Small shelly fauna, small shelly fossils (SSF), or early skeletal fossils (ESF) are types of mineralized fossils. Many are only a few millimeters long and appear in a nearly continuous record from the late Ediacaran period to the end of the Early Cambrian period. These fossils are very diverse, and there is no official definition for "small shelly fauna" or "small shelly fossils." Most of these fossils are found in older rocks than more familiar fossils like trilobites. Since many SSFs were preserved by being quickly covered in phosphate, a method that mainly happened during the late Ediacaran and early Cambrian periods, the animals that made them may have existed before and continued after this time.
Some SSFs show the complete skeletons of small organisms, such as the mysterious Cloudina and snail-like molluscs. However, most SSFs are broken pieces or parts of larger organisms, including sponges, molluscs, slug-like halkieriids, brachiopods, echinoderms, and onychophoran-like organisms that may be related to the ancestors of arthropods.
One early idea for why SSFs appeared—suggesting the evolution of mineralized skeletons—was a sudden increase in ocean calcium levels. However, many SSFs are made of other minerals, like silica. Since the first SSFs appear around the same time as organisms began burrowing to avoid predators, it is more likely that they mark early steps in a competition between predators and prey. On the other hand, mineralized skeletons may have developed because they are stronger and require less energy to create than all-organic skeletons, like those of insects. Still, the animals used the minerals that were easiest to access.
Although the small size and often broken nature of SSFs make them hard to identify and classify, they provide important evidence about how major groups of marine invertebrates evolved, especially the pace and pattern of evolution during the Cambrian explosion. In addition to showing the earliest known examples of some modern animal groups, SSFs are valuable because they offer a nearly continuous record of early Cambrian organisms with hard body parts.
History of discovery
The term "small shelly fossils" was created by Samuel Matthews and V. V. Missarzhevsky in 1975. This term is often shortened to "small shellies" or "SSF." However, this name is misleading because, as Stefan Bengtson explains, "they are not always small, they are commonly not shelly—and the term might equally well apply to Pleistocene periwinkles." Scientists have not found a better term to describe these fossils and have expressed their frustration through jokes like "small silly fossils" and "small smellies."
Most of the physical characteristics seen in later shelled animals first appear among the SSFs. No official definition has been created for phrases like "small shelly fauna" or "small shelly fossils."
Fossils of this type, sometimes in large collections, were found between 1872 and 1967. However, scientists did not realize that the Early Cambrian period had a wide variety of animals beyond the traditionally known trilobites and archaeocyathans. In the late 1960s, Soviet paleontologists discovered even more SSFs in rock layers older than those containing Cambrian trilobites. Unfortunately, the reports of these findings were written in Russian, and the 1975 paper by Matthews and Missarzhevsky was the first to introduce SSFs to scientists who did not read Russian.
At the time, scientists were already discussing how animals evolved early in Earth's history. Preston Cloud argued in 1948 and 1968 that the process was "explosive," meaning it happened quickly. In the early 1970s, Niles Eldredge and Stephen Jay Gould proposed the theory of punctuated equilibrium, which suggests that evolution occurs in long periods of little change interrupted by short bursts of rapid change. Meanwhile, Wyatt Durham and Martin Glaessner, around the same time, believed that animals had a long history during the Proterozoic era, but this was hidden because few fossils from that time were found.
Occurrence
Rich collections of small shelly fauna (SSFs) have been found in China, Russia, Mongolia, Kazakhstan, Australia, and Antarctica. Moderately varied collections have been discovered in India, Pakistan, Iran, Europe, and North America. Scientists have different opinions about when SSFs existed. Russian discoveries from the late 1960s were linked to the Tommotian age of the Cambrian period, and for a time, the term "small shelly fauna" was used only for that age. However, Bengston included Ediacaran fossils like Cloudina and post-Tommotian fossils like Microdictyon from the Maotianshan Shales lagerstätte in the SSF category. SSFs have been found in layers that also contain fossil trilobites. A mass extinction at the end of the Cambrian period's Botomian age was thought to have eliminated most SSFs, except for the halkieriids, wiwaxiids, and Pojetaia.
Mode of preservation
Small shelly fossils are often, but not always, preserved in phosphate. Some of these fossils were originally made of phosphate, but in most cases, the phosphate replaced the original calcite. Scientists usually extract these fossils from limestone by placing the limestone in a weak acid, such as acetic acid. After the rock dissolves, the phosphatized fossils remain. Phosphate preservation of microfossils became less common after the early Cambrian period, possibly because burrowing animals disturbed the seafloor more often. Without this preservation method, many small shelly fossils might not have been saved or could not have been removed from the rock. This suggests that the animals that created these fossils may have lived beyond the early Cambrian period. The apparent disappearance of most small shelly fossils by the end of the Cambrian period may not be real. For many years, scientists believed that halkieriids, which have "armor plates" that are a common type of small shelly fossil, died out during the end-Botomian mass extinction. However, in 2004, halkieriid armor plates were found in Mid Cambrian rocks in Australia, which are about 10 million years younger than the end-Botomian extinction event.
Minerals used in shells
Small shelly fossils are made of different minerals, including silica, calcium phosphate, and calcium carbonate. The type of mineral each organism uses depends on the chemical makeup of the ocean where it first developed. Even if ocean chemistry changes later, organisms often continue using the same minerals. For example, during the Ediacaran period and the Nemakit–Daldynian stage of the Cambrian, animals that used calcium carbonate made their shells from a form called aragonite. In contrast, animals that first appeared in the Tommotian age used a different form, called calcite.
A recently discovered modern gastropod that lives near deep-sea hydrothermal vents shows how both past and present chemical conditions in its environment affect its structure. Its shell is made of aragonite, a material also found in the earliest fossil mollusks. However, it also has armor plates on its foot, which are made of iron sulfides called pyrite and greigite. These materials were never found in any other animal before, but they are released in large amounts by the vents.
The ways organisms build their shells differ greatly among small shelly fossils. In most cases, the exact processes used to create these shells are not fully understood.
Evolution of skeletons and biomineralization
Biomineralization is the process by which living things create parts made of minerals. Scientists have several ideas about why this process evolved. One idea is that it helped organisms adapt to changes in ocean chemistry. Another is that it helped protect them from predators. A third idea is that it allowed them to grow larger. The roles of biomineralization in small shelly fossils (SSFs) vary. Some SSFs are not yet fully understood. Others are parts of armor, and others are skeletons. A skeleton is any fairly stiff structure in an animal, whether or not it has joints, and whether or not it is made of minerals. Although some SSFs may not be skeletons, all SSFs are biomineralized by definition because they are made of shells. Skeletons offer many benefits, such as protection, support, attachment to surfaces, a base for muscles to work from, help with movement, food handling, spaces for filtering water, and storage of important substances.
Some scientists think biomineralization began when ocean water had more calcium, which happened around the Ediacaran–Cambrian boundary. They suggest that storing extra minerals safely was a key benefit, as too much calcium or other minerals could harm an organism’s body. For example, Mikhail A. Fedonkin proposed that longer food chains might have increased the amount of waste and toxins in larger animals, and biomineralization could have helped remove excess carbonates or silicates from their bodies. However, creating a mineral-based skeleton is costly, as most of the expense comes from the organic materials, like proteins and sugars, that combine with minerals to form strong structures. The idea that ocean chemistry changes caused biomineralization is also challenged by the fact that small shelly fossils made of calcite, aragonite, calcium phosphate, and silica appeared at the same time in many different environments.
Around the same time, organisms began burrowing to avoid being eaten. Jerzy Dzik suggested that biomineralized skeletons were a way to protect against predators, starting an evolutionary competition. He noted that early protective "skeletons" sometimes included glued-together inorganic materials, like the early Cambrian worm Onuphionella, which built a tube covered in mica flakes. This strategy required both physical changes to collect and attach materials and a somewhat advanced nervous system to control the behavior.
Bernard Cohen argued that biomineralized skeletons evolved for practical reasons, not just defense. Other defense methods, like speed, strong senses, chemicals, or hiding, were already available. Mineral-organic composites are stronger and require less energy to build than all-organic skeletons. These advantages allowed animals to grow larger and, in some cases, have stronger muscles. For animals larger than a certain size, the force from strong muscles and their leverage would break all-organic skeletons. Modern brachiopods show a shift from all-organic to mineral-organic shells, which may hint at their evolutionary path. The development of rigid biomineralized exoskeletons may have led to a competition where predators evolved tools like drills or chemicals to break shells, and some prey evolved tougher shells in response.
Fedonkin also suggested that the Ediacara biota, which lived in cold waters, had slow metabolisms and lacked the energy needed for biomineralization. However, global warming at the start of the Cambrian may have made biomineralization easier. A similar pattern is seen in today’s marine animals, as biomineralized skeletons are less common and more fragile in cold polar waters compared to warmer tropical regions.
Evolutionary significance
In some areas, up to 20% of Cloudina fossils have holes that may have been made by predators. A similar fossil called Sinotubulites, often found in the same places, does not have these holes. The size of the holes in Cloudina suggests that predators targeted larger shells. This indicates that predation might have led to the development of new species, a possible reason for the quick increase in animal diversity during the early Cambrian.
Small shell fossils provide a continuous record from the early Cambrian, offering better insights into the Cambrian explosion than fossils with exceptional preservation. While many small shell fossils are hard to identify, those linked to modern animal groups or their evolutionary relatives help scientists study how animals evolved. The earliest small shell fossils are the most basic, and as time progresses, they fit into smaller evolutionary groups. Early Ediacaran small shell fossils are likely diploblastic, meaning they have two tissue layers. Later fossils are more clearly triploblastic, like all more complex animals. The first shelly fossils that can be placed in the mollusk group are the Helcionellids. As the fossils get more recent, evidence for their classification becomes stronger. By the Atdabanian period, some small shell fossils can be placed in the modern echinoderm group. This shows that the earliest Cambrian animals were early members of later groups, with the major animal groups appearing in a clear and orderly way over time, rather than suddenly, revealing the actual speed of the Cambrian explosion.
Types of small shelly fossil
Few SSF from the Ediacaran period show a limited variety of forms. Fully and partially mineralized tubes are common, creating a mixed collection. The structures and compositions of their walls vary widely. These fossils have been classified into many groups, including foraminiferans, cnidarians, polychaete and pogonophoran annelids, sipunculids, and others. Cloudina’s "tube," which was 8 to 150 millimeters long, had nested cones made of calcium carbonate with unmineralized gaps between them. Sinotubulites built long, thin, flexible tubes that likely had mineralized ridges.
Namapoikia was probably a sponge or coral-like organism that built dwellings up to 1 meter across using calcium carbonate. Spicules are spines or star-like groups of spines made of silica. These are thought to be remains of sponges. Namacalathus, which may have been a cnidarian related to jellyfish and corals, built goblet-like dwellings with stalks up to 30 millimeters long. This shape is called a "stalked test," as "test" in biology refers to a roughly spherical shell.
In early Cambrian finds, tubes and spicules became more common and diverse, with new types of SSF appearing. Many fossils are linked to well-known groups like molluscs, slug-like halkieriids, brachiopods, echinoderms, and onychophoran-like organisms, which may be related to arthropod ancestors. Problematic tubular fossils, such as anabaritids, Hyolithellus, or Torellella, are found in early Cambrian SSF skeletal assemblages.
Most Cambrian SSF consist of sclerites, which are fragments of external armor from early animals like Halkieria or "scale worms." Rare, complete sets of sclerites are called "scleritomes." Often, the body shapes of sclerites’ creators and how sclerites were arranged on their bodies are unknown. The "coat of mail" usually broke apart after the animal died, with fragments dispersing and sometimes becoming fossils. Reconstructing these elements often depends on finding fully preserved fossils in special rock layers called lagerstätten. These discoveries help scientists understand similar fragments, like those labeled Maikhanella.
Many sclerites are coelosclerites, which have a mineralized shell around a space once filled with organic tissue. There is no clear evidence of growth layers. It is uncertain whether coelosclerites evolved independently in different groups or were inherited from a common ancestor. Halkieriids produced scale- or spine-shaped coelosclerites, and complete specimens show they were slug-shaped with cap-like shell plates at both ends. Chancelloriids made star-shaped coelosclerites and looked like cacti. They may have been related to sponges or halkieriids.
Some sclerites are made of calcium phosphate instead of calcium carbonate. Tommotiids have many different sclerite shapes and structures, possibly representing multiple unrelated groups. They may have developed phosphatic scleritomes independently or inherited them from ancestors of modern brachiopods. Brachiopods resemble bivalve molluscs but have fleshy stalks and different internal anatomy. Some sclerites and small pieces of "debris" are thought to be from echinoderms. Other phosphatic sclerites include tooth-shaped, hook-shaped, or plate-like objects of unknown origin. Some, like Microdictyon, were made by lobopods, worm-like animals with legs that may be related to arthropod ancestors.
Univalved and bivalved shells are common. Some cap-shaped shells may be the only sclerite covering their creators, while others are part of more complex armor systems, like Halkieria’s. Helcionellids are thought to be early molluscs with snail-like shells. Some have horizontal "exhaust pipes" on their shells, and scientists debate whether these pointed forward or backward. Hyoliths left small conical shells and may have been molluscs or worm-like Sipuncula. Other univalved mollusc shells have been found in Canada. Some bivalved shells are still joined and include both brachiopods and bivalve molluscs. Fossils resembling snail-like "lids" are linked to hyoliths.
Small arthropods with bivalve-like shells have been found in early Cambrian beds in China. Other fossils, like Mongolitubulus henrikseni, represent spines that broke off arthropod carapaces.
After the Cambrian, SSF include more recognizable and modern groups. By the mid-Ordovician, most SSF represent larval molluscs, mostly gastropods.