Graptolite

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

These creatures lived in colonies made of tiny, connected rooms called zooids, protected by organic tubes. There were two main types: one that stayed attached to surfaces and looked like bushes, and another that floated freely in the water. Because they were so common and changed over time in clear ways, scientists use these floating graptolites as important clues to date rocks from the Ordovician and Silurian periods.

The word "graptolite" comes from Greek words meaning "written on rock," 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, but we now 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". It was first used by a scientist named Linnaeus in 1735 for certain rock markings that looked like fossils. Over time, scientists studied these markings and realized they were fossils of special colonial animals.

At first, these fossils were grouped with other sea creatures, but later they were recognized as their own unique group. Today, we know they are closely related to some living sea animals. The old name Graptolithus is no longer used by scientists.

Morphology

Graptolites are colonial animals made up of many small individuals called zooids. These zooids live in tubes that are connected together, forming a structure called a tubarium. The tubes are made by glands on a shield-like part of the zooid and are mostly made of special materials that help them stay strong.

Early graptolites were attached to the ocean floor, but later types could float freely in the water. They came in many shapes, from flat and spreading to bushy and branching. These creatures were very important in the ancient seas and were common until they disappeared in the early part of the Devonian period.

Taxonomy

Further information: List of graptolite genera

Graptolites are ancient sea creatures that scientists think are closely 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, including Dendroidea, which lived on the sea floor, and Graptoloidea, which floated in the water. The Graptoloidea group has many different kinds, including Diplograptids and Neograptids, which were very common during a time called the Ordovician.

Ecology

Graptolites were important members of early ocean ecosystems, especially as part of the tiny animals that float in water, called zooplankton. They likely fed by filtering 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 from animals that might want to eat them. Some graptolites lived near the surface, while others were found in deeper water. Exactly how they moved is still a mystery, but they may have floated, swam, or used special body parts to move through the water.

Life cycle

The life cycle of graptolites starts with a tiny, floating larva produced by reproduction. 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, which 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 allows the colony to expand and create more individuals without needing new eggs each time.

Graptolites in evolutionary development

Living graptolites help scientists study how animals change over time. They are especially useful for learning about how left and right sides work in early animals. For example, in one type of graptolite, the parts that make new animals are found on just one side, which might help us understand how this pattern began in early animals.

Scientists also look at special instructions inside these animals, called genes, to see how they work. One important instruction called "hedgehog" acts differently in graptolites compared to some other similar animals. This difference might give graptolites an advantage, even though scientists are still learning why this happens.

Geological relevance

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

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, like ice ages, 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 contributed to the study of these fascinating creatures include Jörg Maletz, Denis E. B. Bates, Alfred C. Lenz, Chris B. Cameron, Adam Urbanek, David K. Loydell, and Hermann Jaeger (1929–1992).