Graptolite

Graptolites are a group of colonial animals that lived long ago. They belong to a special group called Graptolithina within the class Pterobranchia. These animals were filter-feeders, meaning they ate tiny particles from the water. We mostly know about them from fossils found from the Middle Cambrian to the Lower Carboniferous periods.

One possible early graptolite, named Chaunograptus, comes from the Middle Cambrian. Scientists think that a living pterobranch animal called Rhabdopleura might actually be a type of graptolite that has survived until today, diverging from other graptolites in the Cambrian.

Fossil graptolites and Rhabdopleura share a special colony structure made of interconnected parts called zooids inside organic tubes. Most extinct graptolites belong to two main groups: the bush-like Dendroidea and the floating Graptoloidea. These groups probably evolved from encrusting pterobranchs similar to Rhabdopleura. Because graptoloids were very common, lived in the water column, and show clear changes over time, they are useful index fossils for studying the Ordovician and Silurian periods.

The name graptolite comes from Greek words: graptos meaning "written" and lithos meaning "rock". This is because many graptolite fossils look like hieroglyphs written on rock. The famous scientist Linnaeus first thought they were just pictures that looked like fossils, but later scientists thought they might be related to hydrozoans. Today, they are known to be hemichordates.

History

The word "graptolite" comes from an old name, Graptolithus, meaning "writing on the rocks." It was first used by a scientist named Linnaeus in 1735 for certain rock markings that looked like fossils. Over time, scientists realized graptolites were a special group of colonial animals, different from other similar creatures like bryozoans and hydrozoans. Today, we know graptolites are closely related to some living animals called pterobranchs.

Morphology

Graptolites are colonial animals made up of many tiny individuals called zooids. These zooids live in tubes that are connected together by thin threads called stolons. The whole group of tubes forms a structure called a tubarium. Early in their life, the tubarium branches out into several arms, and the tubes are arranged in rows. These features help scientists identify different types of graptolite fossils.

Each zooid has a head-like region, a collar, and a trunk. The collar contains the mouth, anus, and arms with tentacles. Graptolites had a simple nervous system, with a group of nerves similar to the neural tube in some modern animals. While we know a lot about their tube structures, the soft parts of graptolite bodies are still a mystery to scientists.

Taxonomy

Further information: List of graptolite genera

Graptolites are closely related to modern marine animals called pterobranchs. Scientists use special tools to study them and learn about their family tree. Graptolites are mostly found as fossils and come in two main types: benthic, which lived on the ocean floor, and planktic, which floated in the water. The planktic type is very diverse and includes many different groups that evolved over time.

Ecology

Graptolites were important members of early Paleozoic ecosystems, especially as zooplankton. They likely fed by straining water for food such as tiny plants and animals.

Based on studies of similar modern animals, graptolites could move up and down in the water to find food and avoid danger. Some types lived near the surface, while others preferred deeper water. While scientists aren't sure exactly how they moved, ideas include floating, swimming, or using special body parts to push through the water. Graptolites that stayed in one place could still catch food particles using their arms and tentacles near their mouths.

Life cycle

The life cycle of graptolites starts with a tiny, swimming larva produced by sexual reproduction. This larva settles down and becomes the first member of a new colony, called the sicular zooid. In living relatives like Rhabdopleura, fertilized eggs are kept safe inside the mother until they develop into swimming larvae, ready to start their own colonies.

As the colony grows, new members are added through asexual reproduction. This happens at the tip of a special zooid, where new zooids bud off and form their own tubes, helping the colony to expand.

Graptolites in evolutionary development

Living graptolites help scientists study how animals change over time. They are especially useful for learning about asymmetry, which means things are not the same on both sides of the body. For example, in one type of graptolite called Rhabdopleura normani, the testicle is found on just one side of the body. Scientists are interested in this because it might help explain asymmetry in early vertebrates.

Researchers also studied a gene called Hedgehog in graptolites. This gene plays a role in how nerves develop. They found that this gene works differently in graptolites compared to other similar animals. These differences might give graptolites some special advantages that have lasted for millions of years.

Geological relevance

Graptolites are common fossils found all over the world, especially in shales and mudrocks. These rocks form in deep water where there is little oxygen and few scavengers, helping to preserve the graptolites well. They can also be found in limestones and cherts, but these rocks often have more life that might eat the graptolites.

Graptolites are important for dating rocks because they change slowly over time and are found everywhere. They help scientists divide rocks from the Ordovician and Silurian periods into smaller time sections. Some big events, like ice ages, caused many graptolites to disappear, but some survived and evolved into new forms.

Researchers

Many scientists have studied graptolites and pterobranchs. Some well-known researchers include Joachim Barrande, James Hall, and Carl Wiman. Others who made important contributions are Hanns Bruno Geinitz, Frederick M'Coy, and Henry Alleyne Nicholson. These scientists helped us learn more about these fascinating ancient creatures.