The Devil's Toenails — Gryphaea

Take a close look. The strange fossil shown here is not the hoof or claw of some ancient animal. It actually belongs to a real group of bivalves that lived long ago. These organisms belong to the genus Gryphaea, commonly known as the devil's toenails, members of the family Gryphaeidae within the order Ostreida. They were bivalve molluscs that inhabited the oceans from the Triassic to the Jurassic periods.

Figure 1. Lateral and frontal views of Gryphaea arcuata（圖片來源：H. Zell，採用 CC BY-SA 3.0 授權）

Unlike their modern relatives—the oysters we are familiar with today—these animals preferred warm, shallow marine environments with sandy substrates. If one could travel back to the seas of the Mesozoic Era, the shallow seabed would likely appear dotted with many Gryphaea individuals partially buried in sand, quietly feeding on the abundant plankton and organic matter carried by the water.

In contrast to most typical bivalves, the shells of Gryphaea were strongly asymmetrical. During growth, one side of the mantle secreted calcium carbonate and nacre much more rapidly and continuously than the other side. This uneven growth pattern caused one valve to accumulate in a gradually coiling form, while the opposite valve remained comparatively flatter. Over time, this process produced the distinctive curved shell that resembles a twisted claw or fingernail.

Figure 2. An eighteenth-century scientific illustration depicting Gryphaea, currently preserved in the British Library（圖片來源：British Library，CC0 1.0 公共領域）

What purpose did such an unusual shell shape serve? Paleontologists believe the asymmetrical shell was closely related to the animal's lifestyle. As mentioned earlier, Gryphaea lived in soft sandy sediments. However, their growth pattern remained similar to that of oysters, which grow asymmetrically and typically anchor themselves to solid objects. This created a potential problem. Sandy substrates are unstable compared with hard rock surfaces. When waves became energetic, large amounts of sand could be swept away, posing a serious risk to organisms that relied on anchoring themselves.

To cope with this challenge, Gryphaea evolved a distinctive shell architecture: a deeply curved left valve paired with a flatter right valve. This structure helped stabilize the animal in shifting sandy environments. The curved valve functioned somewhat like an anchor, while the overall shell shape reduced the likelihood that waves would dislodge the animal from the substrate.

Figure 3. Fossil specimens of two different species of Gryphaea (Gryphaea arcuata, Gryphaea dilatata) photographed by the author。現藏於於臺灣大學地質標本館，感謝臺灣大學地質系提供拍攝。

Because of their unusual appearance, these fossils captured human imagination long after the animals themselves had vanished. In medieval England, people believed that Gryphaea fossils were the toenails of the devil. They were often carried as protective charms, thought to prevent ailments such as rheumatism or pain from horseback riding. The folklorist Oakley (1974) later suggested that this belief may have arisen because the twisted shape of the fossil shells resembled deformed joints, making them symbolically associated with joint diseases.

Whether or not such interpretations are correct, the peculiar form of Gryphaea certainly adds a distinctive chapter to the history of life on Earth. From the sandy seabeds of ancient oceans to the folklore of medieval Europe, these remarkable fossils preserve more than biological evolution. They also carry traces of human imagination and cultural interpretation, forming a bridge between natural history and the stories people have told about the past.

Author: Rodrigo

References:

1. Hallam, A. (1960). "A quantitative analysis of the evolution of Gryphaea." Journal of Paleontology, 34(3), 497-509.

2. Bottjer, D. J., & Carter, J. G. (1980). Functional and phylogenetic significance of projecting periostracal structures in the Bivalvia (Mollusca). Journal of Paleontology, 54(1), 200–216.