New Research Shows Evidence Of Cooperation In Graptolites

An Australian graptolite, showing its distinctive saw blade shape

Some of the greatest structures ever created in the animal kingdom are the result of cooperation between organisms in a group or colony. Beehives, wasp nests and the maze of underground tunnels built by ants, housing millions of individuals can all reach impressive proportions. Termite nests are even bigger, dominating whatever landscape they find themselves in. The largest, single cooperative structure on Earth, however, is the Great Barrier Reef.

Visible from space, this ecosystem is built on the remains of  ancient calcium carbonate skeletons of coral with younger, living ones layered on top. The entire complex is incredibly diverse, covering a wide range of lifestyles. Rounded brain corals cluster in niches, while large branching ones reach far out into the ocean, absorbing nutrients from the water as fish dart in and out of bright, swaying boughs.

Of course, such structures would not exist without cooperation between members of the reef. Indeed, examples of ancient reef systems, and symbiosis between different organisms, is common throughout the fossil record. Yet recent analysis of the remains of Palaeozoic fretsaw blade-like creatures called graptolites, which evolved during the early Cambrian and vanished without a trace during the mid Carboniferous, show they were as common in the ocean as plankton are today.

One of the colonies showing both hourglass and crosshatching structures

Graptolites existed as free floating colonies and so had a basic degree of organisation, with very similar joins between different members in the conglomerate. Yet the discovery of new fossils shows in their preservation something new. 'The light caught one of the fossils in just the right way, and it showed complex structures I had never seen in a graptolite before,' said Jan Zalasiewicz, a geologist from the University of Leicester, 'it was a sheer stroke of luck, one of those eureka moments.'

Graptolites are animal architects (they build their own living tubes) but these fossils are 'untidy, simple structures in comparison to the finely engineered, tightly organized living quarters of other graptolites,' he added. Differences in joins, some with cross hatch patterns, others similar to hour glasses, show a division in labour and a high degree of cooperation. There were also separate classes of graptolite: some possessed tentacles for snapping up nutrients from the water, while others acted like linchpins, holding the colony together.

'There is clear evidence here of polymorphism that is, of substantially different physical patterns of connection along the colony,' Zalasiewicz said. 'This suggests division of function within the colony also, which might help explain the extraordinary sophisticated building abilities that these animals possessed.' At the end, it was an ice age which dealt the fatal blow to the graptolites. Even with their polymorphic cooperation, the glaciers and freezing oceans whittled away at their numbers. Just a single species remained. Yet this survivor was simply not up to the task of bringing back the group and soon became extinct, consigning the graptolites to the gulf of extinction forever.