What Is a Graptolite? Colonies That Date Half the Palaeozoic
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Flattened marks on dark shale that look like pencil scribbles or tiny saw blades are the remains of colonial animals that drifted through Palaeozoic oceans. They are among the most useful fossils in existence because they evolved fast, spread worldwide and turn up in rocks that hold almost nothing else.
What the animal was
A graptolite was a colony rather than an individual, made up of many tiny animals called zooids each occupying a cup along a shared branching skeleton, comparable in organisation to a coral colony while belonging to a completely different group. The skeleton was built of a protein material closely related to collagen, secreted in overlapping half-rings that are visible under a microscope and that identify the group definitively. Their relationship to living animals was long uncertain and is now settled, since they belong with the pterobranchs, a small and obscure group of living colonial marine animals whose skeletons are built the same way, which places graptolites within the hemichordates and therefore closer to vertebrates than to corals. The earliest forms were attached to the sea floor. The group that matters for dating, the graptoloids, became planktonic, floating or drifting in open water rather than living on the bottom, and it is that shift that made them so useful.
Why they are ideal for dating
The requirements for a good index fossil are demanding and this group meets nearly all of them:
- •Planktonic habit, so they drifted across ocean basins and are found worldwide rather than in one region
- •Rapid evolution, with recognisable species lasting only a few hundred thousand to a couple of million years, which gives fine time resolution
- •Distinctive shapes, since the number of branches, the arrangement of the cups and the overall form differ sharply between species and are identifiable even in flattened specimens
- •Abundance, with rich assemblages in the right rocks and specimens numerous enough to be collected systematically
- •Preservation in deep-water black shales, which are precisely the rocks where shelly fossils are scarce, so they date sequences that nothing else can
- •A well-established zonal scheme, with the Ordovician and Silurian divided into graptolite zones that remain the primary correlation tool for those periods
How they are preserved
The characteristic preservation is a flattened carbonaceous film on the bedding surface of dark shale, which is why they resemble pencil marks and where the name comes from, since it means written stone. That preservation occurs because the organic skeleton was compressed as the sediment compacted, with the original material reduced to a carbon residue. The environment required is specific: quiet deep water with little oxygen at the sea floor, which excludes scavengers and burrowing animals that would otherwise destroy a delicate organic structure, and which is why graptolite shales are black and finely laminated. Three-dimensional specimens do occur, preserved in limestone nodules or in chert, and these are enormously valuable because they preserve the original construction and allowed the relationship to pterobranchs to be established. Specimens can also be released chemically from limestone by dissolving the rock, since the organic skeleton survives acids that destroy the matrix, which produces complete uncrushed colonies.
What they record besides time
Because they drifted in the water column and were widely distributed, these fossils track more than stratigraphy. Their distribution patterns reveal ocean circulation and the positions of continents, since assemblages shared between two regions indicate connected water and distinct ones indicate separation, and this evidence contributed to reconstructing the Palaeozoic arrangement of landmasses. Their abundance and diversity fluctuated with ocean conditions, and a major extinction at the end of the Ordovician, associated with glaciation and falling sea level, is recorded sharply in the group and is used to correlate that event worldwide. The shales they occur in are themselves significant, since organic-rich deep-water shales of this age are source rocks for petroleum and, more recently, targets for extraction, so the fossils are of direct commercial interest to geologists mapping them. The group declined through the Devonian and disappeared, leaving the obscure living pterobranchs as the only survivors of a lineage that once filled the oceans.
The takeaway
The fossils are colonies of tiny animals sharing a branching protein skeleton, related to living pterobranchs and therefore closer to vertebrates than to corals. Drifting in open water spread them worldwide, and fast evolution into sharply distinct forms makes them the primary dating tool for the Ordovician and Silurian. They preserve as carbon films in oxygen-poor black shale, exactly where shelly fossils are absent.