GEOLOGY | FOSSILS | key fossil groups
Graptolites belong to the phylum Hemichordata and class, Graptolithnia, and are an extinct group of marine colonial organisms. They have a thin notochord, suggesting that they may be a primitive ancestor of chordates. Fossil graptolites are often found in shales, rocks laid down in low-energy, deep marine sediments in oxygen-poor conditions. as death assemblages. They are fine, often shiny, etchings on rock surfaces. The fossils resemble pencil sketches, and their name derived from the Greek graptos, meaning 'written' and lithos, meaning 'rock', reflects this.
These intricate markings are the remains of a colonial organism, some of the fossilised skeleton (rhabdosome) accommodated up to 5 000 individual animals (zooids). The skeletal structures were made of collagen.
They existed in the Palaeozoic, from the early Cambrian to the Carboniferous, about 520 to 350 million years ago.
Graptolites were colonial animals that lived in an interconnected system of tubes. From an initial ’embryonic’, cone-like tube (the sicula), subsequent tubes (thecae) are arranged in branches (stipes) to make up the whole colony (rhabdosome). Each individual animal is called a zooid.
Image: Fossil graptolite, Hunneberg, Sweden
Based on similarities with modern Hemichordates such as the worm-like pterobranchs, it is thought that zooids lived in and constructed tubes called thecae.
The first zooid secreted a conical tube called a sicula in which it lived, usually with a long tapering point. Subsequent members of the colony built thecae that were stacked one on top of another, anchored to the sicula. The overlapping stack of thecae is known as a stipe.
As graptolites are extinct and our only evidence comes from interpreting the rock record, it is difficult to ascertain, with certainty, their mode of life and taxonomic affinities. This is partly because the fossils of this significant age, are found as flattened, two-dimensional specimens. Extremely rare three-dimensional graptolite fossils have allowed palaeontologists to reconstruct the rhabdosome, suggesting that it was a colonial organism.
The earliest graptolites (dendroids) were benthonic and sessile, attached to rocks or forming upright cones rooted in the sea-floor substrate.
At the start of the Ordovician graptoloids lived in the water column and were probably pelagic, they were amongst the first animals to have this mode of life. Species evolved and diversified rapidly in order to exploit food in the form of planktonic organisms and to overcome the challenges of floating free in the water column.
Graptolites were common in food-rich environments, for example locations where upwelling currents loaded with nutrients (particularly tropics and at the edge of the continental shelf). Some species of graptolite were deep-water specialists, whilst others were opportunists seeking out temporary supplies of food. Studying the distribution of fossil graptolites, enables geoscientists to reconstruct the palaeo-oceans in which they lived.
Graptolites rapidly evolved and diversified, developing a range of hydrodynamic strategies. Long nemas evolved, possibly to anchor the colony to a floating object. Hooked, spiny and net-like forms appeared, increasing drag in the water so that they moved slowly. Most graptolites appear to have had a planktonic mode of life. However, it is possible zooids may have enabled the colony to move, actively swimming.
Experimental models of graptolites spiral as they sink, which would have improved active filter feeding. It is not clear how they rose back up through the water column, feasibly zooids may have secreted gas or low-density fat to help the colony to rise through the water.
Image: Didymograptus bifidus , from the Ordovician of South Wales, UK. © UKRI.
Biserial: thecae are added on two sides of the stipe.
Horizontal means that the thecae are added outwards from the sicula.
Pendent describes morphology when the thecae are added downwards
from the sicula.
Uniserial means that the thecae are added on one side of the stipe.
The term 'reclined' is used when the the thecae grow upwards or sideways.
Scandent is used to describe when the thecae are growing vertically upwards from the sicula.