Graptolites were extinct marine animals, known from their fossils. These organisms were colonial, living in interconnected groups. Their unique appearance, often resembling hieroglyphs or pencil marks on rocks, has long intrigued scientists and casual observers alike. Studying graptolites offers a window into ancient marine ecosystems and the history of life on Earth.
What Graptolites Were
Graptolites were colonial marine invertebrates, composed of individual animals called zooids. Each zooid resided within a protective casing, a theca. These thecae were interconnected by a common tube, forming the colony’s skeleton, called a rhabdosome. The rhabdosome could exhibit various shapes, from simple, unbranched forms to more intricate, branched or net-like structures, sometimes resembling seaweed or a tuning fork.
The exoskeleton of graptolites was composed of organic material, a protein called periderm, similar to collagen or chitin. This non-mineralized composition is distinctive and aided their preservation. Graptolites are classified within the phylum Hemichordata, a group including modern pterobranchs. This classification is supported by shared features like the colonial structure and feeding arms, though their exact evolutionary relationship is debated.
Where and When They Lived
Graptolites lived in marine environments worldwide, with earliest appearances in the fossil record from the Middle Cambrian Period (542-488 million years ago). While some early forms, known as dendroid graptolites, were sessile and attached to the seafloor, the more common graptoloid graptolites were planktonic, drifting freely in ocean currents. Their planktonic lifestyle led to wide distribution across ancient oceans.
Their diversity peaked during the Ordovician and Silurian Periods (485-419 million years ago), making them abundant and widespread marine zooplankton. They rapidly evolved new forms to exploit food resources in the upper oceans. Although their numbers began to decline after the Silurian, some graptolite lineages persisted into the Early Carboniferous Period (359-318 million years ago).
How They Became Fossils
Graptolite fossilization is distinctive due to their organic skeletons. Unlike many marine organisms with hard shells, graptolites had a chitinous outer covering. This led to their preservation as thin, carbonaceous films or impressions in fine-grained sedimentary rocks, like black shales. Black shales often form in deep, oxygen-poor marine environments where organic matter decomposes less fully.
The organic material compacted into a flattened, two-dimensional film on rock surfaces, often appearing as shiny, dark marks resembling pencil drawings. Most graptolite fossils are flattened, but rare three-dimensional preservation in fine-grained limestones allows reconstruction of their original forms. Studies on modern relatives, pterobranchs, show organic tubes and structures, particularly the pigmented parts, can persist for weeks to months after death, increasing preservation chances. Rapid decay of individual zooids explains why soft body parts are almost entirely absent in the fossil record.
Their Scientific Significance
Graptolites are valuable to paleontologists and geologists, serving as “index fossils” for dating rock layers. Their rapid evolutionary changes and wide geographic distribution make them excellent markers for biostratigraphy, a method to correlate and determine the relative ages of sedimentary rock units across regions. Specific graptolite species can define geological time slices that are short, sometimes less than a million years, providing a high level of accuracy for ancient time scales.
Beyond dating, graptolites offer insights into past ocean conditions and paleogeography. The distribution patterns of different graptolite species can indicate ancient ocean currents, water temperatures, and the arrangement of continents. For example, distinct “Atlantic” and “Pacific” faunal provinces in the Ordovician, with cold and warm water species, help reconstruct ancient ocean circulation. Their presence in specific rock types, like black shales, also suggests periods of low oxygen in ancient marine environments. Studying their evolutionary patterns also contributes to understanding broader evolutionary trends in early marine life.
Their Extinction
Graptolites eventually declined and became extinct, with different groups disappearing at various times within the Carboniferous Period. The planktonic graptoloids, the most diverse and widespread, experienced reductions in diversity and ultimately disappeared in the latest Early Devonian (around 390 million years ago). This decline followed several evolutionary bursts throughout the Ordovician and Silurian.
The more ancient dendroid graptolites, primarily benthic, persisted much longer, surviving until the Late Carboniferous Period (approximately 300 million years ago). Major extinction events, such as the end-Ordovician mass extinction (around 445 million years ago) and other smaller events in the Silurian and Devonian, reduced graptolite diversity. While the exact causes for their final demise are complex, potential factors include reduced planktonic biomass (possibly due to phytoplankton changes) and environmental shifts related to altering sea margins and the rise of land plants.