Swallows and butterflies
Brachiopods are marine animals with shells resembling molluscs but which have a very different internal anatomy. Extremely diverse in the geological past, brachiopods are among the most common of all fossils. In some fossil species the shells are extended at the sides into a pair of wing-like structures.
In China such brachiopods are known as 'Shih-yen or stone swallows. The 5th century Chinese scholar Li Tao-Yuan recorded that during thunderstorms the stone swallows flew about as if they were real swallows (Casanova 1983). A similar myth relating brachiopods to birds also once existed in the European Alps where certain species are known as 'Little Doves (Oakley 1965).
The Devonian slates formerly quarried at Delabole in Cornwall, England, contain a brachiopod closely related to those from China. Here, however, the quarrymen of times past likened the fossils to butterflies, hence the name Delabole Butterfly.
Another kind of butterfly stone consists of the tails of the Cambrian trilobite Drepanura. These Chinese fossils, known as Hu-die-shih, are found in Shantung Province.