Fossil echinoids

Key facts and history

Echinoids have lived in the seas since the Late Ordovician, about 450 million years ago.^[1] Spines usually became detached and fossilised separately, but occasionally test and spines are preserved together.^[1] In the Mesozoic, echinoids evolved into a variety of shapes adapted to burrowing beneath the seabed.^[1] Around 450 million years ago, during the late Ordovician, the oldest echinoid fossils appear.^[2]

Fossils from the Paleozoic era are frequently fragmentary, composed of isolated spines or plate clusters, because those echinoids possessed thin tests.^[2] Rocks from the Jurassic and Cretaceous periods, particularly late Cretaceous chalk, frequently contain echinoid fossils.^[4] In southern England, the research of geologists A W Rowe and R M Brydone demonstrated that distinct echinoid types could split the Chalk into biozones and sub-biozones representing different ages.^[1] For thousands of years, people have regarded these fossils as lucky objects, associating them with deities and magical properties.^[3]

Thunderstones, fairy loaves, and shepherds' crowns are folk names for these fossils.^[5]

What the cited sources add

The BGS account notes that echinoids are marine animals belonging to the phylum Echinodermata and the class Echinoidea.^[1] It explains that calcite also forms the powerful downward-pointing teeth found in some types of living and fossil echinoids.^[1] The BGS source states that some fossil echinoids made themselves less palatable as prey by having large, solid spines.^[1] It adds that echinoids have been used in traditional remedies for stomach disorders and, in Roman times, were considered an antidote to snake bites.^[1]

In Kent, an Acheulean hand axe from the Middle Pleistocene represents the earliest documented human use of such a fossil in England.^[6]