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| Fossil Sea Star |
Their familiar five-pointed form resembles a symbol drawn by hand, yet sea stars are living animals with hundreds of hydraulically operated tube feet, light-sensitive eyespots at the tips of their arms and the remarkable ability to regenerate damaged tissue.
Some can even push their stomach outside their body to digest prey. Nature, as usual, saw no reason to stop after inventing something elegant.
Sea stars belong to the class Asteroidea within the phylum Echinodermata. They are related to brittle stars, sea urchins, sand dollars, sea cucumbers and crinoids. Their closest living relatives are generally considered to be the brittle stars of the class Ophiuroidea; together, sea stars and brittle stars form the Asterozoa.
Sea stars move using a water vascular system that powers their tube feet, while brittle stars generally travel by flexing their slender, sharply defined arms.
The evolutionary story of these animals reaches back nearly half a billion years. Star-shaped echinoderms called asterozoans appear in Lower Ordovician rocks more than 480 million years old. Some early forms belonged to extinct groups such as the somasteroids, which possessed a mixture of features associated with later sea stars and brittle stars.
Fossils from the Early Ordovician Fezouata Biota of Morocco have helped us examine this important stage in asterozoan evolution. Molecular and fossil evidence indicates that the sea-star and brittle-star lineages had separated by approximately 477 million years ago.
Ancient sea stars did not necessarily look exactly like the species crawling through modern tide pools.
Many Palaeozoic forms possessed narrow arms, small central discs and arrangements of skeletal plates unlike those of their living descendants.
Sea stars survived several major extinction events, although the end-Permian mass extinction approximately 252 million years ago profoundly reorganized marine ecosystems. Most of the sea stars alive today belong to evolutionary groups that diversified during the Mesozoic.
Despite their long history, complete fossil sea stars are rare. Their bodies are supported by thousands of small calcite plates called ossicles, connected by soft tissues and ligaments. Soon after death, those tissues decay and the skeleton usually collapses into a jumble of pieces.
Finding an articulated fossil sea star therefore means that the animal was buried quickly—perhaps by a storm deposit, underwater sediment flow or sudden pulse of fine mud—before currents and scavengers could dismantle it. A complete sea star is less a routine fossil than a small geological miracle.
On and around Vancouver Island, fossil sea stars occur within the Upper Cretaceous Nanaimo Group. Specimens have been collected from the Northumberland Formation at Manning and Collishaw Point on Hornby Island (known locally as Boulder Point), where fine marine mudstones and carbonate concretions preserve a diverse deep-water community.
These roughly Late Campanian deposits have produced sea stars alongside ammonites, bivalves, gastropods, sea urchins, fish, and shark teeth.
These fossils tell us that starfish were already established members of the northeastern Pacific ecosystem while mosasaurs still hunted offshore. Their rarity also makes fragments important: isolated marginal plates, arm ossicles and other skeletal pieces may not produce the dramatic outline of a complete star, but they can still reveal which groups lived in an ancient sea. They are a wonderful fossil to find and oh, so pleasing to behold!
Elsewhere in the Pacific Northwest, asteroid fossils and isolated ossicles occur in marine sedimentary formations in Washington and Oregon. Cenozoic units. Washington’s Lincoln Creek Formation preserve evidence of sea stars within the rich marine communities that occupied the northeastern Pacific during the Eocene and Oligocene.
Together with the marine formations of southwestern Vancouver Island, these rocks record changing coastlines, ocean temperatures and seafloor habitats over tens of millions of years.
Worldwide, fossil sea stars have been recovered from Ordovician rocks in Morocco, Europe, North America and Australia; Silurian deposits in Britain; the Devonian Hunsrück Slate of Germany; Jurassic marine beds in Europe; Cretaceous deposits in Lebanon and North America; and younger Cenozoic rocks on several continents. Exceptional deposits sometimes preserve whole animals, while more ordinary marine rocks yield scattered ossicles. Because these tiny elements are easily overlooked, the history of sea stars is undoubtedly richer than the visible fossil record suggests.
Today, approximately 1,900 species inhabit every ocean, from tropical reefs and temperate tide pools to polar waters and the deep sea. They include heavily armoured forms, delicate mud-dwellers, cushion stars and many-armed giants.
Along the British Columbia coast we find the ochre sea star, Pisaster ochraceus; the mottled star, Evasterias troschelii; the leather star, Dermasterias imbricata; and the extraordinary sunflower sea star, Pycnopodia helianthoides, which may possess more than 20 arms.
Sea stars are not merely colourful tide-pool decorations. Many are influential predators that regulate mussels, clams, barnacles, snails and sea urchins.
The ochre sea star helped inspire the ecological concept of a “keystone species”: an animal whose influence on its community is far greater than its abundance might suggest. Sunflower sea stars perform a similarly important role by consuming sea urchins that would otherwise overgraze kelp forests.
That ecological balance was shaken in 2013, when sea star wasting disease swept along the Pacific coast from Mexico to Alaska.
On Vancouver Island, diseased stars were observed around Bamfield and Ucluelet on Vancouver Island's far western shores and later around Nanaimo and Nanoose. More than 20 species were affected. Infected animals developed lesions, twisted their arms unnaturally, lost limbs and eventually disintegrated into pale masses of decomposing tissue.
After more than a decade of investigation, researchers identified the culprit as a strain of the bacterium Vibrio pectenicida, designated FHCF-3, as a causative agent of wasting disease in sunflower sea stars. Scientists detected it in the animals’ coelomic fluid—the internal fluid that functions in some respects like blood—and then cultured the bacterium and reproduced the disease experimentally. The results were published in Nature Ecology & Evolution in 2025.
The sunflower sea star suffered catastrophic losses, with billions believed to have died and populations falling by more than 90 percent in many parts of its range.
Our records and many a beach walk show a striking increase in areas where the species was absent after the outbreak, including waters around Vancouver Island, the Strait of Georgia, Barkley Sound and parts of the central and northern coast. It was a tragic loss. The species is now assessed as Endangered in Canada and Critically Endangered globally.
Warming seas may make conditions more favourable for some Vibrio bacteria and can place additional stress on their hosts, although temperature is not the only factor governing outbreaks. Sea stars also face marine heatwaves, ocean acidification, declining oxygen, pollution, habitat disturbance and changes to their food webs. The loss of major predators such as Pycnopodia helianthoides can allow sea urchin populations to expand, contributing to the destruction of kelp forests and transforming entire coastal ecosystems.
Sea stars have endured almost 500 million years of continental movement, changing oceans and mass extinction. Their fossils prove that the star-shaped body plan is one of evolution’s great survivors. Yet their antiquity does not make them invulnerable.
The same animals whose delicate skeletons so rarely remain intact in stone are now reminding us, in living waters along Vancouver Island and the wider Pacific coast, just how quickly an ancient ecological relationship can come apart.
