Small shelly fauna

Small shelly fauna are tiny fossils, often only a few millimetres long. They help scientists learn about life from very long ago. These fossils come from the latest part of the Ediacaran and through the Early Cambrian Period. They are made of minerals.

Many of these fossils are whole skeletons of small creatures, like Cloudina. But most are pieces of larger animals, such as sponges, molluscs, halkieriids, brachiopods, echinoderms, and onychophoran-like organisms.

Scientists used to think these fossils appeared because there was more calcium in the ocean. But since many are made of other minerals like silica, it seems they evolved as part of an evolutionary arms race. Mineral skeletons might also have been easier to make. These tiny fossils are important because they show how groups of marine animals evolved, especially during the Cambrian explosion. They include the earliest members of some modern animal groups.

History of discovery

The term "small shelly fossils" was created by Samuel Matthews and V. V. Missarzhevsky in 1975. People often call them "small shellies" or "SSF". But this name isn’t perfect because these fossils aren’t always small or made from shells. Paleontologists haven’t found a better name yet.

Many 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 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 learned about them.

At that time, scientists were talking about how animals first evolved. Some, like Preston Cloud, thought evolution happened very quickly. Others, like Niles Eldredge and Stephen Jay Gould, thought there were long periods with little change, followed 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 found in China, Russia, Mongolia, Kazakhstan, Australia, and Antarctica. Other places with these fossils include India, Pakistan, Iran, Europe, and North America. These fossils are often found together with well-known fossils such as trilobites. Many of these small shelly fossils disappeared by the end of the Cambrian period, but some groups like the halkieriids and wiwaxiids survived.

Mode of preservation

Small shelly fossils are often found 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. The most common are silica, calcium phosphate, and calcium carbonate. The minerals an animal uses depend on the ocean's chemistry when it first appeared.

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. 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

Some fossils of Cloudina have holes that might have been made by other animals looking for food. A similar fossil called Sinotubulites, found nearby, does not have these holes. This shows that new animal species might have appeared because they were being hunted.

Small shelly fossils help us learn about what happened during the Cambrian explosion. Some of these fossils can be linked to modern animal groups or their close relatives. This helps scientists understand how animals changed over time. The earliest small shelly fossils are simple, with two layers of tissue. Later ones have three layers, like most animals today. The Helcionellids might belong to the group that includes mollusca. By the Atdabanian, some fossils are in the same group as modern echinoderms. This shows that animal groups appeared one after another.

Types of small shelly fossil

Further information: List of small shelly fossils

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

In the early Cambrian, these fossils became more common and varied. 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 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. 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 start to look more like modern groups. By the mid-Ordovician, many of these fossils are thought to be young molluscs, especially types like gastropods.