Reef builders underwent significant changes throughout geological time. Their evolution led to increasing biodiversity within reef communities, especially during the Paleozoic and Cenozoic eras. These two eras demonstrate different types of reef structures, dominant reef builders, and environmental conditions that influenced reef development.
Paleozoic (Devonian–Permian)
From the Devonian to the present, the scale of reef building has increased, despite periods of decline. The ecological complexity of reef structures and the number of reef-building varieties involved also grew.
In the Paleozoic, attached echinoderms called crinoids contributed to reef formation. However, their contribution to reef building was only secondary. Today, however, many echinoderms like sea stars and some sea urchins destroy reefs.
By the end of the Paleozoic, poorly protected calcified microalgae had declined. Moreover, Paleozoic reefs appear to have grown in the absence of photosymbiosis.1
Devonian
In the Early Devonian (Pragian), the formation of massive barrier reefs, up to 1,000 meters thick, occurred in the Urals. Enormous reef belts stretched across the entirety of Eurasia.
Crinoids increasingly became participants in reef construction. Starting in the Devonian, the role of cyanobacteria in reef building declined, giving way to red and green calcareous algae. It was only in the Middle Devonian that corals, primarily tabulate-like forms, began playing a very significant role in reef formation; for the rest of the Paleozoic, their role remained secondary.
Among reef builders, a significant role was played by hardy stromatoporoids (especially amphiporids), rugose corals, tabulates, and, to a lesser extent, bryozoans and receptaculitids. The robust colonies of rugose corals effectively resisted wave action, contributing to reef expansion. The continuous growth of coral reefs of this type in the Devonian led to the formation of reef limestone deposits up to 1,600 meters thick.
The Middle and Late Devonian (Givetian-Frasnian) marked the peak of reef-building activity and the greatest global reef expansion in the Phanerozoic. During this time, the climate was stable, and sea levels were high. Devonian reefs reached thicknesses exceeding 1,000 meters and spanned areas of hundreds of square kilometers.
Distribution of Devonian Reefs
Extensive Devonian reef complexes, surpassing the size of modern reefs, exist in Canada and Central Asia. Large reefs also occur throughout Europe, including the Alps and the Harz Mountains, in western North Africa, Kazakhstan, Novaya Zemlya, the Urals, southern Tian Shan, the Altai-Sayan and Verkhoyansk-Kolyma regions, Salair, South China, and Southeast Asia. However, the most renowned examples are in the Canning Basin, Western Australia.
In Belgium and Germany, in the region of the Rhenish Slate Mountains, atolls and barrier reefs were widespread during the Late Eifelian–Middle Frasnian. Some of these reefs exceeded 1,000 meters in thickness, and their covered area occasionally reached significant sizes—for example, the Braillon reef complex in Zoerland spanned up to 100 square kilometers. Southwest England has similar reefs, like the Givetian atoll complex in Tor Bay, which covers up to 1,000 square kilometers. In Arctic Canada, the largest barrier and atoll reef complexes cover hundreds of square kilometers. For instance, the Ancient Wall organic complex, with a thickness of up to 500 meters, is located in Alberta, northeastern Canada (Frasnian).2
Reef builders: fossil colonial rugose corals from Michigan and Morocco.
Middle-Late Devonian Reef Builders
Middle-late Devonian reefs were constructed by large, heavily calcified metazoans—stromatoporoid sponges and tabulate corals—together with calcified cyanobacteria and, to a lesser extent, rugose corals. Receptaculitids and lithistid sponges were common in reef communities of the fore-reef slope or deeper waters, as well as in shallow, low-energy environments. Brachiopods were common, sometimes nesting within reef cavities.
Microbialites and calcified cyanobacteria, such as Renalcis and Rothpletzella [Sphaerocodium], played a significant role in many Frasnian reefs. They appeared as isolated mounds, heads, and columns, or as encrusting elements within the reef structures.
Many stromatoporoids and other calcified metazoans attached themselves to these lithified substrates, only occasionally fusing with each other to form a reef framework.
Stromatoporoid-tabulate structures were especially widespread during the Devonian period (430–340 million years ago), reaching impressive dimensions. Their thickness ranged from 50 to 100 meters, with diameters of 1–2 kilometers.
Decline of Reefs in the Late Devonian
At the end of the Devonian, the convergence of continental plates reduced the extent of shallow shelves, leading to a sharp decline in reef formation. During the Famennian age, a mass extinction event affected corals and stromatoporoids. Favosites and alveolitids disappeared; heliolitids and triplasmids became completely extinct. The primary driver of reef loss during this period was global cooling and the formation of glacial ice caps, which significantly impacted all warm-water fauna.
In the Late Devonian, foraminifera began to make a noticeable contribution to reef composition. By the Famennian age, most reef structures consisted largely of mud mounds formed by cyanobacteria, red algae, receptaculitids, and spongiostromids. The role of stromatoporoids diminished dramatically. Many large reef-like structures of the Late Devonian, up to 50 meters thick, were composed almost entirely of algal limestone produced by phylloid red and green algae, closely related to coralline algae and Halimeda. In the absence of competitors, stromatolitic structures once again became more prevalent.
Carboniferous
The Carboniferous period was a time of significant fluctuations in climate and sea level. Polar glaciations and a decrease in global temperature occurred. The arrangement of continents prevented equatorial ocean currents.
Mississippian
Reefs in the Carboniferous were less widespread compared to the Devonian. However, in the mid-Early Carboniferous (Visean), reef formation intensified significantly. The primary reef builders shifted from corals to algae and bryozoans. Stromatoporoids experienced a brief resurgence in the Early Carboniferous but eventually lost their role as reef builders entirely.
In contrast, bryozoans thrived during the Early Carboniferous, actively contributing to the formation of organic buildups. During the Serpukhovian age, algae, microbial communities, and bryozoans mainly formed reefs. Corals, stromatoporoids, crinoids, and attached foraminifera played a smaller role. A variety of organisms, such as chaetetids, brachiopods, and ostracods, inhabited these reefs.
The Late Visean–Serpukhovian stage was the shortest reef-building cycle of the Paleozoic, lasting approximately 16 million years. Reefs from this period exist in the United Kingdom, Spain, Belgium, and France, as well as in the Donbas region, on the slopes of the Urals, in the Caspian Depression, Altai, Eastern Kazakhstan, the Alazeya Folded Region, Sikhote-Alin, and in eastern Canada and the United States, as well as in Queensland, Australia.
Waulsortian Mud Mounds
Deep-water mud mounds, known as Waulsortian mounds, represent a distinct group of reefs that thrived in the Early Carboniferous of Central Europe. However, Late Carboniferous Waulsortian-like reefs also occur on the Ellesmere and Axel Heiberg Islands in the Canadian Arctic. Presumably, these mounds formed in deep waters below wave base, at depths of up to 280 meters.
Unlike a true reef, a mud mound consists almost entirely of uncemented particles, with individual fossils not forming a skeletal framework. Waulsortian mud mounds, with their steep slopes, surround layers rich in disarticulated crinoids, fenestellid bryozoans, as well as gastropods and brachiopods. The Muleshoe Reef in the Sacramento Mountains, New Mexico, is a classic example of a Waulsortian mound.
Pennsylvanian
At the boundary between the Mississippian–Pennsylvanian (Serpukhovian–Bashkirian), up to 80% of corals became extinct, and they virtually lost their role as reef builders during the Late Carboniferous (Pennsylvanian) and Permian periods.
From the Late Carboniferous onwards, the coralline algae Archaeolithophyllum became widespread. These algae had a heavily calcified thallus and formed a thin leafy (phylloid) crust.
During the Late Carboniferous (Pennsylvanian), reef-building activity ceased for an extended period.
Permian
The Permian cycle of Paleozoic reef formation lasted for 45–47 million years. During the Permian, reefs were present in significantly more limited areas. They occur in the Russian Platform, the Caspian Basin, the Urals, Primorye, South America (Venezuela), the Permian Basin of the United States, Northern and Central Europe, the Caucasus, the Pamirs, Iran, Pakistan, China, and Thailand.
Palaeoaplysina. This amazing group of organisms is conventionally classified as hydroid polyps—colonial organisms belonging to cnidarians.
Sponges (primarily the calcareous Sphinctoidae and Pharetronia), red algae Palaeoaplysina (previously considered sponges or hydroid polyps), and, to a lesser extent, bryozoans were the main reef builders. The role of corals significantly declined.3
Early Permian
In the reef limestones of the Early Permian, numerous calcified thalli of green algae, colonies of various bryozoans, shells of brachiopods, bivalve and gastropod mollusks, and fusulinids are found. Sphinctozoa played a major role. These reefs reached thicknesses of 1000 meters or more. Calcareous organisms Tubiphytes4 were widespread. They could even form algal bioherms on their own.
Early Permian reefs existed in Bashkiria. The remains of the Great Bashkir Reef stretch for 20 kilometers. Today, they create solitary conical mounds known as “shikhan,” which means a solitary hill.
They began to form at the end of the Carboniferous period as a chain of reefs. At that time, the Paleo-Ural Ocean remnants were here. By the Early Permian, it had transformed into a narrow strait linking the Panthalassa and Paleotethys oceans. By the end of the Artinskian, it closed off on the southern side, forming the internal Permian Sea. It disappeared during the Kungurian. The main reef builders were Palaeoaplysina, Tubiphytes, and, to a lesser extent, bryozoans, brachiopods, calcareous algae, and rugose corals. Most Paleo-Ural Ocean reefs remain buried underground.
Sylvenian reefs
Sylvenian reefs existed in the late Early Permian to early Late Permian periods, approximately 280–270 million years ago. They now reach heights of 60–70 meters. At that time, the Permian Sea extended over this area. The Sylvenian organogenic structures belong to the category of skeletal mounds. They never rose above the water surface, forming neither islands nor complex landscapes with breakwaters. When the sea retreated, a multi-layered reef ridge was exposed, which now towers over the surrounding area. The main builders of the reefs were ancient organisms that left behind calcareous tubular skeletons. Brachiopods, bryozoans, and other remains of ancient animals can be found on the peaks of the Sylvenian cliffs.
Late Permian
Bryozoans, such as Gonipora and Polypora, and calcified sponges characterize Late Permian reefs. Tubiphytes and various algae, including Archaeolithoporella, continue to play a significant role (Rachel Wood, 2003).
Rapid cementation and encrusting microbialites created wave-resistant reef rock along the shelf edge and slope in places such as Zeichstein Reef of northern England and the Capitan Reef of west Texas and New Mexico.
The cessation of reef formation in the Late Permian is associated with a sea regression.
Capitol Reef
Capitol Reef in Utah stretches over 160 km and has a rich geological history spanning multiple periods. During the early Permian, Utah lay on a continental shelf near the paleoequator, sometimes submerged by a shallow arm of the Panthalassa Ocean. Bright red sandstones, siltstones, conglomerates, and clay-rich limestones formed the Cutler Formation during this time. Later, the Kaibab Sea moved inland, leading to the deposition of up to 60 meters of Kaibab Limestone. This limestone, rich in fossils like brachiopods, conodonts, and trilobites, originated from calcareous mud, organic fragments, and sediments in the shallow marine environment.
Following the mid-Permian, fluctuating sea levels led to alternating marine and non-marine deposits, including evaporites during temporary sea incursions. The formation of Capitol Reef continued through the Triassic and included Jurassic deposits.
In the Cretaceous, the Western Interior Seaway again submerged the area, leaving layers of sandstone and shale. The Capitol Reef region was located on the edge of an epicontinental sea for most of the Cretaceous. However, by the early Cenozoic, the Seaway disappeared. The region’s elevation increased significantly during the late Cretaceous due to the Laramide orogeny, shaping the landscape into its present form.
This geological timeline reveals the interplay of marine invasions, sediment deposition, and tectonic activity that contributed to the striking formations of Capitol Reef.
Captain Reef
Captain Reef is an example of an ancient carbonate shelf and a well-preserved reef. It formed during the Late Guadalupian of the Permian period. The Capitan Barrier Reef, 180 meters thick and 560 kilometers long, is located in western Texas and southeastern New Mexico, underlying El Capitan Peak (2,464 m high). This reef surrounded much of the inland Delaware Sea, which covered parts of modern New Mexico and Texas during the Permian.
The Delaware Sea stretched approximately 150 kilometers long and 75 kilometers wide. Maximum growth of the reef occurred 250–265 million years ago. Over time, the connection with the ocean broke (at the end of the Ochoan epoch). This led the sea to dry up, and sedimentary rocks covered the reef.
1—Gigantospongia discoforma (platy sponge); 2—solitary and branching sphinctozoan sponges; 3—Archaeolithoporella (encrusting ?algae); 4—microbial micrite; 5—cement botryoids. ©Sibbick
Primarily large frondose bryozoans and a scaffolding of sphinctozoan calcified sponges formed the Captain Reef framework. Massive encrusting algae, including Collenella, Parachaetetes, and Solenopora, also played a critical role.
The buried reef began to rise 80 million years ago during the Late Cretaceous. Between 20 and 15 million years ago, it became exposed, ultimately reaching its current height, where it now towers above the valley as it once did above the bottom of the Delaware Sea.
Geologists highlight Captain Reef’s significance through naming conventions: the Capitanian stage of the Permian period takes its name from the reef, while the Guadalupe Mountain Range lends its name to the Late Permian epoch.
Mesozoic
Throughout the Mesozoic and Cenozoic eras, algae and corals became the primary reef builders. Hexacorals (Hexacoralla) replaced tabulate and rugose corals in the Mesozoic seas. Their colonies actively built reefs, which are now widely distributed in the Pacific Ocean.
Triassic
The Extinction of Corals at the Beginning of the Triassic
At the boundary between the Permian and Triassic, a mass extinction event led to the near-total disappearance of corals and the destruction of many Paleozoic reefs, drastically changing marine ecosystems and reducing biodiversity. Tabulate and rugose corals went extinct. During the Early Triassic, no corals were present. The early Triassic seas were very poor in life forms. Microbial mats filled the ecological niche previously occupied by metazoan reefs, leading to the widespread formation of microbialites. Reef communities took a very long time (up to 10 million years) to recover after the mass extinction events.
There is no strict correlation between the cessation of reef building and the moments of extinction. Reef formation stops before the extinction of organisms. Reef builders still existed, but outside the reef ecosystem. The reverse is also true when framework-building organisms appear before the formation of the reefs themselves—forming a reef ecosystem takes some time.
Algae and, to some extent, bryozoans took on the role of reef builders, represented by new species during the Mesozoic. Among algae, red coralline algae (Corallinaceae) played the primary role, with their thalli impregnated with lime. Hydroids and brachiopods also contributed.
Middle–Late Triassic
It was only by the middle of the Triassic that new species of reef-building corals appeared. Since then, and up to the present day, scleractinians have played the role of reef builders. Initially, they became widely distributed as part of the coastal fauna of the Tethys basin. After that, they quickly became an important component of the fauna of the entire tropical shelf. Scleractinians already possessed photosynthetic symbionts in these communities.
In the Triassic, endoliths, which inhabit and destroy reefs, spread.
In the Late Triassic, reefs became widely distributed again, and the role of sphinctozoans increased.
Jurassic
The next period of intensive coral speciation and reef growth occurred in the Jurassic (Obruchev, 1926; Beauvais, Beauvais, 1974). Hydrozoans, including hydroactinoids (Ellipsactinia) and the hydrocorals Milleporidae and Stylasteridae, played a significant role in reef formation starting from the Jurassic. The latter experienced their peak development during the Late Jurassic (Boshma, 1956).
In the middle of the Jurassic period, scleractinians underwent intensive evolution, resulting in the appearance of many new families.
Late Jurassic
In the Late Jurassic, they created massive reef structures over vast areas of the tropical shelf, as well as on the shelves of temperate basins and in cold waters. Sabellariid polychaetes and brachiopod mollusks created large reef formations (up to 50 meters thick) in some regions during the Jurassic period.
In the Late Jurassic, many families of ancient fossil corals, such as Microsolenidae and Amphiasteridae, went extinct, giving rise to new, more modern, and advanced families of scleractinians (Pocilloporidae, Faviidae, and Poritidae).
From the Late Jurassic to the end of the Cretaceous, true reef systems developed widely in Mesozoic seas.
Cretaceous
At the end of the Mesozoic, large reef platforms formed, which later served as the ancient foundation for modern reefs.
During the Late Cretaceous, bivalve mollusks—rudists, with shells shaped like a cup with a lid—proliferated unusually in warm epicontinental seas. They crowded out coral polyps so much that they became the main reef builders of that time, but they did not survive the global extinction at the end of the Cretaceous, just like belemnites, ammonites, and many other marine creatures.
The number of Solenopora (red algae with undifferentiated thalli) sharply decreased in the late Mesozoic/early Cenozoic. This coincided with a reverse increase in the abundance and diversity of coralline algae.
Cenozoic
In the Cenozoic era, the evolution of modern and more complex reef systems occurred, in which scleractinian corals gradually became dominant, adapting to various environmental conditions. These corals are the main reef builders today. They are capable of forming massive structures due to their efficient growth rates and symbiotic relationships with zooxanthellae (photosynthesizing algae).
Paleocene
At the beginning of the Cenozoic era, during the Paleocene, marine ecosystems were relatively sparse in biodiversity. Coral reef growth slowed down again. Some families of ancient scleractinian corals continued to go extinct.
Eocene
A: Bulbous columnar stromatolite 5.5 m high with diameters of 7 m. Layers are composed of one or several types of microbialites. B: Weathered-out giant stromatolite. Cross-section shows a hollow center. Decimeter scale. ©Stanley, Buchheim
The widespread development of reefs began in the middle of the Eocene and continues to the present day. In the Eocene, highly mobile reef fish, which destroy reef structures, appeared. At the same time, branching coral taxa such as Acropora, Porites, and Pocillopora emerged, capable of reattaching fragments, regenerating quickly, and growing, often merging with other colonies.
Oligocene–Miocene
From the Oligocene onward, the significance of delicately branched corallines in the tropics decreased.
In the Oligocene and Miocene, coral growth resumed intensively with the participation of scleractinian families related to modern ones. The coral reefs formed during this period acquired a structure and appearance similar to those of modern reef constructions. In some areas, their thickness reaches up to 600 meters.
At the end of the Tertiary period, a division of scleractinians occurred at the species level, forming Indo-Pacific and Atlantic faunas.
Along with corals, coralline algae and foraminifera played a significant role in reef formation. Materials from these components primarily make up many large reef structures from the Miocene in North America and Europe.
Pleistocene
Intensive growth of reef structures on shallow marine areas in the tropical zone continued during the Pleistocene, with some interruptions during periods of significant ocean level drops associated with periodic glaciations. The thickness of the massive platforms of ancient Pleistocene reefs reached hundreds of meters.
Present
Almost all modern large reef structures formed during the Holocene transgression on ancient Pleistocene reef platforms, the surfaces of which were exposed during periods of ocean level drops. In modern reefs, corals form a symbiosis with green unicellular algae called zooxanthellae, which live in the inner layer of the coral polyps. As a result, the rate of coral calcification increases by 30 times.5
Diversity of modern reef builders
In modern times, the primary reef builders are madrepore corals and calcified red algae. They independently form reefs in the clear waters of the Pacific and Indian Oceans. In contrast, sponges, which are active filter feeders, dominate reefs in the less transparent waters of the Caribbean Sea. Off the coast of Florida, where waters are highly turbid, sponges and tube-building worms create organic buildups without the involvement of corals.
Oyster and bryozoan reefs exist, with the latter being as young as 20 million years old. However, in most modern reefs, bryozoans inhabit areas hidden from direct sunlight, such as beneath rocks, under coral colonies, in crevices, and in voids. Reefs made up of bryozoans and sponges also inhabit deep-water environments, for example, in regions near the shelf of southeastern Australia, where there is little to no light.
The reefs of the Red Sea are predominantly algae-coral reefs, and therefore they are mainly composed of aragonite.
Oyster reefs, often called oyster bars, are a common underwater habitat in the southern United States. Florida’s oyster reefs occur in coastal areas and estuaries along both coasts, but they grow most actively near river mouths.
In modern times, tubeworms also build reef-like structures (Kirtly, Tanner, 1968). Their size can reach hundreds of meters. For example, such a polychaete “reef” is located on the northern coast of the Adriatic Sea near Valli di Comacchio, near Venice.
- Rachel Wood, 2003. The ecological evolution of reefs. ↩︎
- Geldsetzer, 1988. Ancient Wall Reef Complex, Frasnian Age, Alberta. ↩︎
- Kuznetsov V.G., 2020. Asynchronous development of reefs and reef-building biota. Paleozoic. ↩︎
- Tubiphytes are a problematic fossil, maybe Porifera. The skeletal structure of Tubiphytes is similar to the exothecal tissue of archaeocyaths and the tissue that fills sponges. Tubiphytes could colonize deeper and cooler waters compared to the algae they might also be associated with. ↩︎
- A.I. Antoshkina, 2003. Reef formation in the Paleozoic. ↩︎
