Fossil Reefs and Reef Builders: Precambrian–Silurian

The history of fossil reefs reveals the stages of their development, global distribution, and periods of decline and resurgence in reef formation. Over millions of years, various reef builders—from primitive archaeocyaths to modern scleractinian corals—have shaped these underwater structures. In each era, diverse organisms successively dominated reef construction. They succeeded in adapting to changing environmental conditions and creating complex ecosystems in various settings, making reefs a hub of marine biodiversity.

Originally, the term “reef” referred to any rocky shoal posing a hazard to navigation. Today, in geology and paleontology, scientists apply the terms “reef” or “fossil reef” to describe biogenic formations created by organisms with calcareous skeletons.

Fossil Reefs and Organic Buildups

Ancient organogenic formations have long drawn the attention of both paleontologists and other researchers. Scientists commonly use two primary terms to describe them: reef and bioherm (characterized by a mound-like structure), as well as biostrome (featuring a flat, layered structure).

The term reef finds greater use to refer to biogenic constructions that reach the wave-action level and possess a solid framework.

A reef is a geological formation, a massive carbonate structure created by the activity of framework organisms that lived near the water’s surface. It rises above the surrounding sea floor and acts as a breakwater. This structure consists of organism remains and the products of their breakdown. Non-colonial organisms that settle on reefs also contribute to their formation. Nearly all reef-builders form not only fossil reefs but also individual layers and thick deposits of distinctly stratified limestones and dolomites.

A bioherm is a massive, convex body (lens, hill, mound, dome, or oncoid) formed by reef-building sessile organisms (such as corals, stromatoporoids, algae, mollusks, etc.) growing on top of one another, creating a structure that rises above the surrounding rocks. It consists of consolidated, amorphous limestone.

A biostrome consists of the same material, with interlayers of skeletal remains of invertebrates. Unlike a bioherm, a biostrome has a layered structure but lacks a mound-like shape.¹ In fossil form, within sedimentary deposits, biostromes appear as lenses and layers ranging from 2 to 30 meters in thickness.

An oncoid is a small, massive, typically calcareous, irregularly rounded body (nodule). Oncoids can take various shapes, including columnar, cup-like, cylindrical, rounded, tower-like, or resembling hills, mounds, or blocks. They may form coastal reefs or simple massive structures. Oncoids are the result of the activity of presumably cyanobacteria or bacteria, often displaying concentric layering. Essentially, the term oncoid is synonymous with bioherm.

An oncolite is a grain consisting of a nucleus—typically a piece of rock, mineral, or shell fragment—surrounded by concentric carbonate layers produced by cyanobacteria. Oncolite is a type of stromatolite that forms without attaching to the substrate.

A thrombolite is a clotted, accretionary structure that forms in shallow waters through the trapping and cementation of sediment by microbial biofilms, particularly cyanobacteria. Unlike stromatolites, thrombolites lack distinct layers and have a clotted texture. Calcified microbial thrombolites occur in shallow marine sedimentary rocks as early as the Neoproterozoic and Early Paleozoic periods (Kennard et al., 1986).

Organogenic structures such as bioherms and biostromes, composed of the remains of framework organisms, could form at various depths. Corals without zooxanthellae (unlike reef-building corals) can also inhabit environments ranging from the intertidal zone to depths exceeding 6,000 meters.

Reef Structures and Formations

In true reefs, reef-building organisms create a solid framework. These can exist as isolated structures or form extensive reef systems, such as the Great Barrier Reef off northeastern Australia, which stretches for over 2,500 kilometers.

Reef mounds are fossil bioherms where organisms are abundant but do not form rigid, wave-resistant structures due to the lack of overlapping frameworks. Reef mounds develop in deeper waters below the wave base. Microbial-algal communities and sediment-trapping organisms (such as crinoids, bryozoans, sponges, leafy algae, and codiaceans) play a significant role in their formation. Encrusting organisms (the “caps of the mound”) may cover the surface of these reef deposits.

Fossil reef mounds existed in many periods of the Phanerozoic, such as the Early and Middle Devonian coral-crinoid mounds of southeastern Morocco.

In mud mounds, skeletal organisms are present but play a very minor role.

In addition to warm-water reefs located near the sea surface, there are also coral reefs that form in cold waters at depths of up to 1,000 meters.

Major Reef Types

Reefs can form in shallow shelf areas as well as in deep-sea conditions, often surrounding volcanic islands and creating characteristic ring-shaped atolls with inner lagoons. A ring-shaped reef of an atoll may include islands, banks, flat reefs, micro-atolls, and faroes³. At the center of an atoll reef system, there are typically one or more lagoons, separated by patch reefs. Atolls are most common in the Pacific and Indian Oceans.

Barrier reefs form parallel to the coastline and are separated from it by a lagoon. The thickness of a barrier reef can significantly exceed that of fringing reefs.

Fringing reefs are directly adjacent to the shoreline (for example, the coastal reef in Sharm el-Sheikh, Sinai, Egypt). Sometimes, a small depression in the form of a narrow coastal lagoon (“boat channel”) forms between the edge of the reef and the shore.

These reefs act as a buffer against the force of waves.

Patch reefs are smaller, isolated structures within lagoons that rise above the bottom in the form of hills and ridges.

A reef flat is a relatively flat limestone plain that extends from the reef crest toward the shore. During low tide, this shallow platform is often exposed. The roughness of the reef flat is due to the accumulation of broken coral fragments, sand, and other sediment over time.

Cay reefs (sand bar reefs) are relatively small, flat reefs located on shallow-water plateaus. Low-lying sand islands and spits form on their leeward side.

Ribbon reefs are reef ridges that extend for several kilometers in length and have a width of 50 to 100 meters. Some linear reefs have a lagoon-like feature on their leeward side. This lagoon is a depression filled with debris and sand. A zone of patch reefs frequently bordered this lagoon on the leeward side. Linear reefs typically form in areas with strong currents, which influence their shape.

Planular reefs are flat reefs with a highly eroded flat that dries out during low tide and are overgrown with macrophytes, corallines, and seagrasses. The flat of these reefs has steep outer slopes. On the surface of the flat, there are often ridges of sediment, sandbars, and small islands. Planular reefs typically form in areas with calm waters.

Coral banks are reef-like structures, sometimes quite extensive, with an undefined mound-like shape that is almost entirely submerged. In shallow-water coral banks, only small portions of their surface reach the sea level. Deep-water coral banks are common on the continental slope in temperate and subpolar regions at depths ranging from 60 to 1,500 meters.

Pinnacle reefs are isolated, mound-shaped structures created by the accumulation of marine organisms like corals, sponges, and algae. Found in carbonate environments with rapid water depth changes, they form solid, porous structures over millions of years. Their conical shape can trap oil and gas, preventing leakage due to surrounding sedimentary layers.

Reef Builders

Reef-forming organisms extract calcite from seawater to build their skeletons. The resulting accumulations of living and dead reef builders create coral limestone, which forms the reef structures.

Throughout different epochs in Earth’s history, various organisms played the role of primary reef builders. These organisms thrived in diverse ecological conditions, and as these conditions changed, reefs either expanded widely or significantly reduced their distribution. However, reefs have always been hubs of marine life. As early as the Cambrian period, approximately half of all marine animals lived on reefs.

In shallow, but dynamic waters, reef builders include colonial stromatoporoids with layered, lace-like skeletons, corals, and bryozoans. In stagnant waters, below the wave action zone, communities of calcimicrobes and siliceous sponges dominate. Throughout the Phanerozoic, there was an alternation between organic buildups formed by these organisms—carbonate fossil reefs and mud mounds.

Precambrian

Archean

In the Paleoarchean, 3.4 billion years ago, the first organogenic structures in history—stromatolites—appeared, developing on the shelves of warm seas. Communities of blue-green algae and bacteria depositing thin calcareous layers on the surface of stromatolite mats, one above the other, form such structures. There are also large colonies, which may create bioherms.

Their growth was very slow, but it proved to be a highly successful evolutionary innovation, and stromatolites still exist today, albeit only in specific conditions. At that time, as the sole creators of organogenic structures, they built massive formations over 2,500 kilometers long, surpassing even the modern Great Barrier Reef. In the deep parts of the ocean, they formed enormous cones, while in shallow waters, they covered the seabed with crusts.

Algal reefs formed by stromatolites are the oldest known bioherms. From their first appearance and for the next 2.5 billion years, microbialites produced only stromatolites, reflecting their cyanobacterial origin.

Proterozoic

Many stromatolitic formations qualify as true reefs. Fossil stromatolitic reefs could grow from deeper, calmer waters upward into the shallow, wave-active zone. They continued to grow inside the wave zone and expanded in the transverse direction to significant sizes.

Cement-rich stromatolites make up Paleoproterozoic (1.8–1.6 Ga) structures, where carbonate sediments dominate over microbial activity. Stromatolite forms show little variation, primarily including hemispherical, columnar, layered, and conical types. Isolated formations are rare, with most integrated into platforms and ramps as biostromes.

Mesoproterozoic Reefs

Mesoproterozoic reefs experienced a prolonged period of stagnation, with stromatolites largely resembling older formations. However, they are more silt-laden and exhibit an increasing diversity of stromatolite forms. It appears that the roles of abiotic sedimentation and microbial influence were approximately equal in importance.⁴

Riphean

Stromatolitic formations became widespread on continental shelves during the Riphean period, 1,700–670 million years ago (Newell, 1972). Their thickness reached several dozen meters, and their length spanned several hundred meters (Hoffman, 1971). Such formations occur in Proterozoic deposits in North America, Eurasia, and Africa.

Neoproterozoic

The decline of stromatolites at the end of the Proterozoic coincided with an increase in the abundance and diversity of thrombolites and calcified cyanobacteria, as well as the emergence of multicellular animals. This development led to the appearance of the first metazoan reefs in the late Neoproterozoic.⁵

Vendian

Calcareous algae Renalcis and Korilophyton formed the oldest organic buildups of the Vendian period. These structures reached heights of 10–15 cm and widths of 2–3 meters, typically occurring as isolated formations and only occasionally creating larger masses. During this time, the number of stromatolitic formations decreased sharply because fast-growing skeletal organisms displaced stromatolitic bacterial communities into environments with abnormal salinity or strong currents.

Associations of Vendian organisms such as Cloudina and Namacalathus were the earliest multicellular animals with calcified skeletons. However, they also inhabited ecosystems of stromatolitic reefs, alongside sessile, colonial animals capable of creating limited topographic relief, including reefs as long as 7 kilometers and as wide as 300 meters.

Early Paleozoic (Cambrian–Silurian)

During the Early Paleozoic, the first significant reef formations emerged.

During this period, reefs mainly developed in shallow marine environments, typically in warm, tropical waters. Abundant sunlight allowed photosynthesizing organisms to thrive. These environments frequently had low sedimentation rates, resulting in clearer water that facilitated coral growth.

In the Paleozoic, calcareous algae, including red algae, constructed most of the reef frameworks, making up to 70% of the volume. Sponges, bryozoans, and only then corals (tabulates and heliolitids, and to a lesser extent rugosans) followed. Tabulates were true corals, although they had the simplest calcareous skeletons.

In the early Paleozoic, reef-like structures composed of bioherms were widespread. Bryozoans such as Trepostoma and Cystoporata, which had calcareous skeletons, created their skeletal framework (Cuffey, 1977). Ancient bryozoan limestones became widespread, as bryozoans were capable of forming coastal, barrier reefs, and even atolls.

Cambrian

Early Cambrian

At the very beginning of the Cambrian (early Fortunian or Tommotian), the first animals with mineralized skeletons appeared in the Aldan River on the Siberian Platform, among them the archaeocyaths, “ancient cup.” They became the first true reef builders, quickly spreading across the globe. They settled in dense communities at depths of 20–50 meters. Together with algae—bushy calcibionts (Renalcis, Epiphyton, Girvanella)—they created the first bioherms, biohermal masses, and even massive reef systems with steep slopes, although gentler slopes were more typical for them.

By the middle of the Tommotian, additional sessile calcified organisms joined the archaeocyath-cyanobacterial reefs. These included radiocyaths, a variety of simple cup-shaped forms known as “coralomorphs,” globally rare but locally abundant large skeletal tabulate corals, and other cnidarians, as well as stromatoporoid sponges. Possible calcarean sponges also appeared in the early mid-Tommotian. Vertically growing and branching forms of calcibionts became the primary factor in shaping the distinct relief of the reefs. Alongside them, oncolites, trilobites, and brachiopods inhabited the reefs. Ridges of such structures formed entire systems like atolls or barrier reefs up to 10 m thick on the expansive shallow seas of warm oceans.⁶

In the Altai-Sayan region, such organic masses created true fossil reefs, reaching thicknesses of up to 2,000 meters.

In the Early and Middle Cambrian, the earliest polychaetes, Scolithos, which lived in calcareous tubes, also formed reef structures.

The Cambrian reef-building cycle was relatively short—lasting 18–19 million years. The peak of reef formation occurred in the Early Cambrian (Atdabanian–Botomian, also known as Stage 2–Stage 3), when massive constructions emerged across all ancient platforms. These fossil reefs occur in Mongolia, Canada, Australia, Sardinia, southern Spain, northern Morocco, the Altai-Sayan region, and the Siberian Platform.

Boring sponges, known as reef destroyers, were already present by the middle of the Early Cambrian. After the extinction of archaeocyaths in the Toyonian or early Middle Cambrian, reef construction in these early seas virtually ceased until the Ordovician.

Middle–Late Cambrian

After the disappearance of archaeocyaths at the boundary of the Early and Middle Cambrian, reef building ceased for a long time, and only microbial-algal structures arose. The Middle-Late Cambrian and Early Ordovician were eras of bacterial organic buildup. Starting from the Drumian age (Middle Cambrian), spicular sponges (lithistid and axinellid) replaced archaeocyaths in reef ecosystems, forming mud mounds on the outer shelf. Stromatolites once again gained prominence.

Ordovician

Early Ordovician

In the Early Ordovician, true reefs were still absent, but potential reef-builders had already appeared: stromatoporoids in the Tremadocian, bryozoans by the end of the Early to the beginning of the Middle Ordovician, and the first corals (initially tabulates, followed by rugosans) in the Early Floian. However, corals still played a minor role at this time.

In the Early Ordovician, calcareous sponges (Archaeoscyphia) and green calcareous algae (Celathum) formed small, isolated structures that rose above the seafloor as mounds and ridges, also known as patch reefs. During this period, only mud mounds a few meters thick or stromatolitic bioherms up to 5 meters high are known, often inhabited by gastropods, trilobites, and brachiopods.

Middle Ordovician

Reef-like structures of bryozoans are characteristic of the Middle Ordovician. From the late Middle Ordovician (Darriwilian) to the end of the Devonian (Famennian), the Middle Paleozoic phase of reef development occurred. Ordovician reef formation lasted 24–25 million years. Fossil reefs from this period occur in Canada, parts of the United States, the Baltoscandian and British regions, China, the Urals, Kazakhstan, northeastern Siberia, Chukotka, Taimyr, and Australia.

The shelf area expanded, and warm epicontinental seas spread further, creating favorable conditions for reef builders. From the Middle Ordovician onward, stromatoporoids (with their robust calcareous skeletons) became one of the primary reef-building groups. They could form their own colonies but often settled on the shells of large mollusks or on colonies of tabulate corals, which also contributed to reef construction. Bryozoans acted more as reef-building assistants, though they could dominate in bioherms. Rugosans (Tetracorallia) and lithistid sponges played a lesser role.

In the Middle Ordovician (West Virginia), algal reef mounds formed in the shallow carbonate deposits of the Great American Bank, growing up to 14 meters thick. However, frame-builders were absent in these structures.

Late Ordovician

The oldest fossil reefs from the Late Ordovician (Caradoc) are clearly visible today. They are often composed of fine-grained limestones containing siphonous green algae, red algae (Solenopora), sponges, bryozoans, and less frequently tabulate corals and thecoids. By the Ashgill age (late Katian), their thickness had already reached 150–500 meters. Their inhabitants included a significant number of crinoids, ostracods, receptaculitids, and cephalopods. Many of these fossil reefs developed in seas dotted with numerous volcanic islands and shoals.

Overall, Ordovician organic buildups reached greater thicknesses than Cambrian reefs and featured a higher diversity of contributing organisms.

At the end of the Late Ordovician, glaciation caused widespread regression. This led to a sharp drop in sea levels, the drying of many shallow basins, and the collapse of fossil reefs communities. Stromatolitic formations and oolitic limestones, indicative of sedimentation under regressive conditions, became widespread.

Silurian

In the Silurian, reefs were more widespread than in the Ordovician, as the transgression increased the number of shallow-water basins. The Silurian-Devonian reef formation cycle lasted for 69 million years. It was the most extensive cycle, with a decline in the early Devonian (Lochkovian–Pragian) and ended at the Frasnian-Famennian boundary. Reefs of this stage are distributed almost all over the globe: East European (Russian) Platform and the Urals, North American Platform and the Cordillera, Volhynia and Podolia, Kazakhstan, Southern Tian Shan, Altai-Sayan and Verkhoyansk-Kolyma regions, Chukotka, Mongolia, China, and Australia.

At the beginning of the Early Silurian, a powerful transgression created new shallow-water basins, leading to the expansion of reef community ranges. The thickness of the Altai Silurian reef massifs reached 400–450 meters, and their length extended up to 100 kilometers. In the Early Silurian (Early Rhuddanian), a reef in North America (Michigan) had a length of over a thousand kilometers. It was a local island area within the Paleozoic North American continent. The total length of the Late Silurian (Ludlow) reef belt in southeastern Alaska exceeds 800 kilometers.

The reef builders include algae, stromatoporoids, tabulates, and to a lesser extent, rugosans and bryozoans. Algae and microbial communities were important suppliers of material and its cementation. Brachiopods, crinoids, and receptaculites inhabited the reefs.

Associations of stromatoporoids and corals created the skeletal framework of reef structures. This was typical of Silurian reefs. The growth of rugosans and tabulates occurred simultaneously with stromatoporoids, forming a single colony. The structures were filled with calcareous material produced by blue-green and green algae, as well as various invertebrates: bryozoans, sponges, and echinoderms. Large Silurian reefs rarely contain bryozoans.

In areas of intensive reef formation, the thickness of the complex could reach 1500 meters. The reef belt that began in the Silurian in the Northern Urals lasted until the end of the Middle Devonian, spanning 40–50 million years. During the Late Silurian (Ludlow), afrosalpingoids emerged in stromatolite bioherms.

At the end of the Silurian and the beginning of the Devonian, there was a slight decrease in the rate of reef formation, caused by regression and cooling.

1. Heindel et al., 2018. The formation of microbial-metazoan bioherms and biostromes following the latest Permian mass extinction. ↩︎
2. Epeiric is a shallow sea that covers a large part of a continent while remaining connected with the ocean. ↩︎
3. The term “Faroes” mentioned in the context of coral reefs refers to cold-water coral reefs with a unique marine ecosystem found in the deep waters surrounding the Faroe Islands, an archipelago in the North Atlantic. ↩︎
4. Grotzinger, James. Precambrian carbonates evolution of understanding. ↩︎
5. Rachel Wood, 2003. The ecological evolution of reefs.  ↩︎
6. Yu. I. Sorokin, 1990. Ecosystems of coral reefs. ↩︎