Graptolites are small colonial organisms, each made up of many tiny, genetically identical zooids joined together by tubes. They've been around since the Cambrian and at times in Earth's history have been very morphologically and taxonomically diverse. Now there is just one living genus, but they are very common in the fossil record, often appearing as a 'sawtooth' pattern flattened on surfaces of deep sea sedimentary rocks.

In this episode Laura talks to Dr David Bapst, a postdoctoral scholar at UC Davis and adjunct assistant professor at the South Dakota School of Mines, about extinct graptolites - the Graptoloidea - and how these animals have changed in the 520 million years since they originated. We find out about major events in their evolutionary history including the transition from sea-floor dwelling benthic species to plankton that floated in the water column, and the reduction through geological time of the number of branches from many branching dendritic forms to the single 'stick' monograptids.