The fossil record for Lepidoptera is lacking in comparison to other winged species, and tends not to be as common as some other insects in habitats that are most conducive to fossilization, such as lakes and ponds; their juvenile stage has only the head capsule as a hard part that might be preserved. Lepidopteran bodies tend to come apart after death, and decompose quickly, so fossil remains are often extremely fragmentary. Of the fossils known, only an estimated 7% have been described. The location and abundance of the most common moth species are indicative that mass migrations of moths occurred over the Palaeogene North Sea, which is why there is a serious lack of moth fossils. Yet there are fossils, some preserved in amber and some in very fine sediments. Leaf mines are also seen in fossil leaves, although the interpretation of them is tricky.