Sponges are about as simple as animals get: no nerves, no muscles, no gut, just cells organised around a system of water channels. That simplicity has made them a natural candidate for the oldest animals on Earth, the living descendants of the first experiments in multicellular animal life. The fossil record mostly supports that story, but with some important twists. Sponges have a long, rich fossil history, they built some of the earliest reefs, and their remains are hidden in rocks most people would never think to look at, including the flint nodules in English chalk.

As a reef keeper who has spent decades building living reefs in glass boxes, I find it humbling that sponges were building reefs more than half a billion years ago. This page sets out what the fossils show, what remains controversial, and why the “first animal” question is still open.

How sponges fossilise

Soft-bodied animals rarely fossilise, but sponges have an advantage: many have mineral skeletons.

Most sponges build their skeletons from tiny needles called spicules, made of silica or calcium carbonate; see sea sponge structure. When a sponge dies, its soft tissue rots, and loose spicules scatter into the sediment, where they can be preserved for hundreds of millions of years. Sponges whose spicules interlock or fuse into rigid frameworks, such as many glass sponges, can be preserved whole. Some extinct groups built massive calcium carbonate skeletons that fossilised as easily as coral. Under exceptional conditions, such as in the Burgess Shale of Canada or the Chengjiang deposits of China, even sponges without hard skeletons have been preserved with remarkable detail. And sponges can leave chemical fossils: distinctive molecules from their cell membranes that survive in rocks long after the sponge is gone.

The oldest possible sponges

Several lines of evidence suggest sponges existed well before the Cambrian, but each is disputed.

In 2009 a team led by Gordon Love reported a distinctive steroid molecule, 24-isopropylcholestane, in rocks from Oman more than 635 million years old, from the Cryogenian period. That molecule is produced today mainly by demosponges, and the find was hailed as chemical evidence of the oldest sponges. Later work suggested some algae might produce similar molecules, and the debate continues. Possible body fossils have also been reported, including a tiny sponge-like fossil called Eocyathispongia from about 600 million years ago in China, and in 2021 the geologist Elizabeth Turner described possible sponge-like structures in reef rocks about 890 million years old in northwestern Canada. If confirmed, those would be by far the oldest animal fossils known. Many paleontologists remain cautious, because microbial structures can look surprisingly similar.

The Cambrian: sponges everywhere

By the early Cambrian, around 535 million years ago, the evidence for sponges becomes clear and abundant.

Isolated sponge spicules appear in rocks of this age around the world, and by the time of the Cambrian explosion, when most major animal groups appear in the fossil record, sponges were diverse and widespread. The Burgess Shale of British Columbia, about 508 million years old, contains many beautifully preserved sponges, such as Vauxia, Choia and Hazelia, alongside the strange arthropods and worms that made the site famous. The Chengjiang biota of southern China, slightly older, also preserves a variety of sponges.

The Cambrian also saw the first animal-built reefs, and sponges built them. The archaeocyaths were cup- or vase-shaped animals with porous, double-walled calcium carbonate skeletons, now generally regarded as an extinct group of sponges. Together with microbial mats, they formed reefs in warm, shallow seas during the early Cambrian. Archaeocyaths declined sharply within a few tens of millions of years and disappeared before the end of the Cambrian, but they were the pioneers of reef-building.

Stromatoporoids and the great Paleozoic reefs

From the Ordovician to the Devonian, reefs were built largely by another group of sponges, the stromatoporoids.

Stromatoporoids built layered, mound-like skeletons of calcium carbonate, often with small star-shaped canal markings on the surface. Together with early corals, they constructed enormous reefs during the Silurian and Devonian periods, some of which can still be seen as limestone ranges, such as the Devonian reef complexes of the Canning Basin in Western Australia. For a long time their identity was disputed, but they are now generally classed as sponges with heavily calcified skeletons. Our page on stromatoporoids covers them in detail. They declined sharply during the Late Devonian mass extinctions, and reef ecosystems took millions of years to recover.

Collectors often find other Paleozoic sponges too. Astraeospongia, a saucer-shaped sponge with star-shaped spicules from Silurian rocks of Tennessee, and Hydnoceras, a knobbly, vase-shaped glass sponge from the Devonian of New York, are two well-known examples sold in fossil shops.

Sponge reefs, flint and chalk

Sponges continued to build reefs after the Paleozoic, and their silica still shapes the rocks of Europe.

During the Late Jurassic, siliceous sponges formed extensive reefs and mounds across a shallow sea covering much of Europe, from Spain to Poland; their remains form limestone cliffs in places such as the Swabian Alb in southern Germany. In the Cretaceous chalk seas of northern Europe, sponges were abundant on the seafloor. When their silica spicules dissolved and recrystallised, they helped form the flint nodules found in chalk cliffs from Dover to Normandy. Many flint nodules have the shape of a sponge, and fossil sponges such as Ventriculites are common in English chalk. Stone Age people who knapped flint into tools were, in a sense, working with sponge silica.

Remarkably, a type of glass sponge reef that was thought to have vanished with the Jurassic still exists. In 1987 and 1988, Canadian scientists discovered living glass sponge reefs on the seafloor of Hecate Strait off British Columbia, some built up over thousands of years. They are often described as living fossils, a window onto the Jurassic seafloor; see glass sponges.

Finding sponge fossils yourself

Fossil sponges are more common than most people realise, and you do not need to be a paleontologist to find one.

The easiest place to start is a chalk coastline. On beaches below chalk cliffs in southern England and northern France, flint pebbles often preserve the shapes of sponges, and broken flints sometimes show sponge canal structures inside. Look for flints shaped like vases, cups or branching fingers, or for small round pores on the surface. In North America, Silurian and Devonian limestones in states such as Tennessee, Indiana and New York yield fossil sponges and stromatoporoids, and stromatoporoid “heads” with their layered, wavy internal structure are common in Devonian reef limestones. Loose spicules are microscopic, but sieved sediment samples from some limestones and cherts are full of them. Always check local rules before collecting, since many fossil sites are protected, and stay well away from the base of crumbling cliffs.

The living equivalents are easier to find. Every live rock I have put into a reef tank has been, in effect, a small piece of reef limestone, and the sponges that sometimes grow on it are distant relatives of the animals that built reefs half a billion years ago.

Sponges or comb jellies: which came first?

The fossils show sponges are very old. Whether they were the first animals to branch off is a question for genetics, and the answer is not settled.

For most of the 20th century, sponges were considered the earliest-branching animals, the sister group to all others, because of their simplicity and because their collar cells closely resemble choanoflagellates, the single-celled organisms most closely related to animals. Since the late 2000s, large genetic studies have repeatedly suggested instead that comb jellies, the ctenophores, branched off first, which would mean either that comb jellies evolved nerves and muscles independently, or that sponges lost them. A 2023 study comparing the order of genes on chromosomes, rather than gene sequences, came down on the side of comb jellies. Other researchers still favour sponges, and the debate is lively. Either way, sponges sit very close to the root of the animal tree, and studying them remains one of the best ways to understand how animal life began. Our page are sponges multicellular? explains what makes them animals at all, and deep sea sponge lifespan covers the living sponges that rival their ancestors for longevity.