were deposited in ancient swamps and rivers. These curious fossils range from about a millimetre in diameter up to the size of a fingernail (Fig. 1). When well preserved, they are egg shaped, but, in most cases, they have been flattened to a thin flake in the rock by the weight of the overlying strata. Some specimens appear to have a net-like coating on the surface but, otherwise, they offer few clues as to their identity. Indeed palaeontologists have reported these objects for over 150 years and have variously interpreted them 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.
These strange objects occur mostly in sediments deposited in continental settings, and they have been reported widely in the residues left over after palaeobotanists have dissolved rock samples in the search for fossil spores, pollen and leaf cuticle. Two conclusions can be gleaned from these occurrences:
- The mysterious fossils likely belong to a land- or freshwater-based organism; and
- These objects are composed of a material that is extraordinarily resistant, even to some of the strongest acids used in the laboratory.
The mystery of their origin was finally solved in 1991, when Norwegian palaeontologist Svein Manum and his colleagues recognised that these fossils match the characteristics of the egg-bearing cocoons of modern leeches and their relatives. Our further investigations have revealed that the link to leeches had been identified by the German naturalist, F Gergens, all the way back in 1861, but his report was largely overlooked by later researchers, probably because it was published as a brief communication in the German language. These strange fossils are now known to have a record extending back to the Triassic, but DNA-dating of modern cocoon-producing annelids (earthworms, leeches and their relatives) suggests that this group has an even earlier origin – perhaps in the early Palaeozoic.
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Stephen McLoughlin, Benjamin Bomfleur and Thomas Mörs