Archaeocidaris sp.

Archaeocidaris is an extinct genus of primitive sea urchins (echinoids) that lived from the Late Devonian through to the end of the Permian period. It belongs to the phylum Echinodermata, class Echinoidea, and is one of the earliest well-documented representatives of the echinoids (sea urchins). Members of the genus Archaeocidaris are characterised by their rigid, globular tests (skeletons) and long, robust spines, which make them distinctive in the fossil record. They also have many backwards-facing barbs along their spines that make them quite recognisable.

The genus was particularly widespread during the late Palaeozoic, with fossils found in marine limestones across Europe, North America, and Asia. Archaeocidaris was among the dominant echinoids, thriving in shallow, warm, carbonate-rich environments alongside crinoids, brachiopods, and corals. These habitats provided abundant surfaces for attachment and feeding opportunities within reefal and near-reefal ecosystems.

Morphologically, Archaeocidaris possessed a spherical to slightly flattened test constructed from tightly interlocking plates. The most striking feature of this genus is its spines, which could be very long relative to body size with backwards-facing barbs. These spines were likely used for defence against predators, as well as for stabilisation on soft substrates. Unlike many modern sea urchins, which exhibit varied modes of locomotion and feeding, Archaeocidaris is thought to have been primarily a grazer or detritivore, feeding on organic material and microbial films coating seafloor sediments.

As an early echinoid, Archaeocidaris retains primitive features compared to its later relatives. Its arrangement of plates and tubercles reflects a simpler structure, placing it within the order Cidaroida, a group often referred to as the “pencil urchins.” Remarkably, cidaroids are one of the few groups of echinoids to have persisted to the present day, with modern cidaroids still inhabiting tropical and subtropical seas.

Fossils of Archaeocidaris are often preserved as disarticulated plates and spines, which are commonly found in Carboniferous limestone deposits. Complete tests are much rarer. The durable spines are especially frequent in the fossil record and are sometimes mistaken for other skeletal elements.

In terms of evolutionary significance, Archaeocidaris illustrates the early diversification of echinoids during the Palaeozoic. Whereas other early echinoid groups went extinct, cidaroids successfully survived multiple mass extinction events, including the devastating Permian–Triassic extinction.