Crinoids are marine animals, not plants, though their appearance has earned them the common names “sea lilies” and “feather stars.” Their bodies consist of a cup-like structure called a calyx which houses their organs, a set of feathery arms for filter-feeding, and often a stalk for attachment to the seafloor. First appearing hundreds of millions of years ago, crinoids became so widespread in Paleozoic Era oceans that some limestone deposits are composed almost entirely of their fossilized remains.
Early Paleozoic Crises
The long history of crinoids includes several encounters with extinction events, starting with crises in the Paleozoic Era that reshaped their populations. During the Ordovician-Silurian extinction event (440-450 million years ago), widespread glaciation caused sea levels to drop dramatically. This shift severely impacted the shallow marine habitats where many crinoid species lived.
The Late Devonian extinction was another challenge, representing a series of environmental changes rather than a single catastrophe. These changes included periods of widespread anoxia, where ocean waters became depleted of oxygen, which was detrimental to most marine life. Although many crinoid families did not survive these Paleozoic crises, the group as a whole proved resilient, recovering and diversifying after each event.
The Great Dying
The most severe test for crinoids came with the Permian-Triassic extinction about 252 million years ago. Known as “The Great Dying,” this was the most catastrophic mass extinction in Earth’s history, eliminating the vast majority of marine species and nearly wiping out crinoids entirely. The fossil record shows a dramatic drop in their diversity, bringing the group to the brink of total extinction.
The likely cause was immense volcanic eruptions in Siberia, which released massive amounts of greenhouse gases. This led to rapid global warming and caused ocean waters to become acidic and low in oxygen, devastating stationary organisms like crinoids. So few crinoid lineages survived that all crinoids living today are believed to be descended from just one or two surviving groups.
Mesozoic Recovery and Setbacks
Following the Permian-Triassic extinction, the recovery for crinoids was slow. The few surviving lineages began to diversify during the Triassic period, leading to an evolutionary shift with the rise of modern, unstalked crinoids known as feather stars. Unlike their stalked ancestors, these new forms were capable of movement, possibly as an adaptation to increased pressure from new predators like echinoids.
The Mesozoic Era presented its own challenges, including the Triassic-Jurassic and Cretaceous-Paleogene (K-Pg) extinction events. While the K-Pg event ended the age of dinosaurs, it did not impact crinoids as severely as the Permian-Triassic extinction. By this point, the more mobile feather stars had become dominant over the sessile, stalked forms, and the lineage persisted through these later extinctions.
Modern Crinoids as Living Fossils
The descendants of these ancient survivors inhabit today’s oceans. Free-swimming feather stars are now far more common and diverse than their stalked relatives, the sea lilies. Sea lilies are now mostly found in deep-sea environments, a remnant of their former abundance in shallower Paleozoic waters.
Their long history of survival, particularly through the Permian-Triassic bottleneck, makes modern crinoids “living fossils.” The approximately 600 species alive today are a testament to the resilience of a lineage that has survived some of the most profound crises in Earth’s history.