Graptolite

Graptolites are an interesting group of animals that lived long ago. They belong to a group called Pterobranchia and mostly ate tiny particles from the water. We know about them from fossils found from the Middle Cambrian to the Lower Carboniferous periods.

These animals lived in groups inside tiny, connected rooms called zooids. They were protected by special tubes. There were two main types: one that stayed on surfaces and looked like bushes, and another that floated freely in the water. Scientists use these floating graptolites to help date rocks from the Ordovician and Silurian periods because they were common and changed over time in clear ways.

The word "graptolite" comes from Greek words meaning "written on rock." This is because their fossils often look like ancient writing carved into stone. At first, even famous scientists like Linnaeus thought they were just pictures, not real fossils. Today we know they were living animals related to a group called hemichordates.

History

The word "graptolite" comes from an old name, Graptolithus, meaning "writing on the rocks". A scientist named Linnaeus first used this name in 1735 for special markings in rocks that looked like fossils. Scientists later learned these markings were fossils of unique colonial animals.

At first, these fossils were grouped with other sea creatures. But then they were recognized as their own special group. Today, we know they are related to some sea animals that are still alive. The old name Graptolithus is no longer used by scientists.

Morphology

Graptolites are animals that lived together in groups. Each small part, called a zooid, lived in a tube. All the tubes connect to make one shape, called a tubarium. The tubes are made by special parts on the zooid and are strong.

Early graptolites stuck to the ocean floor. Later ones could float in the water. They had many shapes, like flat or branching. These creatures were common in ancient seas until they disappeared in the early Devonian period.

Taxonomy

Further information: List of graptolite genera

Graptolites are ancient sea creatures. Scientists think they are related to a rare group of modern sea animals called pterobranchs. These creatures live in colonies, meaning they stick together in groups. They are part of a group called Graptolithina.

One big difference between graptolites and another similar group, Cephalodiscida, is that graptolites live together in colonies, while Cephalodiscida do not. Graptolites come in different types. Some lived on the sea floor, and others floated in the water. The floating group has many kinds, including Diplograptids and Neograptids. These were very common during a time called the Ordovician.

Ecology

Graptolites were important in early ocean ecosystems. They were part of tiny animals that float in water, called zooplankton. They likely ate small particles, such as plankton, from the water.

Scientists think graptolites could move up and down in the water to find food and stay safe. Some graptolites lived near the surface, while others lived in deeper water. Exactly how they moved is still a mystery, but they may have floated, swam, or used special body parts to move.

Life cycle

The life cycle of graptolites starts with a tiny, floating larva. This larva grows into a special zooid that begins a new colony. In modern relatives, called Rhabdopleura, fertilized eggs are kept safe inside the parent until they become larvae. The larvae then swim away to start their own colonies.

As the colony grows, new zooids form from the tip of a special zooid through a process called budding. This helps the colony grow bigger.

Graptolites in evolutionary development

Living graptolites help scientists study how animals change over time. They are useful for learning about how animals develop. In some graptolites, the parts that make new animals are found on just one side. This can help us understand how patterns in early animals began.

Scientists also study special instructions inside these animals, called genes. One important instruction called "hedgehog" works differently in graptolites than in some other animals. This difference might give graptolites an advantage, but scientists are still learning why.

Geological relevance

Graptolites are common fossils found all over the world. They are often found in shales and mudrocks. These rocks form in deep water where few animals live, which helps preserve the graptolites well. They can also be found in limestones and cherts, but these rocks usually have more animals.

These fossils are useful for dating rocks because they change slowly over time and are found everywhere. Geologists use them to divide rock layers into sections that help tell the age of the Earth. Some big changes in Earth's climate affected graptolite numbers, but many graptolite fossils remain and help scientists understand the past.

Researchers

Many scientists have studied graptolites and pterobranchs over the years. Some well-known researchers include:

• Joachim Barrande (1799–1883)
• Hanns Bruno Geinitz (1814–1900)
• James Hall (1811–1898)
• Henry Alleyne Nicholson (1844–1899)
• John Hopkinson (1844–1919)
• Sven Axel Tullberg (1852–1886)
• Carl Wiman (1867–1944)
• Thomas Sergeant Hall (1858–1915)
• Noel Benson (1885–1957)
• Vladimir Nikolayevich Beklemishev (1890–1962)
• Michael Sars (1805–1869)
• George Ossian Sars (1837–1927)
• William Carmichael M'Intosh (1838–1931)
• Nancy Kirk (1916–2005)
• Roman Kozłowski (1889–1977)
• Gertrude Elles (1872–1960)

Other researchers who have helped us learn more about these interesting creatures include Jörg Maletz, Denis E. B. Bates, Alfred C. Lenz, Chris B. Cameron, Adam Urbanek, David K. Loydell, and Hermann Jaeger (1929–1992).