What Built Reefs Before Corals? A Different Cast in the Same Role
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Reefs existed for hundreds of millions of years before modern corals evolved, built by sponges, algae and extinct groups nobody expects. The best exposed example is now a desert mountain range in Texas.
A reef without corals
A reef is a structure built up from the seafloor by organisms and resistant enough to stand against waves, and nothing in that definition requires corals. Through the Palaeozoic the builders were a changing cast including calcareous sponges, calcified algae, bryozoans, and extinct groups with no modern equivalent, which between them produced structures comparable in scale to modern reefs and quite different in construction. The reefs of the Permian period, the last before the largest extinction in the history of life, are particularly well studied because several are exceptionally exposed, and they were built largely by sponges and by encrusting organisms with abundant carbonate precipitated directly from seawater filling the spaces, which makes them as much a chemical structure as a biological one.
Who did the building
The organisms responsible differ from anything on a reef today:
- •Calcareous sponges, which formed rigid frameworks and are the main structural element
- •Calcified algae, binding and encrusting the framework
- •Bryozoans, colonial filter feeders forming delicate fenestrate sheets
- •Brachiopods, abundant shelled animals occupying roles bivalves fill today
- •Crinoids in large numbers on the slopes
- •Directly precipitated carbonate cement, which filled voids and made the structure rigid
The mountain that was a reef
The Capitan reef complex in west Texas and New Mexico is among the best exposed ancient reefs anywhere and has shaped how geologists think about such structures. It formed along the margin of a basin roughly two hundred and sixty million years ago, was buried, and has since been uplifted and eroded so that a cross-section through the whole system stands exposed in mountain faces, showing the reef itself, the shallow lagoon behind it and the steep slope and basin deposits in front. That three-dimensional exposure allowed the relationships between environments to be worked out directly rather than inferred, and the terminology developed there is applied to reef systems worldwide. The same rocks host substantial petroleum reservoirs, which is why the system has been studied with unusual intensity and funding.
The long series of builders
Reef building has been taken up and dropped by a remarkable succession of groups over the last half billion years, and listing them makes the point better than any generalisation. The earliest reef-like structures were built by microbes producing layered mounds, which dominated for well over a billion years before animals existed. Archaeocyathids, a short-lived sponge-like group, built the first animal reefs in the Cambrian and vanished within it. Stromatoporoids and tabulate and rugose corals, all now extinct, built the great reefs of the Silurian and Devonian. Sponges and algae took over in the Permian. Bivalves with strange conical shells built reefs through much of the Cretaceous before disappearing. Modern corals have held the role since, which by this record is a relatively recent arrangement rather than a permanent one.
How the story ended
Reef systems of this kind disappeared at the end of the Permian and their absence afterwards is informative. The extinction at that boundary was the most severe known, removing a very large share of marine species, and reef-building organisms were hit particularly hard, with the sponges and other builders vanishing or being reduced to insignificance. The following interval shows a gap of several million years with essentially no reefs anywhere, which is the clearest single illustration of how completely the ecosystem collapsed. When reefs returned, they were built by different organisms, and the corals that dominate modern reefs appeared in the Triassic and are not descended from the Palaeozoic corals that preceded them. The pattern of loss, gap and replacement by an unrelated group recurs at several extinction boundaries.
The takeaway
Reefs are defined by being wave-resistant structures built by organisms, which does not require corals, and Palaeozoic ones were built by sponges, algae and bryozoans with directly precipitated cement. The Capitan complex in Texas is exposed in cross-section through reef, lagoon and basin. Reef building ceased entirely for several million years after the end-Permian extinction.