Small shelly fauna

Small shelly fauna are tiny fossils, often only a few millimetres long, that help scientists understand life from very ancient times. These fossils come from the latest part of the Ediacaran and through the Early Cambrian Period. They are made of minerals and are very diverse, though there is no strict definition of what counts as small shelly fauna.

Many of these fossils are the whole skeletons of small creatures, like the mysterious Cloudina and some snail-like molluscs. But most are pieces or parts of larger animals, such as sponges, molluscs, halkieriids, brachiopods, echinoderms, and onychophoran-like organisms that might be close to the ancestors of arthropods.

Scientists used to think that these fossils appeared because there was suddenly more calcium in the ocean. But since many are made of other minerals like silica, it seems more likely they evolved as part of an evolutionary arms race between animals that were eaten and those that developed better defenses. Mineral skeletons might also have been easier and stronger to make than all-organic ones. These tiny fossils are very important because they give clues about how major groups of marine invertebrates evolved, especially during the busy time known as the Cambrian explosion. They include the earliest known members of some modern animal groups and offer a nearly continuous record of early Cambrian life with hard parts.

History of discovery

The term "small shelly fossils" was created by Samuel Matthews and V. V. Missarzhevsky in 1975. People often shorten this to "small shellies" or "SSF". But this name isn’t perfect because these fossils aren’t always small or made from shells. Paleontologists haven’t been able to come up with a better name.

Many examples of these fossils were found between 1872 and 1967, but people didn’t realize that the Early Cambrian had many different animals besides the well-known trilobites and archaeocyathans. In the late 1960s, Soviet scientists found even more fossils in layers older than those with Cambrian trilobites. However, their papers were written in Russian, so it wasn’t until Matthews and Missarzhevsky’s 1975 paper that the rest of the world took notice.

At that time, scientists were already discussing how animals first evolved. Some, like Preston Cloud, thought evolution happened very quickly. Others, like Niles Eldredge and Stephen Jay Gould, proposed the idea of long periods with little change, broken by short bursts of rapid change. Meanwhile, Wyatt Durham and Martin Glaessner suggested that animals had a long history before the fossils we find today.

Occurrence

Rich collections of small shelly fossils have been discovered in China, Russia, Mongolia, Kazakhstan, Australia, and Antarctica. Moderately diverse finds have also appeared in India, Pakistan, Iran, Europe, and North America. These fossils are found in layers that also contain well-known fossils such as trilobites. While many of these small shelly fossils disappeared by the end of the Cambrian period, some groups like the halkieriids and wiwaxiids managed to survive.

Mode of preservation

Small shelly fossils are often kept safe in a special kind of rock called phosphate. Sometimes, these tiny fossils were made of a material called calcite, but later they changed into phosphate. Scientists can find these fossils by putting limestone rocks in a weak acid, like acetic acid, which dissolves the rock but leaves the fossils behind.

After the early Cambrian time, it became rarer for fossils to be preserved this way. This might be because animals started stirring up the ocean floor more, making it harder for fossils to stay intact. Because of this, many small shelly fossils might have actually lived longer than we think. For a long time, people believed that creatures called halkieriids, known for their armored plates, disappeared at the end of the Botomian mass extinction. However, in 2004, scientists found halkieriid plates in rocks from Australia that are 10 million years newer than previously thought.

Minerals used in shells

Small shelly fossils are made from different minerals, with the most common being silica, calcium phosphate, and calcium carbonate. The minerals an animal uses depend on the chemistry of the ocean when it first appeared, even if the ocean changes later. For example, during the Ediacaran period and the Nemakit–Daldynian age of the Cambrian, animals using calcium carbonate used a form called aragonite. Later, in the Tommotian age, animals used another form called calcite.

A modern gastropod living near deep-sea hydrothermal vents shows how both old and new chemical environments affect shell building. Its shell is made of aragonite, like the earliest fossil molluscs, but it also has armor plates made from iron sulfides pyrite and greigite. These minerals had never been found in any animal before, but they are released in large amounts by the vents.

The ways animals build their shells differ greatly among small shelly fossils, and in most cases, we do not know exactly how they did it.

Evolution of skeletons and biomineralization

Biomineralization is when living things make hard parts using minerals. Scientists have different ideas about why this happened. Some think it was to protect against being eaten, while others believe it helped animals grow bigger or deal with changes in the ocean.

Skeletons can help in many ways, like giving support, protecting the body, or helping muscles work. Some small shelly fossils were part of an animal's armor, while others were actual skeletons. Even though we don't know everything about them, these tiny fossils were made by living things and can tell us a lot about life from long ago.

Evolutionary significance

In some places, up to 20% of Cloudina fossils have holes that may have been made by other animals looking for food. Another similar fossil called Sinotubulites, often found nearby, does not show these holes. The pattern of holes in Cloudina suggests that larger shells were targeted more often. This shows that new animal species might have appeared because they were being hunted, which could explain why many animal groups appeared quickly during the early Cambrian.

The small shelly fossils give us a clear view of what happened during the Cambrian explosion, more so than other special fossil finds like exceptional preservation. Even though many of these fossils are hard to classify, some can be linked to modern animal groups or their close relatives. This helps scientists understand how animals evolved step by step. The earliest small shelly fossils seem to be simple, with two main layers of tissue. Later ones have three layers, like most animals today. The Helcionellids are the first that might belong to the group that includes mollusca. By the time of the Atdabanian, some fossils can even be placed in the same group as modern echinoderms. This shows that animal groups appeared one after another in a clear order, rather than all at once, giving us a better sense of how fast the Cambrian explosion really was.

Types of small shelly fossil

Further information: List of small shelly fossils

The small shelly fossils from the Ediacaran period show a variety of shapes. Many are tubes made of minerals, like Cloudina, which had nested cones made of calcium carbonate. Others, like Sinotubulites, built long, thin tubes with mineralized ridges. Namapoikia might have been a sponge or coral-like organism, creating large structures from calcium carbonate. Spicules, which are star-like spines made of silica, are thought to come from sponges.

In the early Cambrian, these fossils became more common and diverse. New types appeared, including ones linked to groups like molluscs, brachiopods, and early relatives of arthropods. Many fossils from this time are small pieces called sclerites, which were part of the armor of early animals. These pieces often break apart after the animal dies, making it hard to know what the whole animal looked like. Some sclerites have a hollow space inside, and it’s unclear if they evolved separately in different animals or came from a common ancestor.

Some of these fossils are made of calcium phosphate instead of calcium carbonate. Tommotiids have many different shapes of sclerites and might be related to the ancestors of modern brachiopods. Other fossils include shells that look like they belonged to early molluscs, with some having special “exhaust pipes” on their edges. Small arthropods with shell-like covers have also been found, along with spines that broke off from these shells.

After the Cambrian, small shelly fossils begin to resemble more modern groups. By the mid-Ordovician, many of these fossils are thought to be young molluscs, especially types like gastropods.